|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online 10 July 2006
doi: 10.1242/dev.02459
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334, USA.
* Author for correspondence (e-mail: David.Stock{at}Colorado.edu)
Accepted 25 May 2006
 |
SUMMARY |
|---|
 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Key words: Zebrafish, Astyanax, Medaka, Catfish, Dlx, Fgf, Constraint
|
INTRODUCTION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
; Stock,
2001). Reduction of dentition from this state has occurred
repeatedly, whereas the gain of teeth has been less common
(Huysseune and Sire, 1998;
Stock, 2001;
Sire, 2001). Two potential
explanations for this macroevolutionary trend are that (1) reduced dentitions
are adaptively superior to more complete ones and (2) mechanisms of tooth loss
constrain the ability to regain teeth. A potential example of tooth loss
constraining dental evolution is the loss of oral teeth in cypriniform fishes,
which include the zebrafish. The cypriniform fossil record
(Cavender, 1991) suggests that
oral teeth were lost at least 50 million years ago. Since then, the group has
diversified into over 3000 extant species in five families
(Nelson, 1994), and included
within this diversity are feeding modes, such as predation on fishes, for
which oral teeth are likely adaptive
(Sibbing, 1991;
Portz and Tyus, 2004) (D.W.S.,
unpublished). This raises the possibility that no cypriniform has regained
oral teeth because of developmental genetic constraints on the ability to do
so, which would be further supported by finding a complex series of genetic
modifications associated with tooth loss.
We investigated the developmental genetic mechanism of cypriniform tooth
loss by comparing oral development in the zebrafish and a member of the
closest lineage that retains oral teeth - the characiform Astyanax
mexicanus, or Mexican tetra (Fink and
Fink, 1996; Jeffery,
2001). Cypriniforms and characiforms are members of the Superorder
Ostariophysi (Nelson, 1994);
we extended some comparisons to an additional ostariophysan (the siluriform
cuckoo catfish, Synodontis multipunctatus) and to an outgroup (the
Japanese medaka, Oryzias latipes, a member of the Superorder
Acanthopterygii), both of which possess oral teeth.
Tooth development has been studied most extensively in the mouse
(Jernvall and Thesleff, 2000;
Tucker and Sharpe, 2004;
Zhang et al., 2005), in which
the earliest sign of initiation is thickening of the oral epithelium to form
the dental lamina. This epithelium then invaginates into the underlying
mesenchyme to form a bud. Mesenchyme condenses around the bud and folding of
the epithelium occurs, prefiguring the crown shape of the tooth. Tooth
initiation and morphogenesis are similar in larval teleost fishes, although
mesenchymal condensation has been more difficult to document, perhaps because
of the small number of cells involved
(Huysseune et al., 1998;
Sire et al., 2002).
Because no morphological evidence of oral tooth development has been
observed in zebrafish (Huysseune et al.,
1998), we focused our comparisons of gene expression and function
on those involved in the earliest stages of mammalian tooth development. Even
before dental lamina formation in the mouse, signaling from odontogenic
epithelium through the Fibroblast growth factor (Fgf) pathway induces the
expression of multiple transcription factors in the underlying mesenchyme
(Neubüser et al., 1997;
Trumpp et al., 1999). Although
pathways regulating dental lamina formation are less well understood
(Jernvall and Thesleff, 2000;
Tucker and Sharpe, 2004),
several genes have been shown to mark this structure, including Shh
(Dassule and McMahon, 1998;
Hardcastle et al., 1998),
Dlx2 (Thomas et al.,
2000; Zhao et al.,
2000) and Pitx2
(Mucchielli et al., 1997;
Keränen et al.,
1999).
We examined expression in the zebrafish and Astyanax of the Fgf pathway ligand Fgf8, its putative downstream targets Pax9 and Lhx6, and the dental lamina markers Shh, Pitx2, Dlx2a and Dlx2b. Fgf signaling from oral epithelium to mesenchyme appears conserved in both species, although this pathway may not be involved in tooth development, as it is in the mouse. Dental lamina markers are expressed in the odontogenic epithelium of Astyanax in a pattern similar to that of the mouse. However, whereas pitx2 and shh are expressed in zebrafish oral epithelium, dlx2a and dlx2b are not. Examination of Dlx2 ortholog expression in additional fish species suggests that oral epithelial expression was lost in the zebrafish lineage rather than gained in Astyanax. Inhibition of Fgf signaling in Astyanax produced a partial phenocopy of the gene expression characteristics of zebrafish oral epithelium. We hypothesize that multiple Fgf signaling events occur in the jaw of toothed fishes, and that alteration of one of these led to the loss of cypriniform oral teeth.
|
MATERIALS AND METHODS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
). cDNAs were
cloned from an eyed population collected in Texas
(Dowling et al., 2002). In situ
hybridization and histology were performed with blind larvae from a commercial
population originating from La Cueva Chica, while imaging of live larvae and
inhibition of Fgf signaling employed a population from La Cueva de El
Pachón (Jeffery and Martasian,
1998). No features examined differed significantly among these
populations. Japanese medaka were of the Cab strain
(Wittbrodt et al., 2002).
Synodontis embryos and larvae were from a colony maintained by
Alexander Cruz (University of Colorado). Embryos and larvae of all species
were staged by hours post-fertilization (hpf). Zebrafish were raised at
28.5°C, Astyanax at 25°C and medaka at 26°C; rearing
temperatures of Synodontis were not closely controlled. For zebrafish
larvae examined in whole mount, pigmentation was inhibited with 0.003%
1-phenyl-2-thiourea.
Cloning and sequence analysis
RNA was isolated from larvae (or adult ovary) of Astyanax, medaka
and Synodontis using the Ultraspec System (Biotecx). Reverse
transcriptase-mediated PCR was carried out with the following degenerate
primer pairs (added restriction sites are underlined):
dlx2a, GCCGGGATCCATGACNGGNGTNTTYGAYAG, GCCGGAATTCAADATNGTNCCNGCRCTNAC;
dlx2b, GCCGGGATCCATNGTNAAYGGNAARCCNAA, GCCGGAATTCTGRAACCADATYTTNACYTG;
fgf8, GCCGGGATCCACNAGYGGNAARCAYGTNCA, GCCGGAATTCGGNARNCKYTTCATRAARTG;
lhx6, GCCGGGATCCGCNTGYTTYGCNTGYTTYTC, GCCGGAATTCARTTYTGRAACCANACYTG;
pax9, GCCGGGATCCGCNTTYGGNGARGTNAAYCA, GCCGGAATTCACNGARTGNGANGANGGCCA;
pitx2, GCCGGGATCCARMGNACNCAYTTYACNAG, GCCGGAATTCTTRCANGTRTCNCKRTANAC.
