|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online 16 May 2007
doi: 10.1242/dev.002618
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Laboratory for Evolutionary Morphology, Center for Developmental Biology
(CDB), RIKEN, 2-2-3 Minatojima-minami, Kobe 650-0047, Japan.
2 Sado Marine Biological Station, Faculty of Science, Niigata University, 87
Tassha, Sado, Niigata 952-2135, Japan.
* Author for correspondence (e-mail: saizo{at}cdb.riken.jp)
Accepted 29 March 2007
 |
SUMMARY |
|---|
 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Key words: Turtle, Rib, Somite, Tissue interactions, Signaling molecule, Evolutionary innovation
|
INTRODUCTION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
;
Gilbert et al., 2001). This
shift seems to have brought about a change in the topographical relationship
between the ribs and the scapula in this animal. The scapula of the turtle is
ventral to the ribs and is not found in the superficial position outside the
rib cage as in other amniotes (reviewed by
Burke, 1989;
Hall, 1998;
Rieppel, 2001).
These anatomical differences in the turtle support the concept of
`evolutionary innovation' or novelty proposed by Mayr
(Mayr, 1960). In particular,
the disturbance of the morphological homology in the skeletal elements of the
turtles relative to those of other amniotes implies that the ancestral
developmental constraints have been overridden, consistently with the stricter
definition of novelty (Müller and
Wagner, 1991) (see also
Shigetani et al., 2002).
Turtle evolution might also have been salutatory, because there is no known
intermediate fossil record linking the turtle-specific and ancestral positions
of the ribs (reviewed by Rieppel,
2001). The earliest fossil turtle, Proganochelys, already
possessed a shell outside the scapula and was anatomically almost identical to
modern turtles (see Gaffney,
1990).
It seems likely that the turtle-specific changes in developmental pathways
are associated with the mesenchymal component of the region defined as
`thoracic' by the Hox code (Ohya et al.,
2005). In this region, the turtle-specific embryonic structure,
the carapacial ridge (CR), has been proposed as a possible candidate for these
changes (Burke, 1989).
Appearing as a longitudinal ridge on the lateral aspect of the flank at the
late pharyngula stage (Fig.
1A,B), the CR forms the leading edge of the laterally expanding
carapacial primordium, which is followed by the growth of the rib primordia
(Ohya et al., 2005;
Burke, 1989). The CR comprises
the condensation of undifferentiated mesenchyme underlying a thickened
epidermis. Because it resembles the apical ectodermal ridge (AER) of the limb
bud, the patterning of the ribs is assumed to be caused by similar inductive
activity (reviewed by Burke,
1989; Burke, 1991;
Hall, 1998;
Loredo et al., 2001;
Gilbert et al., 2001;
Vincent et al., 2003;
Cebra-Thomas et al., 2005).
Turtle embryology has not been studied extensively [see Nagashima et al.
(Nagashima et al., 2005) and
references therein], and it is difficult to apply to turtles the experimental
embryological techniques established in the chicken
(Yntema, 1964;
Yntema, 1970;
Burke, 1991). Although
experimental manipulation of the CR alters the rib pattern
(Burke, 1991), no histological
data have been presented. Interspecies transplantation may also be a potential
strategy, but the interactions of tissues from turtle and other amniotes, such
as the chicken, are thought to be incompatible and appear to disturb the
developmental potential of tissues in chimeras. For example, our previous
transplantation study using the Chinese soft-shelled turtle, Pelodiscus
sinensis, showed that P. sinensis sclerotome cannot chondrify
normally. This may derive from a difference in signaling molecules involved in
chondrogenesis. It is unknown whether this incompatibility is relevant to
turtle-specific rib growth (Nagashima et
al., 2005).
We have previously used microbead-based differential cDNA screening, a
molecular-level strategy, to identify some CR-specific genes from P.
sinensis (Kuraku et al.,
2005). Interestingly, these genes (LEF-1, APCDD1, CRABP1
and SP5) are orthologs of other vertebrate cognates, the regulation
of which has changed specifically in the turtle lineage
(Kuraku et al., 2005). The
co-expression of the transcriptional factor-encoding gene, LEF-1, and
transcriptional targets of the LEF-1/ß-catenin complex, APCDD1
and SP5 (Takahashi et al.,
2002; Takahashi et al.,
2005), and the nuclear localization of ß-catenin or the
co-factor of LEF-1 suggest the involvement of a Wnt-signaling pathway, which
has not been identified in the CR or its vicinity
(Kuraku et al., 2005). Thus,
the function of CR-specific genes and their regulation, which may explain the
mechanism of CR development itself, remain enigmatic.
