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Developing Tardigrades as
a New
Model for Studying the Evolution of Development  Studying
development in diverse taxa can address a central issue in evolutionary
biology: how morphological diversity arises through the evolution of
developmental mechanisms. Two of the best-studied developmental model
organisms, the arthropod Drosophila
and the nematode C. elegans,
have
been found to belong to a single protostome superclade, the
Ecdysozoa. This finding suggests that a closely related
ecdysozoan phylum could serve as a valuable model for studying how
developmental mechanisms evolve. Other ecdysozoan phyla have been
used as study organisms, but their use has been limited by a paucity of
information on developmental genes and of basic developmental data such
as cell lineages in systems with stereotyped development.
Tardigrades, also called water bears, make up a phylum of microscopic
ecdysozoan animals. We have studied a tardigrade, Hypsibius dujardini, to determine
if it can be a useful model
for studies of how development evolves. We
developed immunostaining methods for
tardigrade embryos, and we used cross-reactive antibodies to
investigate the expression of homologs of the pair-rule gene paired
(Pax3/7) and the segment polarity gene engrailed in H. dujardini. We
find that H. dujardini Pax3/7
protein localizes not in a pair-rule
pattern but in a segmentally iterated pattern, after the segments are
established, in regions of the embryo where neurons later arise.
Engrailed protein localizes in the posterior ectoderm of each segment
before ectodermal segmentation is apparent. Together with previous
results from others, our data support the conclusions that the
pair-rule function of Pax3/7 is specific to the arthropods, that some
of the ancient functions of Pax3/7 and Engrailed in ancestral
bilaterians may have been in neurogenesis, and that Engrailed may have
a function in establishing morphological boundaries between segments
that is conserved at least among the Panarthropoda (Gabriel and
Goldstein, 2007).
Long-term culturing of tardigrades historically has been a challenge, and there have been few studies of tardigrade development. We and our collaborators reported that the tardigrade Hypsibius dujardini can be cultured continuously for decades and can be cryopreserved. H. dujardini has a compact genome, a little smaller than that of C. elegans or Drosophila, and sequence evolution has occurred at a typical rate. H. dujardini has a short generation time, 13-14 days at room temperature. We found that the embryos of H. dujardini have a stereotyped cleavage pattern with asymmetric cell divisions, nuclear migrations, and cell migrations occurring in reproducible patterns. We presented a cell lineage of the early embryo and an embryonic staging series. These data are intended to serve as a platform for using H. dujardini as a model for studying the evolution of developmental mechanisms (Gabriel et al., 2007). back |