PCR products were cloned into pCR4-TOPO (Invitrogen) and subjected to automated sequencing. The SMART RACE Kit (BD Clontech) was used to generate complete cDNA sequences for Astyanax dlx2a, dlx2b, fgf8, pax9 and pitx2, medaka dlx2a, and Synodontis dlx2a and dlx2b. A partial cDNA sequence for Astyanax lhx6 was obtained similarly. All sequence positions reported (GenBank Accession numbers DQ822509-DQ822517) were determined for at least five independent clones, representing both strands.
The sequences were translated and potential orthologs identified by BLAST
searches of GenBank and the zebrafish and Fugu genome databases
(http://www.ensmbl.org).
Alignments of amino acid sequences were performed with the Clustal X program
(Thompson et al., 1997), and
phylogenetic analyses were carried out with the neighbor-joining algorithm of
MEGA version 2.0 (Kumar et al.,
2001).
In situ hybridization
Whole-mount in situ hybridization and sectioning followed the protocol of
Jackman et al. (Jackman et al.,
2004), with proteinase K pre-treatment at 2.5-25 µg/ml. Each
gene was examined in zebrafish fixed at 4-hour intervals between 28-60 hpf and
at 12-hour intervals thereafter through 120 hpf. All Astyanax genes
were examined in specimens aged 30, 36, 42, 48, 60 and 72 hpf.
Digoxigenin-labelled antisense riboprobes were synthesized from cloned cDNA
fragments as follows: Astyanax dlx2a (nucleotides 190-1019 of GenBank
DQ822509), dlx2b (13-587 of DQ822510), fgf8 (898-2103 of
DQ822511), lhx6 (56-897 of DQ822512), pax9 (189-775 of
DQ822513), pitx2 (427-958 of DQ822514) and shh (194-1187 of
AY661433); zebrafish dlx2b
(Ellies et al., 1997),
dlx2a, fgf8, lhx6, pax9, pitx2
(Jackman et al., 2004) and
shh (918-1573 of NM_131063); O. latipes dlx2a (158-987 of
DQ822515); and Synodontis dlx2a (199-1028 of DQ822516) and
dlx2b (264-955 of DQ822517).
Morphological analysis and histology
Astyanax oral development was examined using differential
interference contrast (DIC) microscopy in anaesthetised specimens embedded in
0.5% agarose. Astyanax and zebrafish were cleared and stained with
Alcian green (Jackman et al.,
2004), or Alcian blue and Alizarin red
(Hanken and Wassersug, 1981),
to visualize teeth. In addition, 2 µm serial sections of
glycol-methacrylate-embedded zebrafish and Astyanax larvae were cut
with glass knives and stained with 0.1% Toluidine blue.
SU5402 treatment
SU5402 (Mohammadi et al.,
1997) was used to inhibit signaling through Fgf receptors in the
zebrafish and Astyanax. Zebrafish were dechorionated and treated with
25 µM SU5402 in 0.5% DMSO from 24 or 32 hpf through 56 hpf, followed by
fixation and in situ hybridization
(Jackman et al., 2004). A
range of SU5402 concentrations was examined in Astyanax to find one
affecting teeth while minimizing other phenotypic abnormalities; the
concentration used in the data presented was 10 µM. Larvae were raised in
SU5402 beginning at 30 or 36 hours. Specimens for in situ hybridization were
fixed at 84 hpf and additional embryos were allowed to develop to 108 hpf for
skeletal staining. Control embryos of both species were raised in 0.5% DMSO.
Any larvae showing gross morphological abnormalities or severe developmental
retardation were excluded from consideration.
|
RESULTS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
), adult A. mexicanus possess
teeth attached to premaxillary, maxillary and dentary bones
(Fig. 1A). Trapani et al.
(Trapani et al., 2005)
identified the first teeth to appear in larvae as one attached to each
premaxillary bone and one on each side of the dentary bone midline
(Fig. 1B,C). Germs of these
could not be identified reliably in histological sections at 60 hpf, whereas
most 72 hpf specimens exhibited bell-shaped germs
(Fig. 1D-F), corresponding to
ongoing morphogenesis or cytodifferentiation stages in zebrafish pharyngeal
tooth development (Van der heyden and
Huysseune, 2000).
The absence of oral teeth in adult cypriniforms generally
(Nelson, 1994) and in
zebrafish specifically (Kimmel et al.,
1995; Huysseune et al.,
1998) is well known (Fig.
1G). Similarly, extensive skeletal staining of zebrafish larvae
(Schilling, 2002) failed to
reveal mineralized oral teeth (Fig.
1H). To search for arrested oral tooth germs in zebrafish larvae,
we produced serial histological sections of several individuals each of 60,
72, 84, 96, 108 and 120 hpf. No evidence for mandibular tooth germs was found
in any specimen (Fig.
1I-L).
Gene orthology
It has been suggested that genome duplication occurred in ray-finned fishes
before the divergence of the Ostariophysi (zebrafish, Astyanax and
Synodontis) and the Acanthopterygii (medaka and Fugu)
(Meyer and Van de Peer, 2005).
Of the genes we examined, two zebrafish orthologs are known for tetrapod Shh
(Zardoya et al., 1996) and
Dlx2 (Stock et al., 1996).
Yamamoto et al. (Yamamoto et al.,
2004) established the orthology of Astyanax shh, while
phylogenetic analysis (Fig. 2A)
identified Dlx2a and Dlx2b orthologs in Astyanax and
Synodontis, and dlx2a in medaka. Despite extensive
sequencing of Dlx homeodomains, we found no evidence for medaka
dlx2b. This gene may be absent from acanthopterygians, as BLAST
searches of the Fugu genome with the zebrafish homeodomain failed to detect
it.
A single zebrafish ortholog is known for tetrapod Fgf8, Pax9, Lhx6 and Pitx2. Phylogenetic analyses of Fgf8 and Pax9 (Fig. 2B,C) identify the Astyanax genes as orthologs rather than recently formed paralogs of their zebrafish counterparts because of concordance between gene and species relationships. Although phylogenetic analysis of Lhx6 and Lhx8 sequences has identified the Astyanax gene cloned as lhx6 (Fig. 2D), we cannot rule out the possibility that it is a recently formed paralog of zebrafish lhx6 because of lack of other teleost sequences.
|
) and
phylogenetic analyses (not shown) clustered the Astyanax gene with,
but outside of, Pitx2 genes from other vertebrates. Pitx2 is alternatively
spliced (Arakawa et al., 1998;
Essner et al., 2000), with the
cDNA we characterized corresponding to Pitx2a. We searched the zebrafish
genome with each Astyanax exon separately, but did not find any genes
other than pitx1, pitx2 and pitx3. Alignment of Pitx genes
revealed no similarity between exon 2 of Pitx2 and any region of Pitx1 or
Pitx3. By contrast, exon 2 exhibited a single amino acid difference (out of
fifteen) between the Astyanax gene and zebrafish pitx2
(Fig. 2E). We conclude that the
Astyanax gene is orthologous to zebrafish pitx2, but has
undergone substantial sequence divergence.