|
) and Burke and Nowicki
(Burke and Nowicki, 2003); also
see Discussion]. The CR is incomparable to any other embryonic ridges found in
nonturtle amniote embryos and seems to play a role in the development of the
fan-shaped pattern of the ribs that is characteristic of turtles. |
MATERIALS AND METHODS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
)
(TK stages). Fertilized eggs of the chicken Gallus gallus and the
Japanese quail Coturnix coturnix were also obtained from local
suppliers. The eggs were incubated at 38°C, and the embryos were staged
according to Hamburger and Hamilton
(Hamburger and Hamilton,
1951).
Immunohistochemistry and histochemistry
Histological observations were made on hematoxylin and eosin (HE)-stained
sections (5-6 µm), some of which were stained further with 0.1% Alcian Blue
(Nowicki et al., 2003) to show
the cartilage in older embryos. To detect quail cells in chicken-quail
chimeras, the quail-specific antibody QCPN (1:5; Developmental Studies
Hybridoma Bank, Iowa City, IA, USA) was applied to embryos fixed with Serra's
fixative (Serra, 1946).
Biotin-conjugated anti-mouse IgG1 (Zymed Laboratories, South San Francisco,
CA, USA) was used as the secondary antibody. The Vectastain ABC Elite Kit
(Vector Laboratories, Burlingame, CA, USA) was used to visualize the
immunoreaction. All histological images were recorded with a DP70 digital
camera (Olympus, Shinjyuku, Tokyo, Japan) attached to a light microscope.
In situ hybridization
In situ hybridization was performed using either the method described by
Kuraku et al. (Kuraku et al.,
2005) for embryos fixed with 4% paraformaldehyde in
phosphate-buffered saline (PFA/PBS) or Discovery XT (Ventana Automated
Systems, Tucson, AZ, USA) for embryos fixed with Serra's fixative. Riboprobes
for P. sinensis CRABP1, APCDD1 and LEF-1 were generated
based on the nucleotide sequences AB124564-AB124566 deposited in GenBank,
respectively. Embryos were embedded in paraffin and sliced in 5 µm thick
sections.
DiI labeling
Focal dye injection was performed based on the method of Shigetani et al.
(Shigetani et al., 2000).
Fertilized P. sinensis eggs that had been incubated for 4 days
(corresponding to TK stages 11-11+) were used. To label the embryo,
a solution of CM-DiI (C-7000, Molecular Probes, Eugene, OR, USA) diluted in
dimethylsulfoxide (2 mg/ml) was injected focally into either the lateral part
of the dermomyotome or the medial part of the lateral-plate (somatic) mesoderm
at the flank levels using a fine glass pipette with a Pneumatic PicoPump
(PV830; World Precision Instruments, Sarasota, FL, USA). Some of the injected
embryos were fixed immediately with 4% PFA/PBS overnight at 4°C and
processed for frozen sectioning. Other embryos were incubated for 5-7 days
before fixation. Sections were cut to thicknesses of 12-16 µm and stained
with Hoechst 33258 (Molecular Probes) before observation under a fluorescence
microscope.
In ovo electroporation
The dominant-negative form of LEF-1 (provided by Dr S. Nakagawa,
RIKEN, Wako, Saitama, Japan) (Kubo et al.,
2003) was co-electroporated with pCAGGS-GFP (provided by Dr Y.
Takahashi, Nara Institute of Science and Technology, Ikoma, Nara, Japan)
(Niwa et al., 1991;
Sato et al., 2002) into stage
14 P. sinensis embryos. The positive platinum electrode was
positioned under the embryo, and the negative tungsten electrode was placed
onto the CR. The DNA solution, diluted to 5 µg/µl with 1% Fast Green
(Sigma, St Louis, MO, USA), was applied to the CR using a glass capillary
while five electric pulses of 5 V and 25 ms duration were applied to the
electrodes.
|
). The
embryos were incubated for 1-12 days before fixation. To implant the CR
ectopically, the CR graft was excised from a TK stage 14- donor
using a sharpened tungsten needle and immersed in a mixture of CM-DiI solution
(see above) and F-12 culture medium (1:9) for 10-15 minutes at room
temperature (Ambler et al.,
2001). The graft was implanted into a scar made dorsally to the CR
of the host embryo (TK stage 13+). The chimeric embryos were then
incubated for 6 hours to 5 days and then fixed. The whole-mount staining of
the skeletal system was based on the method by Ehehalt et al.
