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Fig. 8. Crustacean fate maps and various cell lineages. (A,C,E,G,I) Fate maps from
crustacean taxa that possess total cleavage. The fate maps show the
arrangements of the mesoderm, endoderm and germ cells at the time of the
gastrulation. The fate map of Parhyale is derived from the lineage
tracing data described here. All other fate maps were conceived from staining
whole embryos and looking at the differential morphology and location of
cells. By definition, the position of initial cell ingression is defined as
the blastopore. The blastopore, however, is at different places in different
crustaceans. The blastopore is anterior in Parhyale, posterior in
barnacles, shrimps and copepods, and ventral in waterfleas. Therefore, the
panels show ventral views, anterior upwards in A,I, and posterior views,
dorsal upwards in C,E,G. Although Parhyale is a malacostracan
crustacean like shrimps, its fate map (A) is less similar to that of shrimps
(E) and more similar to those of non-malacostracans (C,G,I). In all four taxa,
the endoderm progenitors (gray cells with blue nuclei) or joint
endoderm+germline progenitors (gray cells with yellow nuclei) are situated in
front of the mesoderm progenitors (green cells). Moreover, in
Parhyale (A), Cyclops (G) and the waterfleas (I), the
endoderm and mesoderm encircle the germ cells (white cells with blue nuclei).
The fate map of the malacostracan shrimps (E) places the endoderm dorsal of
the mesoderm. Most other malacostracans have superficial cleavage and the
mesoderm is positioned anterior of the endoderm (not shown). (B,D,F,H,J)
Crustacean cell lineages. Again, other than for the work reported here for
Parhyale, cell fate in the crustacean cell lineages has been inferred
from cell morphologies and is not based on tracing experiments. The number of
divisions before the putative progenitors for mesoderm, endoderm and germ
cells (m, en, g) are specified varies across the taxa from three in the
amphipod, four in barnacles and five in the copepod, to seven in shrimps. The
germline emerges as a sister of either the endoderm or the mesoderm but not
the ectoderm (ec), but has not been recognized in early barnacle embryos.
(K,L) Nematode and spiralian cell lineages. In C. elegans, the
endoderm is specified after the third division. In Patella, the
primary mesoderm is specified after the sixth division. Data are based on the
following: (A,B) malacostracan amphipod Parhyale (this study); (C,D)
maxillopodan barnacles (Bigelow,
1902 ; Shiino,
1957); (E,F) malacostracan shrimps (Kajishima, 1951;
Hertzler, 2002); (G,H)
maxillipodan copepod (Fuchs,
1914) (the relationship between the AP axis and the endoderm and
germline that is shown here is modeled after other crustaceans); (I,J)
branchiopodan waterfleas (Grobben,
1879; Kühn,
1913); (K) nematode C. elegans (Sulston et al., 1983);
and (L) limpet snail Patella
(Dictus and Damen, 1997).
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100 superficially located cells of roughly the same size. (D-G)
DAPI stained embryos. (D) The early germband at day 3. Ventral view, anterior
upwards. The first landmarks of the germ band are the head lobes (arrows). At
this stage, the trunk ectoderm is organizing itself into a remarkably precise
grid of rows and columns, with each initial row giving rise eventually to a
single parasegment of the animal. Cells are still being added to the germband
at the posterior (asterisk). The overall organization shows a marked AP
gradient of development. (E-H) Lateral views, anterior leftwards. (E) Germband
extension at day 4. As the germband extends, it acquires a sharp ventral
infolding (arrowhead; head indicated by arrow, telson by asterisk). In this
embryo, the segments anterior to the fifth thoracic segment and posterior to
approximately the middle of the abdomen are at the ventral surface of the egg,
while the remaining thoracic and abdominal segments are within the infolded
region. (F) The extended germband at day 5. The infolding has extended to the
point where only the telson and segments anterior to the mandible still lie at
the ventral surface of the egg. At this stage, the appendages are distinct
from the body wall (second antenna marked by arrow, telson by asterisk). (G)
The embryo at day 9. By this time, the adult morphology has been established
as Parhyale is a direct developer (compare with H). (H) A living
gravid adult female carrying eggs in her ventral brood pouch (arrow). Scale
bar: 100 µm in A-G; 2 mm in H.