Dlx2 orthologs mark Astyanax odontogenic epithelium
Jackman et al. (Jackman et al.,
2004) identified the homeodomain transcription factor
dlx2b as a marker of zebrafish pharyngeal tooth epithelium. We
examined dlx2b expression in Astyanax to determine if it
also marked oral tooth epithelium. At 72 hpf, staining is present in localized
domains on either side of the midline in upper and lower jaws
(Fig. 3H). Sectioning revealed
that each corresponds to a tooth germ, based on epithelial morphology and
mineralization (Fig. 3I,M,N).
Expression is strongest in the epithelial layer, but prolonged staining
revealed expression in tooth germ mesenchyme (not shown).
To determine whether dlx2b marks earlier odontogenic epithelium,
we traced the tooth germ expression domains to earlier stages. In the
specimens examined, the oral plate was intact at 42 hours and ruptured by 48
hours (Fig. 3A,B). Between 48
and 72 hours, the mouth opening enlarges substantially and moves from a
midventral to a terminal location (Fig.
3B-D), similar to that of the zebrafish
(Kimmel et al., 1995). We
detected epithelial expression domains of dlx2b reminiscent of tooth
germs as early as 36 hpf (Fig.
3E-G,J-L). These move medially between 42 and 60 hpf
(Fig. 3E-G), and are likely to
correspond to odontogenic epithelium.
In addition to expression in odontogenic epithelium and mesenchyme, mouse
Dlx2 is expressed in mandibular arch mesenchyme outside tooth germs
(Qiu et al., 1997;
Thomas et al., 2000;
Zhao et al., 2000). This
latter expression is lacking for Astyanax dlx2b, but present for
dlx2a (lateral to tooth germs). At 72 hpf, dlx2a was
expressed in the epithelium and mesenchyme of all four tooth germs
(Fig. 3R,S). Tooth germ
expression appeared at 60 hpf (Fig.
3Q), while lateral (non-dental) mesenchyme expression could be
detected at 30 hours, the earliest stage examined
(Fig. 3O,P).
Loss of epithelial Dlx2 ortholog expression is associated with cypriniform tooth loss
Initial characterization of zebrafish dlx2a
(Akimenko et al., 1994) and
dlx2b (Ellies et al.,
1997) identified mandibular arch expression of the former but not
the latter. We closely examined the expression of both genes in zebrafish aged
28-120 hpf. dlx2b was not expressed in mandibular arch epithelium or
mesenchyme at any stage (Fig.
4A-C), whereas mandibular arch expression of dlx2a was
limited to lateral mesenchyme, corresponding to the non-dental expression of
its Astyanax ortholog (Fig.
4D-F).
We next investigated whether Dlx2 ortholog expression was gained in the
lineage leading to Astyanax or lost from that of zebrafish. We
isolated dlx2a and dlx2b orthologs
(Fig. 2A) from the catfish
Synodontis, an ostariophysan more closely related to
Astyanax than is the zebrafish
(Saitoh et al., 2003).
Synodontis dlx2a and dlx2b expression closely
matched that in Astyanax, with both being present in odontogenic
epithelium but only dlx2a present laterally in jaw mesenchyme
(Fig. 4G-L). We were only able
to isolate dlx2a (Fig.
2A) from the medaka, an outgroup of the Ostariophysi. As in the
other teleosts with oral teeth, medaka dlx2a is expressed both
laterally in jaw mesenchyme and in oral epithelium
(Fig. 4M-O). These data suggest
a loss of epithelial Dlx2 ortholog expression in association with cypriniform
oral tooth loss.
|
|
;
Dassule and McMahon, 1998;
Keränen et al., 1999).
Pitx2 is expressed broadly in stomodeal epithelium well before the
appearance of tooth germs, becoming progressively restricted to odontogenic
epithelium. Astyanax pitx2 is expressed similarly. Oral epithelial
expression was detected at the earliest stage examined and progressed from a
broad uniform domain (Fig.
5A,D) to more intense expression in the tooth germs themselves
(Fig. 5B,C,E,F).
Mouse Shh is localized to the epithelium of initiating tooth
germs, but at earlier stages is expressed broadly in pharyngeal endoderm and
stomodeal ectoderm (Keränen et al.,
1999; Sarkar et al.,
2000; Cobourne et al.,
2004; Jeong et al.,
2004; Moore-Scott and Manley,
2005). Faint mandibular arch expression of Astyanax shh
could be detected at 42 hours, and, by 48 hours, expression was strong in
patches on either side of the midline of upper and lower jaws
(Fig. 4G,J). Although tooth
germ expression was detected at 72 hpf
(Fig. 5I,L), it remains unclear
whether expression at earlier stages includes odontogenic epithelium
(Fig. 5G,H,J,K). In general,
shh was more posteriorly (lingually) restricted in expression than
pitx2 and may be adjacent but lingual to dlx2b expression
before definitive tooth germ expression at 72 hpf
(Fig. 3K,L;
Fig. 5D,E,J,K).
|
;
Miller et al., 2004). We
examined the expression of these genes in detail to determine whether it
resembled expression in Astyanax tooth germs. Both shh and
pitx2 exhibited broad expression that was continuous across the
medial-lateral axis at 60 hpf (Fig.
5M,P). Sagittal sections at a variety of stages revealed that
shh expression was lingual to that of pitx2, with little, if
any, overlap (Fig. 5N,Q).
Transverse sections showed shh expression throughout the mediolateral
axis of the lower jaw and in a position dorsal to Meckel's cartilage
(Fig. 5R). This pattern
persisted through the latest stage examined (120 hpf), with no evidence of
focal expression resembling tooth germs. pitx2 was similarly
expressed across the mediolateral axis of the lower jaw
(Fig. 5O), with no evidence of
focal expression, before becoming undetectable at 120 hpf. Interestingly,
expression was ventral to Meckel's cartilage, suggesting the absence of a
domain co-expressing pitx2 and shh that characterizes
Astyanax tooth germs.