(Ehehalt et al., 2004).
Bromodeoxyuridine (BrdU) incorporation
With a glass capillary pipette, 10 µl of 10 mg/ml (w/v) BrdU (Sigma)/PBS
was injected into the allantoic vein of TK stage 16 P. sinensis
embryos, and the eggs were reincubated for 2 hours. The embryos were fixed in
Carnoy's fixative, and BrdU-labeled cells were detected using an anti-BrdU
monoclonal antibody (Becton Dickinson Biosciences, San Jose, CA, USA).
|
RESULTS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
). At
the early pharyngula stage (Fig.
1D and Fig. 3A,
top), in somites that had already differentiated into the sclerotome and
dermomyotome, and where the dermatome had begun to produce dermis, both
embryos show an indentation on the lateral aspect of the flank. This
indentation or the adjacent bulge correspond to the position at which the
lateral body wall (somatopleure) attaches to the main body of the embryo,
which will be referred to as the axial part
(Fig. 1D and
Fig. 3A, arrows;
Fig. 3B). Although the
indentation between the axial and body wall parts of the embryo correspond to
the earlier boundary between paraxial (somitic) and lateral mesodermal
components, the somite derivatives later invade the body wall as primaxial
elements in amniote development (Nowicki
et al., 2003) (see Fig.
3C, left side). Thus, the boundary of somite-derived cell lineage
[the `lateral somitic frontier' of Burke and Nowicki
(Burke and Nowicki, 2003)] does
not correspond to the postpharyngular anatomy of amniote embryos, which
consists of the axial part and the body wall (compare
Fig. 3B with 3C). This indentation constantly delineate the axial and body wall parts, and corresponds to the lateral somitic frontier only of the superficial dermis (Fig. 1C and Fig. 3C). Lateral and ventral to this notch, a bulge in the body wall called the Wolffian ridge is often recognized in these animals (Fig. 1D and Fig. 3A, top). By this stage of development, the ventrolateral lip of the dermomyotome has reached the above boundary, and the overall embryonic morphologies are nearly identical in both species.
Species-specific differences become clear in subsequent stages (Fig. 1D and Fig. 3A, middle). In chicken embryos, the indentation becomes shallower, and both the muscle plate and the mesenchymal condensation representing the rib primordium have migrated into the lateral wall (Fig. 1D and Fig. 3A, left middle). By contrast, the previously observed indentation in P. sinensis is still clearly visible, showing the boundary of the axis and body wall. Furthermore, the ventrolateral part of the dermis in the axial domain has swollen laterally to form another ridge (Fig. 1D and Fig. 3A, right middle). The latter corresponds to the initial appearance of the CR. The muscle plate has now invaded the lateral wall, as in the chicken, and there is no dense mesenchymal condensation growing into this wall (Fig. 1D and Fig. 3A, right middle).
In older embryos, in which overt rib primordia can be seen (Fig. 1D and Fig. 3A, bottom), the chicken and P. sinensis embryos begin to show conspicuously different patterns. The ribs in chicken embryos now grow extensively into the lateral body wall, together with the massive hypaxial muscles (data not shown), whereas in P. sinensis the ribs never invade the wall, which contains only a thin thread of myofibers representing the hypaxial muscles of this animal (Fig. 1D and Fig. 3A, bottom). The latter muscles represent the migration of somitic elements into the lateral body wall. By this stage, the lateral surface of the chicken embryonic flank has flattened (Fig. 1D and Fig. 3A, left bottom). In the P. sinensis embryos, the CR and junction remain, and the junction persists as long as the carapace enlarges laterally. These observations suggest that, morphologically, the CR mesenchyme in turtle embryos is a primaxial element forming the ventrolateral portion of the axial domain. Consistently in chicken-quail chimeras, in which quail somites have replaced those of the host chicken, we could confirm that the boundary of dermis corresponds to the junction of the lateral body wall and the axial domain (Fig. 1C, left).
|
|
), who showed the
possibility that the entire mesenchymal component of the carapace (including
ribs and CR) is of somite origin.
Only in the turtle does the somite-derived dermis make a ridge on the
flank, suggesting that the CR is an embryonic novelty unique to turtles. This
conclusion leads us to infer that the turtle ribs are confined within the
axial domain and never invade the lateral body wall
(Fig. 1D and
Fig. 3C)
(Burke, 1989;
Nagashima et al., 2005).