Induction of mesenchymal Pax9 and Lhx6 expression by Fgf signaling occurs in zebrafish and Astyanax
Absence of evidence for a dental lamina in the zebrafish oral cavity led us
to examine an initiation pathway that acts in the mouse before the physical
appearance of tooth germs. In this pathway, epithelially expressed
Fgf8 induces the expression of multiple transcription factors in the
underlying mesenchyme, including the paired domain-containing Pax9
and the LIM-homeodomain-containing Lhx6
(Neubüser et al., 1997;
Trumpp et al., 1999). We found
fgf8 expression in the Astyanax oral region from the
earliest stage examined. This expression was located in the lateral epithelium
of presumptive upper and lower jaws (Fig.
6A,D,G,J). As had been observed for dlx2b, the left and
right fgf8 expression domains move medially between 42 and 48 hpf.
However, fgf8 expression was more lateral than that of
dlx2b, being adjacent to dlx2b expression through 48 hpf and
discontinuous with this dental epithelial marker after 60 hpf
(Fig. 6M).
Astyanax pax9 and lhx6 are expressed in the mesenchyme of upper and lower jaws (Fig. 6B,C,E,F,H,I,K,L,N,O). Although their expression underlies that of fgf8 and extends further medially, it is concentrated laterally to developing tooth germs. In the case of pax9, simultaneous analysis with shh or dlx2b as markers of the dental epithelium revealed that pax9 expression at 60-72 hpf was very weak in the tooth germ mesenchyme (Fig. 6N). Finally, we found that Astyanax pax9, but not lhx6, was expressed in oral epithelium (Fig. 6H,I,K,L,N).
Expression of zebrafish fgf8 has been described in the early
stomodeum (Eberhart et al.,
2006). We examined its later expression, and that of zebrafish
lhx6 and pax9, finding no significant differences with
Astyanax. Expression of all three zebrafish genes persisted in oral
jaws through to the latest stage examined
(Fig. 6P-R).
|
Inhibition of Fgf signaling in Astyanax partially phenocopies zebrafish oral epithelium
Pharyngeal tooth expression of zebrafish dlx2a and dlx2b
is Fgf dependent (Jackman et al.,
2004), whereas oral epithelial expression of mouse Fgf8,
Shh and Pitx2 is not
(Mandler and Neubüser,
2001). As dlx2a and dlx2b expression is absent
from zebrafish oral epithelium, while fgf8, shh and pitx2
expression is present, we explored the hypothesis that the loss of Fgf
signaling led to the loss of oral Dlx2 expression in cypriniforms. We examined
whether Fgf signaling is required for the development of oral teeth in
Astyanax by treating larvae with SU5402 from 30 or 36 hpf through 108
hpf. Both treatments blocked tooth formation
(Fig. 7A,F), as assayed by
skeletal staining (n=9/12 and 11/13 completely lacking oral teeth
from the treatments, respectively, versus n=1/12 in DMSO-treated
controls). Varying degrees of cranial cartilage reduction and malformation
were also observed, with effects being more severe with early treatments. For
this reason, reported gene expression data are from treatments at 36 hpf,
unless otherwise indicated.
SU5402 treatment did not prevent oral expression of shh
(n=7/7) or pitx2 (n=6/6) in Astyanax,
although focal expression corresponding to tooth germs was completely
eliminated for shh and reduced (n=3/6) for pitx2
(Fig. 7M,N,R,S). Treatment at
30 hpf completely eliminated focal expression of pitx2
(n=6/6). Similar results were obtained by treating zebrafish with
SU5402 from 24 to 56 hpf, which failed to eliminate oral shh
(n=10/10) or pitx2 (n=9/9) expression.
Unexpectedly, given that oral Fgf8 expression in mouse was found to
be Fgf independent (Mandler and
Neubüser, 2001), SU5402 treatment resulted in a severe
reduction or absence of oral Fgf8 expression in Astyanax
(Fig. 7E,J; n=4/4 with
treatment at 30 hpf, 6/8 at 36 hpf) and zebrafish (n=9/10).
Finally, we found oral expression of dlx2a and dlx2b to be Fgf dependent in Astyanax (Fig. 7K,L,P,Q). SU5402 treatment eliminated epithelial dlx2b (n=6/7) and dlx2a (n=8/8) expression, with both results being significantly different from controls (n=2/8, P<0.05; n=1/6, P<0.01, respectively; Fisher's exact test). Although neither gene is expressed in zebrafish oral epithelium, SU5402 treatment reduced or eliminated lateral mesenchymal expression of Dlx2a in both Astyanax (Fig. 7K,P; n=8/8) and zebrafish (Fig. 7O,T; n=6/6).
|
DISCUSSION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
;
Zhang et al., 2005).
Histological analysis failed to reveal such thickening in the zebrafish oral
cavity at any developmental stage. For the dental lamina markers examined, the
most distinctive difference between zebrafish and Astyanax was a
complete absence of Dlx2 ortholog expression in zebrafish oral epithelium. Of
these genes, at least dlx2b is expressed in Astyanax oral
epithelium well before tooth germs appear, suggesting that differences between
the two species arise early in development.
|
;
Keränen et al., 1999;
Sarkar et al., 2000;
Cobourne et al., 2004) and
rainbow trout (Fraser et al.,
2004), and we found a similar pattern in Astyanax.
Expression of zebrafish pitx2 and shh was broadly
distributed at all stages, with no obvious focal expression corresponding to
initiating tooth germs. We suggest that this expression is unrelated to tooth
formation, but rather reflects other known functions of these genes in
vertebrate craniofacial development (e.g.
Lu et al., 1999;
Liu et al., 2003;
Hu et al., 2003;
Moore-Scott and Manley, 2005;
Wada et al., 2005).
Fgf signaling to oral mesenchyme occurs in zebrafish but is independent of tooth initiation
We found Fgf signaling to mandibular mesenchyme, an event considered to be
the earliest stage of mouse tooth initiation
(Neubüser et al., 1997;
Tucker and Sharpe, 2004), to
be conserved in the zebrafish oral region. Zebrafish fgf8 is
expressed in oral epithelium, and orthologs of mesenchymal transcription
factor targets in the mouse (Pax9 and Lhx6) are expressed in
mandibular arch mesenchyme in an Fgf-dependent fashion. However, although Fgf8
and Pax9 are required for mammalian tooth development
(Peters et al., 1998;
Trumpp et al., 1999;
Stockton et al., 2000), our
analysis of their expression in Astyanax suggests this may not be the
case in teleosts.
Both Fgf8 and Pax9 are expressed in rodents in regions considered to be
presumptive dental epithelium and mesenchyme, respectively, and their
expression persists in tooth germs through the bud stage
(Neubüser et al., 1997;
Peters et al., 1998;
Keränen et al., 1999). We
found Astyanax fgf8 expression to be consistently lateral to the
expression of dlx2b, a probable marker of odontogenic epithelium, and
absent from tooth germs once they became morphologically apparent. Although
pax9 expression does extend into early tooth germ mesenchyme, it does
so only weakly, with stronger expression located laterally. This lack of
association between fgf8 and pax9 expression and tooth
development in Astyanax parallels the absence of expression of both
genes in zebrafish pharyngeal teeth
(Jackman et al., 2004). We
speculate that fgf8 and pax9 expression in the zebrafish
oral region does not represent residual tooth initiation, but rather reflects
other roles in jaw skeletal development
(Peters et al., 1998;
Trumpp et al., 1999).
Interestingly, Pax9 is required for dental mesenchyme condensation in
the mouse (Peters et al.,
1998), a process that may not occur in some teleosts
(Huysseune and Sire, 1997;
Huysseune et al., 1998).
Loss of epithelial Dlx2 expression is associated with cypriniform oral tooth loss, but is unlikely to be the sole cause
Oral epithelial expression of Dlx2 orthologs is absent from zebrafish but
present in two ostariophysans with oral teeth (Astyanax and
Synodontis), and in two outgroups, the medaka and the mouse
(Thomas et al., 2000). These
data are consistent with loss of cypriniform dlx2a and dlx2b
expression in association with oral tooth loss. Several lines of evidence
suggest that loss of Dlx2 expression was not the sole cause of tooth loss,
however. Dlx2 does not have a unique function in mouse tooth
development, as its inactivation has no effect on dentition
(Qiu et al., 1995).
Simultaneous inactivation of Dlx1 and Dlx2 leads to an early
arrest of upper molar teeth, but this effect is believed to reflect their
requirement in mesenchyme (Thomas et al.,
1997). However, other Dlx genes are expressed in mouse dental
epithelium (Zhao et al.,
2000), where they may function redundantly with Dlx2. Similarly,
injection of morpholino antisense oligonucleotides targeting zebrafish
dlx2a and dlx2b altered cartilage but not pharyngeal tooth
development (W.R.J. and D.W.S., unpublished). Finally, coordinate loss of
dlx2a and dlx2b requires only one change in a (common)
trans-acting regulator, as opposed to independent changes in the
cis-regulatory region of each gene.
|
), and we
found a similar dependence in Astyanax oral teeth. Furthermore, Shh
and Pitx2 expression persists after SU5402 treatment in both species, as it
does in the mouse (Mandler and
Neubüser, 2001). In Astyanax, focal expression
frequently did not appear, leaving expression domains similar to those found
in untreated zebrafish. These data are consistent with the hypothesis that a
loss of Fgf signaling caused the loss of Dlx2 ortholog expression in
cypriniform oral epithelium without abolishing shh and pitx2
expression. Our results with other genes assayed after SU5402 treatment
suggest, however, that global loss of Fgf expression did not occur in the
cypriniform mouth. Mandibular arch expression of dlx2a, lhx6 and
pax9 in mesenchyme, and fgf8 in epithelium, has been
retained in zebrafish but is Fgf dependent in both zebrafish and
Astyanax. Mandler and Neubüser
(Mandler and Neubüser,
2001) found a similar Fgf dependence for mouse Dlx2, Lhx6
and Pax9, but not Fgf8. However, the different results for
Fgf8 may reflect differences in the developmental stage of treatment.
|
; Trumpp et al.,
1999; Abu-Issa et al.,
2002). We speculate that a separate conserved signal inducing
epithelial Fgf8 originates from the mesenchyme based on the mesenchymal
dependence of oral Fgf8 expression in the mouse
(Creuzet et al., 2004) and
induction by mouse mesenchymal cells of ectopic Fgf8 expression in
chick facial primordia (Mitsiadis et al.,
2003). The Fgf signal we propose to have been lost from
cypriniforms, one inducing epithelial Dlx2a and Dlx2b expression, may
originate medial to the former signals based on our gene expression
comparisons. We speculate that the source of this is the mesenchyme, through
analogy with feather development, which exhibits numerous similarities to that
of teeth (Pispa and Thesleff,
2003). Specifically, Fgf signaling from the feather mesenchyme to
epithelium is required for organ initiation
(Mandler and Neubüser,
2004) and the induction of epithelial Dlx2 expression
(Rouzankina et al., 2004).
Multiple paths to tooth loss
The developmental genetic basis of tooth loss has been studied in the
chicken (Chen et al., 2000;
Mitsiadis et al., 2003;
Harris et al., 2006) and the
diastema (gap in dentition) of rodents
(Keränen et al., 1999).
Cypriniform tooth loss appears to differ in mechanism from both of these
examples. In contrast to the zebrafish oral region, bud stage rudiments appear
in the rodent diastema. In addition, diastemal epithelium exhibits discrete
Shh and Pitx2 expression at late stages
(Keränen et al., 1999),
and Dlx2 expression at early stages
(Thomas et al., 2000). Various
(and not mutually exclusive) mechanisms have been proposed for tooth loss in
birds, including loss of Bmp4 signaling from the epithelium
(Chen et al., 2000), loss of a
mesenchyme-to-epithelium signal (Mitsiadis
et al., 2003) and change in the position of an epithelial
signaling center (Harris et al.,
2006). Although the mandibular arch expression pattern of
Dlx2 has not been described in detail in the chicken, this species
differs from the zebrafish in the presence of a rudimentary dental lamina
expressing Shh and Pitx2
(Helms et al., 1997;
Chen et al., 2000;
Harris et al., 2006). We
conclude that tooth development proceeds to later stages in birds and the
rodent diastema than in cypriniform oral jaws, although such differences may
have accumulated after the initial loss of functional teeth.
Is cypriniform tooth loss reversible?
It has been argued that tooth loss in birds is evolutionarily irreversible
because of the genetic drift-induced inactivation of tooth-specific genes
(Marshall et al., 1994).
Consistent with this view, ectopic protein expression and a single gene
mutation can produce tooth rudiments in chickens
(Chen et al., 2000;
Harris et al., 2006) but not
fully formed teeth. Even associations of mouse and chicken tissues that form
tooth-like structures with dentine fail to form enamel
(Mitsiadis et al., 2003).