Functions of CR-expressed regulatory genes
The possibility that the CR is unique to the turtle led us to question
whether its formation requires a turtle-specific developmental program. In a
previous study, we identified four genes, LEF-1, APCDD1, CRABP1 and
SP5, expressed specifically in the CR around the time of CR formation
(Kuraku et al., 2005). These
genes are not expressed in the corresponding area in the chicken, the ventral
lateral limit of the axial domain, suggesting that the CR is a true
evolutionary novelty that is not comparable to the flank region of other
amniotes. In the previous study, we also used immunohistochemical analysis to
detect the nuclear localization of ß-catenin protein and found that, in
the CR epithelium, ß-catenin was specifically localized in nuclei,
suggesting that this phenomenon was associated with the development of the
CR.
To confirm the involvement of these molecules in CR development in the
present study, we introduced the dominant-negative LEF-1 ectopically
into the epidermis of CR using in ovo electroporation
(Fig. 4A). The protein product
lacks the region that interacts with ß-catenin, and others have shown
that the protein inhibits the canonical Wnt pathway in several developmental
contexts (Kengaku et al.,
1998; Aoki et al.,
1999; Vleminckx et al.,
1999). Our electroporation strategy caused the exogenous genes to
be overexpressed only in the CR epidermis
(Fig. 4A). Four days after
injection, the embryos treated with the dominant-negative LEF-1
showed an overt indentation of the CR (Fig.
4C) corresponding to the injection site, which was visualized with
green fluorescent protein (GFP) signals. By contrast, no morphological changes
were detected in the control embryos that received only GFP
(Fig. 4B). Thus, the LEF-1
activity in the CR epidermis is likely to be essential for the formation and
maintenance of the CR, probably in association with the nuclear-localized
co-factor, ß-catenin, in the CR epithelium.
|
The CR was ablated in stage 14- to 14 embryos using a microcautery unit (purchased from the workshop of the Medical College of Georgia, Augusta, GA, USA); the CR was ablated at various longitudinal levels (Fig. 5A-C). Cauterizing the AER on the ipsilateral limb bud simultaneously always resulted in a malformed autopod (Fig. 5O-R), showing that this method removes the inductive function of local cell populations on the embryonic surface. After CR microcauterization, the histological analysis and CR-specific gene expression showed that nine of 12 embryos exhibited an apparently regenerated CR if fixed fewer than 24 hours after the surgery (see below). The CR was eliminated partially at the site of the cauterization in 17 of 70 embryos fixed more than 2 days after the surgery, as evaluated by histology and embryonic morphology. In these embryos, the CR-specific expression of CRABP1, APCDD1 and LEF-1 was absent or lower in intensity (Fig. 5D-G).
In the CR-less embryos, the local thickening of the epidermis and mesenchymal condensation were missing from the flank where the CR would have developed, as observed in transverse sections (Fig. 5H-J). By contrast, no change occurred in the dorsal position of the rib primordia (Fig. 5I); the ribs on the operated side were restricted to the axial domain, as were those on the control side, and they never invaded the body wall (Fig. 5I). Only when observed in whole-mount embryos did partial arrest of the characteristic fan-shape of the ribs become apparent at the site of ablation; the ribs were slightly shorter on the operated site than on the adjacent site (Fig. 5K-N).
|
Our results suggest that the CR is not responsible for the axially restricted pattern of rib growth unique to the turtles but instead seems to function in the concentric marginal growth of the developing carapace, resulting in the fan-shaped arrangement of the ribs (Fig. 6O). Consistent with this, the application of BrdU resulted in the localized intake of BrdU at high levels in the CR mesenchyme (Fig. 6N). The CR appears to act within a certain developmental window between about stage 14, when the fan shape of the ribs is not yet apparent, and stage 18, when this pattern becomes conspicuous. During this time, the CR seems to be responsible for shaping the carapace by activating the marginal growth of the mesenchyme along the CR.
|
DISCUSSION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
;
Nowicki et al., 2003;
Burke and Nowicki, 2003).
Experimental evidence shows that the somite derivatives are not always
confined to the axial domain and that the dorsal ribs and the related
intercostal muscles migrate, along with some connective tissues, as compact
masses of cells from the axial to the lateral body wall domains as `primaxial
elements' (Nowicki et al.,
2003). Some other somite-derived elements are called `abaxial'.
The ventralmost part of the ribs and the ventrally migrating myoblasts of the
tongue and limbs fall into this category.