In contrast to birds, zebrafish retain the genetic information necessary to
make pharyngeal teeth and the data available to date suggest that this
information is similar to that used in controlling oral tooth development
(Jackman et al., 2004;
Fraser et al., 2004). The
number of genes that must be redeployed in the cypriniform oral region to
produce teeth remains unknown, however. We documented differences in the
expression of pitx2, shh, dlx2a and dlx2b in zebrafish and
in teleosts with oral teeth, and showed that all could potentially result from
a single upstream genetic change, namely a loss of Fgf signaling. Such a
change might be reversible in evolution; for example, through the de novo
appearance of an enhancer in a gene in the Fgf pathway. This potential ease of
tooth re-acquisition conflicts with evidence for tooth loss having constrained
the evolution of cypriniform feeding mechanisms. Although cypriniform fishes
exhibit a diversity of feeding modes, large fish-eating forms are rare
(Sibbing, 1991), and those
that exist may be less efficient predators than teleosts retaining oral teeth
(Portz and Tyus, 2004).
Investigation of additional genetic pathways of tooth development in the
zebrafish oral region and particularly gain-of-function experiments should
further help to distinguish between adaptation and developmental constraint as
explanations for the pattern of cypriniform dental evolution.
|
ACKNOWLEDGMENTS |
|---|
|
Footnotes |
|---|
Present address: Department of Paleobiology, National Museum of Natural
History, Smithsonian Institution, NHB, MRC 121, Washington, DC 20013-7012,
USA 
|
REFERENCES |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Abu-Issa, R., Smyth, G., Smoak, I., Yamamura, K. I. and Meyers,
E. N. (2002). Fgf8 is required for pharyngeal arch
and cardiovascular development in the mouse.
Development 129,4613
-4625.
Akimenko, M.-A., Ekker, M., Wegner, J., Lin, W. and Westerfield, M. (1994). Combinatorial expression of three zebrafish genes related to Distal-less: part of a homeobox gene code for the head. J. Neurosci. 14,3475 -3486.[Abstract]
Arakawa, H., Nakamura, T., Zhadanov, A. B., Fidanza, V., Yano,
T., Bullrich, F., Shimizu, M., Blechman, J., Mazo, A., Canaani, E. et al.
(1998). Identification and characterization of the ARP1
gene, a target for the human acute leukemia ALL1 gene.
Proc. Natl. Acad. Sci. USA
95,4573
-4578.
Cavender, T. M. (1991). The fossil record of the Cyprinidae. In Cyprinid Fishes: Systematics, Biology and Exploitation (ed. I. J. Winfield and J. S. Nelson), pp.34 -54. New York: Chapman & Hall.
Chen, Y., Zhang, Y., Jiang, T.-X., Barlow, A. J., St Amand, T.
R., Hu, Y., Heaney, S., Francis-West, P., Chuong, C.-M. and Maas, R.
(2000). Conservation of early odontogenic signaling pathways in
Aves. Proc. Natl. Acad. Sci. USA
97,10044
-10049.
Cobourne, M. T., Miletich, I. and Sharpe, P. T.
(2004). Restriction of sonic hedgehog signaling during early
tooth development. Development
131,2875
-2885.
Creuzet, S., Schuler, B., Couly, G. and Le Douarin, N. M.
(2004). Reciprocal relationships between Fgf8 and neural crest
cells in facial and forebrain development. Proc. Natl. Acad. Sci.
USA 101,4843
-4847.
Dassule, H. R. and McMahon, A. P. (1998). Analysis of epithelial-mesenchymal interactions in the initial morphogenesis of the mammalian tooth. Dev. Biol. 202,215 -227.[CrossRef][Medline]
Dowling, T. E., Martasian, D. P. and Jeffery, W. R.
(2002). Evidence for multiple genetic forms with similar eyeless
phenotypes in the blind cavefish, Astyanax mexicanus. Mol. Biol.
Evol. 19,446
-455.
Eberhart, J. K., Swartz, M. E., Crump, J. G. and Kimmel, C.
B. (2006). Early Hedgehog signaling from neural to oral
epithelium organizes anterior craniofacial development.
Development 133,1069
-1077.
Ellies, D. L., Stock, D. W., Hatch, G., Giroux, G., Weiss, K. M. and Ekker, M. (1997). Relationship between the genomic organization and the overlapping embryonic expression patterns of the zebrafish dlx genes. Genomics 45,580 -590.[CrossRef][Medline]
Essner, J. J., Branford, W. W., Zhang, J. and Yost, H. J. (2000). Mesendoderm and left-right brain, heart and gut development are differentially regulated by pitx2 isoforms. Development 127,1081 -1093.[Abstract]
Fink, S. V. and Fink, W. L. (1996). Interrelationships of ostariophysan fishes (Teleostei). In Interrelationships of Fishes (ed. M. L. J. Stiassny, L. R. Parenti and G. D. Johnson), pp. 209-249. New York: Academic Press.
Fraser, G. J., Graham, A. and Smith, M. M. (2004). Conserved deployment of genes during odontogenesis across osteichthyans. Proc. R. Soc. Lond. B Biol. Sci. 271,2311 -2317.[Medline]
Gage, P. J., Suh, H. and Camper, S. A. (1999). The bicoid-related Pitx gene family in development. Mamm. Genome 10,197 -200.[CrossRef][Medline]
Hanken, J. and Wassersug, R. (1981). The visible skeleton. Funct. Photog. 16, 22-26, 44.
Hardcastle, Z., Mo, R., Hui, C.-C. and Sharpe, P. T. (1998). The Shh signalling pathway in tooth development: defects in Gli2 and Gli3 mutants. Development 125,2803 -2811.[Abstract]
Harris, M. P., Hasso, S. M., Ferguson, M. W. J. and Fallon, J. F. (2006). The development of archosaurian first-generation teeth in a chicken mutant. Curr. Biol. 16,371 -377.[CrossRef][Medline]
Helms, J. A., Kim, C. H., Hu, D., Minkoff, R., Thaller, C. and Eichele, G. (1997). Sonic hedgehog participates in craniofacial morphogenesis and is down-regulated by teratogenic doses of retinoic acid. Dev. Biol. 187, 25-35.[CrossRef][Medline]
Hu, D., Marcucio, R. S. and Helms, J. A.
(2003). A zone of frontonasal ectoderm regulates patterning and
growth in the face. Development
130,1749
-1758.
Huysseune, A. and Sire, J.-Y. (1997). Structure and development of first-generation teeth in the cichlid Hemichromis bimaculatus (Teleostei, Cichlidae). Tissue Cell 29,679 -697.[CrossRef][Medline]
Huysseune, A. and Sire, J.-Y. (1998). Evolution of patterns and processes in teeth and tooth-related tissues in non-mammalian vertebrates. Eur. J. Oral Sci. 106 Suppl 1,437 -481.
Huysseune, A., Van der heyden, C. and Sire, J.-Y. (1998). Early development of the zebrafish (Danio rerio) pharyngeal dentition (Teleostei, Cyprinidae). Anat. Embryol. 198,289 -305.[CrossRef][Medline]
Jackman, W. R., Draper, B. W. and Stock, D. W. (2004). Fgf signaling is required for zebrafish tooth development. Dev. Biol. 274,139 -157.[CrossRef][Medline]
Jeffery, W. R. (2001). Cavefish as a model system in evolutionary developmental biology. Dev. Biol. 231,1 -12.[CrossRef][Medline]
Jeffery, W. R. and Martasian, D. P. (1998). Evolution of eye regression in the cavefish Astyanax: apoptosis and the Pax-6 gene. Am. Zool. 38,685 -696.
Jeong, J., Mao, J., Tenzen, T., Kottmann, A. H. and McMahon, A.
P. (2004). Hedgehog signaling in the neural crest cells
regulates the patterning and growth of facial primordial. Genes
Dev. 18,937
-951.
Jernvall, J. and Thesleff, I. (2000). Reiterative signaling and patterning during mammalian tooth morphogenesis. Mech. Dev. 92,19 -29.[CrossRef][Medline]
Keränen, S. V. E., Kettunen, P., Aberg, T., Thesleff, I. and Jernvall, J. (1999). Gene expression patterns associated with suppression of odontogenesis in mouse and vole diastema regions. Dev. Genes Evol. 209,495 -506.[CrossRef][Medline]
Kimmel, C. B., Ballard, W. W., Kimmel, S. R., Ullmann, B. and Schilling, T. F. (1995). Stages of embryonic development of the zebrafish. Dev. Dyn. 203,253 -310.[Medline]
Kumar, S., Tamura, K., Jakobsen, I. B. and Nei, M.
(2001). MEGA2: molecular evolutionary genetics analysis software.
Bioinformatics 17,1244
-1245.
Liu, W., Selever, J., Lu, M.-F. and Martin, J. F.
(2003). Genetic dissection of Pitx2 in craniofacial
development uncovers new functions in branchial arch morphogenesis, late
aspects of tooth morphogenesis and cell migration.
Development 130,6375
-6385.
Lu, M.-F., Pressman, C., Dyer, R., Johnson, R. L. and Martin, J. F. (1999). Function of Reiger syndrome gene in left-right asymmetry and craniofacial development. Nature 401,276 -278.[CrossRef][Medline]
Mandler, M. and Neubüser, A. (2001). FGF signaling is necessary for the specification of the odontogenic mesenchyme. Dev. Biol. 240,548 -559.[CrossRef][Medline]
Mandler, M. and Neubüser, A. (2004). FGF
signaling is required for initiation of feather placode development.
Development 131,3333
-3343.
Marshall, C. R., Raff, E. C. and Raff, R. A.
(1994). Dollo's law and the death and resurrection of genes.
Proc. Natl. Acad. Sci. USA
91,12283
-12287.
Meyer, A. and Van de Peer, Y. (2005). From 2R to 3R: evidence for a fish-specific genome duplication (FSGD). BioEssays 27,937 -945.[CrossRef][Medline]
Miller, C. T., Schilling, T. F., Lee, K.-H., Parker, J. and Kimmel, C. B. (2000). sucker encodes a zebrafish Endothelin-1 required for ventral pharyngeal arch development. Development 127,3815 -3828.[Abstract]
Miller, C. T., Maves, L. and Kimmel, C. B.
(2004). moz regulates Hox expression and pharyngeal
segment identity in zebrafish. Development
131,2443
-2461.
Mitsiadis, T. A., Chéraud, Y., Sharpe, P. and
Fontaine-Pérus, J. (2003). Development of teeth in
chick embryos after mouse neural crest transplantations. Proc.
Natl. Acad. Sci. USA 100,6541
-6545.
Mohammadi, M., McMahon, G., Sun, L., Tang, C., Hirth, P., Yeh,
B. K., Hubbard, S. R. and Schlessinger, J. (1997). Structures
of the tyrosine kinase domain of fibroblast growth factor receptor in complex
with inhibitors. Science
276,955
-960.
Moore-Scott, B. A. and Manley, N. R. (2005). Differential expression of Sonic hedgehog along the anterior-posterior axis regulates patterning of pharyngeal pouch endoderm and pharyngeal endoderm-derived organs. Dev. Biol. 278,3232 -3235.
Mucchielli, M.-L., Mitsiadis, T. A., Raffo, S., Brunet, J.-F., Proust, J.-P. and Goridis, C. (1997). Mouse Otlx2/RIEG expression in the odontogenic epithelium precedes tooth initiation and requires mesenchyme-derived signals for its maintenance. Dev. Biol. 189,275 -284.[CrossRef][Medline]
Nelson, J. S. (1994). Fishes of the World (3rd edn). New York: John Wiley & Sons.
Neubüser, A., Peters, H., Balling, R. and Martin, G. R. (1997). Antagonistic interactions between FGF and BMP signaling pathways: a mechanism for positioning the sites of tooth formation. Cell 90,247 -255.[CrossRef][Medline]
Peters, H., Neubüser, A., Kratochwil, K. and Balling,
R. (1998). Pax9-deficient mice lack pharyngeal pouch
derivatives and teeth and exhibit craniofacial and limb abnormalities.
Genes Dev. 12,2735
-2747.
Pispa, J. and Thesleff, I. (2003). Mechanisms of ectodermal organogenesis. Dev. Biol. 262,195 -205.[CrossRef][Medline]
Portz, D. E. and Tyus, H. M. (2004). Fish humps in two Colorado River fishes: a morphological response to cyprinid predation? Environ. Biol. Fishes 71,233 -245.[CrossRef]
Qiu, M., Bulfone, A., Martinez, S., Meneses, J. J., Shimamura,
K., Pedersen, R. A. and Rubenstein, J. L. R. (1995). Null
mutation of Dlx-2 results in abnormal morphogenesis of proximal first
and second branchial arch derivatives and abnormal differentiation in the
forebrain. Genes Dev. 9,2523
-2538.