In the late pharyngula of amniotes, only the superficial layer of the
dermis reflects its embryonic origin; in avian chimeric embryos, the boundary
between the primaxial and abaxial dermis corresponds to the junction of the
body axis and body wall (Fig.
1C and Fig. 3C)
(also see Nowicki et al.,
2003). The leading edge of this primaxial cell mass is called the
`lateral somitic frontier' (Nowicki and
Burke, 2000; Nowicki et al.,
2003; Burke and Nowicki,
2003). Based on this scheme, the turtle ribs are both primaxial in
their cell lineage and entirely axial in their position. The CR, which
develops at the lateral limit of the axial domain, is co-extensive with the
growth of the turtle ribs (Fig.
3C). The uniqueness of the turtle body plan can now be described
anew as the arrested growth of the lateral somitic frontier. The axially
restricted CR mesenchyme may represent, in part, the cells that would have
migrated deeper into the body wall in other amniotes
(Fig. 3C). Thus, the
turtle-specific developmental pattern is based primarily on the developmental
changes associated with the mesenchyme, including its distribution and gene
regulation.
The DiI injection into the dorsomedial edge of the lateral mesoderm also labeled the CR epidermis over the nonlabeled CR mesenchyme (Fig. 2G), showing that the epidermis over the lateral mesoderm in the early pharyngula moves medially relative to mesenchymal components, even crossing over the lateral somitic frontier. By contrast, our preliminary data show that the epidermis over the somite moves in the reverse direction, that is, towards the body wall, raising the possibility that the thickened epidermis on the CR apex is formed by coalescing epidermal cells from both the dorsal and ventral direction. Answering this question, however, will belong to our future project.
Developmental function of the CR and its evolutionary significance
Overexpression of the dominant-negative LEF-1 in the CR confirmed
that the expression of the gene, especially in the epidermis, is essential to
the formation and maintenance of the CR
(Fig. 4). This shows that the
unique gene expression profile of the turtle actually plays a role in the
development of the CR, confirming the novel nature of the CR. Because of its
topographical correlation, the CR has been assumed to possess inductive
activity associated with the turtle-specific patterning of the ribs
(Burke, 1989;
Burke, 1991;
Cebra-Thomas et al., 2005)
(reviewed by Gilbert et al.,
2001).
To eliminate the function of the CR, we experimentally removed the CR from
P. sinensis embryos (Fig.
5). The CR was very regenerative, as has been reported previously
(Burke, 1991). This frequent
regeneration implies that this structure is developmentally established as a
`field' induced by its environment at specific sites in the embryo, as seen in
the organizer (Nieuwkoop,
1969; Nakamura and Takasaki,
1970) (reviewed by Gilbert,
2003; Kimelman and Bjornson,
2004) and the neural crest
(Moury and Jacobson, 1989;
Dickinson et al., 1995;
Selleck and Bronner-Fraser,
1995). To avoid regeneration, we resorted to microcauterization
(see Kirby et al., 1985),
after which the cauterized cells remain in the wound and are not replaced
easily by surrounding cells. This method was effective in removing the AER of
the limb bud and constantly resulted in a disfigured autopod
(Fig. 5O-R).
In a few embryos, the morphological analysis showed that the CR was
eliminated. The expression of CR-specific genes, CRABP1, APCDD1 and
LEF-1, was either downregulated or its intensity decreased in the
cauterized CR epidermis, whereas it was maintained in the mesenchyme
(Fig. 5D-G). It is unclear
whether this expression pattern can be ascribed to the residual CR cells or to
the cells recruited secondarily from surrounding tissue. However, as in the
overexpression of dominant-negative LEF-1
(Fig. 4), the presence of the
thickened ectoderm with normal gene expressions is essential for the
maintenance of the CR. In this context, it is also worth mentioning that the
nuclear localization of ß-catenin, the co-factor of LEF-1, is restricted
to the CR epidermis (Kuraku et al.,
2005). Taken together, these data clearly show that the CR is
established through a series of complicated reciprocal interactions between
the epithelium and mesenchyme, as has been previously shown by Burke
(Burke, 1991), who eliminated
the CR by blocking the interaction between somites and prospective CR domain.
Thus the development of the CR is highly reminiscent of tooth or scale/hair
patterning (reviewed by Pispa and
Thesleff, 2003).