Qiu, M., Bulfone, A., Ghattas, I., Meneses, J. J., Christensen, L., Sharpe, P. T., Presley, R., Pedersen, R. A. and Rubenstein, J. L. R. (1997). Role of the Dlx homeobox genes in proximodistal patterning of the branchial arches: mutations of Dlx-1, Dlx-2, and Dlx-1 and -2 alter morphogenesis of proximal skeletal and soft tissue structures derived from the first and second arches. Dev. Biol. 185,165 -184.[CrossRef][Medline]
Rouzankina, I., Abate-Shen, C. and Niswander, L. (2004). Dlx genes integrate positive and negative signals during feather bud development. Dev. Biol. 265,219 -233.[CrossRef][Medline]
Saitoh, K., Miya, M., Inoue, J. G., Ishiguro, N. B. and Nishida, M. (2003). Mitochondrial genomics of ostariophysan fishes: perspectives on phylogeny and biogeography. J. Mol. Evol. 56,464 -472.[CrossRef][Medline]
Sarkar, L., Cobourne, M., Naylor, S., Smalley, M., Dale, T. and
Sharpe, P. T. (2000). Wnt/Shh interactions regulate
ectodermal boundary formation during mammalian tooth development.
Proc. Natl. Acad. Sci. USA
97,4520
-4524.
Schilling, T. F. (2002). The morphology of larval and adult zebrafish. In Zebrafish: A Practical Approach (ed. C. Nüsslein-Volhard and R. Dahm), pp.59 -94. New York: Oxford University Press.
Sibbing, F. A. (1991). Food capture and oral processing. In Cyprinid Fishes: Systematics, Biology and Exploitation (ed. I. J. Winfield and J. S. Nelson), pp.377 -412. New York: Chapman & Hall.
Sire, J.-Y. (2001). Teeth outside the mouth in teleost fishes: how to benefit from a developmental accident. Evol. Dev. 3,104 -108.[CrossRef][Medline]
Sire, J.-Y., Davit-Beal, T., Delgado, S., Van der heyden, C. and Huysseune, A. (2002). First-generation teeth in nonmammalian lineages: evidence for a conserved ancestral character. Microsc. Res. Tech. 59,408 -434.[CrossRef][Medline]
Stock, D. W. (2001). The genetic basis of
modularity in the development and evolution of the vertebrate dentition.
Philos. Trans. R. Soc. Lond. B Biol. Sci.
356,1633
-1653.
Stock, D. W., Ellies, D. L., Zhao, Z., Ekker, M., Ruddle, F. H.
and Weiss, K. M. (1996). The evolution of the vertebrate Dlx
gene family. Proc. Natl. Acad. Sci. USA
93,10858
-10863.
Stockton, D. W., Das, P., Goldenberg, M., D'Souza, R. N. and Patel, P. I. (2000). Mutation of PAX9 is associated with oligodontia. Nat. Genet. 24, 18-19.[CrossRef][Medline]
Thomas, B. L., Tucker, A. S., Qiu, M., Ferguson, C. A., Hardcastle, Z., Rubenstein, J. L. R. and Sharpe, P. T. (1997). Role of the Dlx-1 and Dlx-2 genes in patterning of the murine dentition. Development 124,4811 -4818.[Abstract]
Thomas, B. L., Liu, J. K., Rubenstein, J. L. R. and Sharpe, P. T. (2000). Independent regulation of Dlx2 expression in the epithelium and mesenchyme of the first branchial arch. Development 127,217 -224.[Abstract]
Thompson, J. D., Gibson, T. J., Plewniak, F., Jeanmougin, F. and
Higgins, D. G. (1997). The CLUSTAL_X windows interface:
flexible strategies for multiple sequence alignment aided by quality analysis
tools. Nucleic Acids Res.
25,4876
-4882.
Trapani, J., Yamamoto, Y. and Stock, D. W. (2005). Ontogenetic transition from unicuspid to multicuspid oral dentition in a teleost fish: Astyanax mexicanus, the Mexican tetra (Ostariophysi: Characidae). Zool. J. Linn. Soc. 145,523 -538.[CrossRef]
Trumpp, A., Depew, M. J., Rubenstein, J. L. R., Bishop, J. M.
and Martin, G. R. (1999). Cre-mediated gene inactivation
demonstrates that FGF8 is required for cell survival and patterning of the
first branchial arch. Genes Dev.
13,3136
-3148.
Tucker, A. and Sharpe, P. (2004). The cutting-edge of mammalian development; how the embryo makes teeth. Nat. Rev. Genet. 5,499 -508.[Medline]
Valdéz-Moreno, M. E. and Contreras-Balderas, S. (2003). Skull osteology of the characid fish Astyanax mexicanus (Teleostei: Characidae). Proc. Biol. Soc. Wash. 116,341 -355.
Van der heyden, C. and Huysseune, A. (2000). Dynamics of tooth formation and replacement in the zebrafish (Danio rerio) (Teleostei, Cyprinidae). Dev. Dyn. 219,486 -496.[CrossRef][Medline]
Wada, N., Javidan, Y., Nelson, S., Carney, T. J., Kelsh, R. N.
and Schilling, T. F. (2005). Hedgehog signaling is required
for cranial neural crest morphogenesis and chondrogenesis at the midline in
the zebrafish skull. Development
132,3977
-3988.
Wittbrodt, J., Shima, A. and Schartl, M. (2002). Medaka - a model organism from the Far East. Nat. Rev. Genet. 3,53 -64.[CrossRef][Medline]
Yamamoto, Y., Stock, D. W. and Jeffery, W. R. (2004). Hedgehog signaling controls eye degeneration in blind cavefish. Nature 431,844 -847.[CrossRef][Medline]
Zardoya, R., Abouheif, E. and Meyer, A. (1996).
Evolutionary analyses of hedgehog and Hoxd-10 genes in fish species
closely related to the zebrafish. Proc. Natl. Acad. Sci.
USA 93,13036
-13041.
Zhang, Y. D., Chen, Z., Song, Y. Q., Liu, C. and Chen, Y. P. (2005). Making a tooth: growth factors, transcription factors, and stem cells. Cell Res. 15,301 -316.[CrossRef][Medline]
Zhao, Z., Stock, D. W., Buchanan, A. V. and Weiss, K. M. (2000). Expression of Dlx genes during the development of the murine dentition. Dev. Genes Evol. 210,270 -275.[CrossRef][Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  R. Britz, K. W. Conway, and L. Ruber Spectacular morphological novelty in a miniature cyprinid fish, Danionella dracula n. sp. Proc R Soc B, June 22, 2009; 276(1665): 2179 - 2186. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. B. Gross, M. Protas, M. Conrad, P. E. Scheid, O. Vidal, W. R. Jeffery, R. Borowsky, and C. J. Tabin Gene Networks in Development and Evolution Special Feature Sackler Colloquium: Synteny and candidate gene prediction using an anchored linkage map of Astyanax mexicanus PNAS, December 23, 2008; 105(51): 20106 - 20111. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. R. Jackman and D. W. Stock Transgenic analysis of Dlx regulation in fish tooth development reveals evolutionary retention of enhancer function despite organ loss PNAS, December 19, 2006; 103(51): 19390 - 19395. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