Our study does not provide any supportive evidence for the idea that the CR
induces the turtle-specific dorsally shifted rib pattern, although the CR is
truly a unique novelty of turtles. As observed histologically, neither the
ablation nor the addition of the CR resulted in the dorsoventrally shifted
growth of the ribs in P. sinensis. Rather, at least in this species,
the CR is more likely to function in the development of another feature of the
ribs, namely, the fan-shaped patterning, through the marginal growth of the
carapace primordium. This is consistent with the specific incorporation of
BrdU in this tissue, as well as the slight shortening of the ribs on CR
ablation (Fig. 5M,N). As for
the relatively more active cell proliferation in the CR and its significance
in the lateral growth of the carapacial primordium; a supportive observation
has already been made by Burke (Burke,
1989), who showed, in Chelydra serpentina, the
incorporation of tritiated thymidine by the CR, although this author assumed
inductive roles of the CR in the turtle-specific patterning of the rib
primordium (Burke, 1989;
Burke, 1991). Burke
(Burke, 1991) found, in
Chelydra, that CR removal caused ribs to be deflected into intact CR
domains; by contrast we found that the ribs approached distally at the level
of ablation. Difference in methods of CR inactivation may explain this
difference. At any rate, it appears that the growth pattern of the carapace is
unanimously attributable to the proliferative activity in the CR.
Alternatively, this difference may also be associated with the
species-specific morphological patterns in the carapace: the carapace of
P. sinensis assumes an exceptionally round shape with ribs fanning
conspicuously compared with the shape in other chelonian species such as
Pelusios or Caretta
(Ruckes, 1929) (reviewed by
Ewert, 1985). One can imagine
that embryos with various growth rates could end up with different
morphological patterns, even after the same experimental treatment. More
extensive comparative studies of turtle development are needed to answer this
question.
Our hypothesis about the function of the CR does not rule out the
possibility that it also functions in dorsoventral patterning of the ribs at
earlier stages than those available for manipulative experiments.
Nevertheless, the idea that rib patterning requires environmental induction
does not appear relevant to the primaxial structures, which are generally
recognized as being cell autonomous
(Nowicki and Burke, 2000;
Burke and Nowicki, 2003).
Instead, we support the view that the turtle ribs and the CR mesenchyme
together represent a single primaxial unit, which fails to penetrate the body
wall ventrally, although no common upstream factor has yet been identified.
The uniqueness of the turtle can be seen, at least in part, in the arrest of
the lateral somitic frontier (Fig.
3C). The changes in the developmental program responsible for this
may simultaneously cause the shifted growth of the ribs and the appearance of
the CR. This arrest should result in `morphologically short' ribs [in the
sense that they never invade the body wall (see also
Burke, 1989)], which might
allow the ventral shift of the scapula anlage, the universal feature of the
turtles. We still do not understand the most fundamental factors behind the
turtle body plan, the factors that should lead to the dorsal dislocation of
the turtle ribs. The hypothetical second phase probably depended upon
invention of the CR to allow the lateral expansion of the dorsally dislocated
ribs, which is seen more or less in the carapaces of all the turtle species.
The key to understanding the factor responsible for the first phase must lie
in the mesenchymal behavior at the stage at which chelonian and avian embryos
still look alike. This is an even more intriguing issue.
|
ACKNOWLEDGMENTS |
|---|
|
REFERENCES |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Ambler, C. A., Nowicki, J. L., Burke, A. C. and Bautch, V. L. (2001). Assembly of trunk and limb blood vessels involves extensive migration and vasculogenesis of somite-derived angioblasts. Dev. Biol. 234,352 -364.[CrossRef][Medline]
Aoki, M., Hecht, A., Kruse, U., Kemler, R. and Vogt, P. K.
(1999). Nuclear endpoint of Wnt signaling: neoplastic
transformation induced by transactivating lymphoid-enhancing factor 1.
Proc. Natl. Acad. Sci. USA
96,139
-144.
Burke, A. C. (1989). Development of the turtle carapace: implications for the evolution of a novel bauplan. J. Morphol. 199,363 -378.[CrossRef]
Burke, A. C. (1991). The development and evolution of the turtle body plan. Inferring intrinsic aspects of the evolutionary process from experimental embryology. Am. Zool. 31,616 -627.
Burke, A. C. and Nowicki, J. L. (2003). A new view of patterning domains in the vertebrate mesoderm. Dev. Cell 4,159 -165.[CrossRef][Medline]
Cebra-Thomas, J., Tan, F., Sistla, S., Estes, E., Bender, G., Kim, C., Riccio, P. and Gilbert, S. F. (2005). How the turtle forms its shell: a paracrine hypothesis of carapace formation. J. Exp. Zoolog. B Mol. Dev. Evol. 304,558 -569.[Medline]
Dickinson, M. E., Selleck, M. A. J., McMahon, A. P. and Bronner-Fraser, M. (1995). Dorsalization of the neural tube by the non-neural ectoderm. Development 121,2099 -2106.[Abstract]
Ehehalt, F., Wang, B. G., Christ, B., Patel, K. and Huang, R. J. (2004). Intrinsic cartilage-forming potential of dermomyotomal cells requires ectodermal signals for the development of the scapula blade. Anat. Embryol. 208,431 -437.[Medline]
Ewert, M. A. (1985). Embryology of turtles. In Biology of the Reptilia. Vol.14 (ed. C. Gans, F. Billet and P. F. A. Maderson), pp.74 -255. New York: John Wiley.
Gaffney, E. S. (1990). The comparative osteology of the Triassic turtle Proganochelys. Bull. Am. Mus. Nat. Hist. 194,1 -263.
Gilbert, S. F. (2003). Developmental Biology (7th edn). Massachusetts: Sinauer Associates.
Gilbert, S. F., Loredo, G. A., Brukman, A. and Burke, A. C. (2001). Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. Evol. Dev. 3, 47-58.[CrossRef][Medline]
Hall, B. K. (1998). Evolutionary Developmental Biology (2nd edn). London: Chapman & Hall.
Hamburger, V. and Hamilton, H. L. (1951). A series of normal stages in the development of the chick embryo. J. Morphol. 88,49 -91.[CrossRef]
Kengaku, M., Capdevila, J., Rodriguez-Esteban, C., De La Pena,
J., Johnson, R. L., Belmonte, J. C. and Tabin, C. J. (1998).
Distinct WNT pathways regulating AER formation and dorsoventral polarity in
the chick limb bud. Science
280,1274
-1277.
Kimelman, D. and Bjornson, C. (2004). Vertebrate mesoderm induction. In Gastrulation from Cells to Embryo (ed. C. D. Stern), pp. 363-372. Cold Spring Harbor: Cold Spring Harbor Laboratory Press.
Kirby, M. L., Turnage, K. L., III and Hays, B. M. (1985). Characterization of conotruncal malformations following ablation of "cardiac" neural crest. Anat. Rec. 213,87 -93.[CrossRef][Medline]
Kubo, F., Takeichi, M. and Nakagawa, S. (2003).
Wnt2b controls retinal cell differentiation at the ciliary marginal zone.
Development 130,587
-598.
Kuraku, S., Usuda, R. and Kuratani, S. (2005). Comprehensive survey of carapacial ridge-specific genes in turtle implies co-option of some regulatory genes in carapace evolution. Evol. Dev. 7,3 -17.[CrossRef][Medline]
Loredo, G. A., Brukman, A., Harris, M. P., Kagle, D., Leclair, E. E., Gutman, R., Denny, E., Henkelman, E., Murray, B. P., Fallon, J. F. et al. (2001). Development of an evolutionarily novel structure: fibroblast growth factor expression in the carapacial ridge of turtle embryos. J. Exp. Zool. 291,274 -281.[CrossRef][Medline]
Mayr, E. (1960). The Evolution of Life: Its Origin, History, and Future. Chicago, IL: University of Chicago Press.
Moury, J. D. and Jacobson, A. G. (1989). Neural fold formation at newly created boundaries between neural plate and epidermis in the axolotl. Dev. Biol. 133, 44-57.[CrossRef][Medline]
Müller, G. F. and Wagner, G. P. (1991). Novelty in evolution: restructuring the concept. Annu. Rev. Ecol. Syst. 22,229 -256.[CrossRef]
Nagashima, H., Uchida, K., Yamamoto, K., Kuraku, S., Usuda, R. and Kuratani, S. (2005). Turtle-chicken chimera: an experimental approach to understanding evolutionary innovation in the turtle. Dev. Dyn. 232,149 -161.[CrossRef][Medline]
Nakamura, O. and Takasaki, H. (1970). Further studies on the differentiation capacity of the dorsal marginal zone in the modula of Triturus pyrrhogaster. Proc. Jpn. Acad. 46,700 -705.
Nieuwkoop, P. D. (1969). The formation of the mesoderm in urodele amphibians. I. Induction by the endoderm. Wilhelm Rouxs Arch. Entwicklungsmech. Org. 162,341 -373.[CrossRef]
Niwa, H., Yamamura, K. and Miyazaki, J. (1991). Efficient selection for high-expression transfections with a novel eukaryotic vector. Gene 108,193 -199.[CrossRef][Medline]
Nowicki, J. L. and Burke, A. C. (2000). Hox genes and morphological identity: axial versus lateral patterning in the vertebrate mesoderm. Development 127,4265 -4275.[Abstract]
Nowicki, J. L., Takimoto, R. and Burke, A. C. (2003). The lateral somitic frontier: dorso-ventral aspects of anterior-posterior regionalization in avian embryos. Mech. Dev. 120,227 -240.[CrossRef][Medline]
Ohya, K. Y., Kuraku, S. and Kuratani, S. (2005). Hox code in embryos of Chinese soft-shelled turtle Pelodiscus sinensis correlates with the evolutionary innovation in the turtle. J. Exp. Zoolog. B Mol. Dev. Evol. 304,107 -118.[Medline]
Pispa, J. and Thesleff, I. (2003). Mechanisms of ectodermal organogenesis. Dev. Biol. 262,195 -205.[CrossRef][Medline]
Rieppel, O. (2001). Turtles as hopeful monsters. BioEssays 23,987 -991.[CrossRef][Medline]
Ruckes, H. (1929). Studies in chelonian osteology part II, The morphological relationships between the girdles, ribs and carapace. Ann. N. Y. Acad. Sci. 31, 81-120.[CrossRef]
Sato, Y., Yasuda, K. and Takahashi, Y. (2002).
Morphological boundary forms by a novel inductive event mediated by Lunatic
fringe and Notch during somitic segmentation.
Development 129,3633
-3644.
Selleck, M. A. J. and Bronner-Fraser, M. (1995). Origins of the avian neural crest: the role of neural plate-epidermal interactions. Development 121,525 -538.[Abstract]
Serra, J. A. (1946). Histochemical tests for protein and amino acids: the characterization of basic proteins. Stain Technol. 21,5 -18.
Shigetani, Y., Nobusada, Y. and Kuratani, S. (2000). Ectodermally-derived FGF8 defines the maxillomandibular region in the early chick embryo: epithelial-mesenchymal interactions in the specification of the craniofacial ectomesenchyme. Dev. Biol. 228,73 -85.[CrossRef][Medline]
Shigetani, Y., Sugahara, F., Kawakami, Y., Murakami, Y., Hirano,
S. and Kuratani, S. (2002). Heterotopic shift of
epithelial-mesenchymal interactions for vertebrate jaw evolution.
Science 296,1316
-1319.
Takahashi, M., Fujita, M., Furukawa, Y., Hamamoto, R.,
Shimokawa, T., Miwa, N., Ogawa, M. and Nakamura, Y. (2002).
Isolation of a novel human gene, APCDD1, as a direct target of the
ß-catenin/T-cell factor 4 complex with probable involvement in colorectal
carcinogenesis. Cancer Res.
62,5651
-5656.
Takahashi, M., Nakamura, Y., Obama, K. and Furukawa, Y. (2005). Identification of SP5 as a downstream gene of the ß-catenin/Tcf pathway and its enhanced expression in human colon cancer. Int. J. Oncol. 27,1483 -1487.[Medline]
Tokita, M. and Kuratani, S. (2001). Normal embryonic stages of the Chinese softshelled turtle Pelodiscus sinensis.Zool. Sci. 18,705 -715.[CrossRef]
Vincent, C., Bontoux, M., Le Douarin, N. M., Pieau, C. and Monsoro-Burq, A. H. (2003). Msx genes are expressed in the carapacial ridge of turtle shell: a study of the European pond turtle, Emys orbicularis. Dev. Genes Evol. 213,464 -469.[CrossRef][Medline]
Vleminckx, K., Kemler, R. and Hecht, A. (1999). The C-terminal transactivation domain of ß-catenin is necessary and sufficient for signaling by the LEF-1/ß-catenin complex in Xenopus laevis. Mech. Dev. 81,65 -74.[CrossRef][Medline]
Yntema, C. L. (1964). Procedurement and use of turtle embryos for experimental procedures. Anat. Rec. 149,577 -586.[CrossRef][Medline]
Yntema, C. L. (1970). Extirpation experiments on the embryonic rudiments of the carapace of Chelydra serpentina.J. Morphol. 132,235 -244.[CrossRef][Medline]
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
