Patterns of Distal-less Gene Expression and Inductive Interactions in the Head of the Direct Developing FrogEleutherodactylus coqui

AbstractThe direct developing frogEleutherodactylus coquiexhibits radical changes in its embryogenesis. A frog-like head forms directly with no appearance of a cement gland or several jaw cartilages characteristic of tadpoles, and limbs appear early in development. The numerous differences in the embryogenesis ofE. coquiprovide an opportunity to examine developmental causes for the evolutionary shift from biphasic to direct development. We have cloned DNA fragments corresponding to fourE. coquigenes related to theDrosophiladistal-less geneDll.While the expression patterns of the distal-less genes are generally conserved, there are some spatiotemporal differences when embryos ofE. coquiare compared to those ofXenopus laevis.The changes in gene expression are correlated with the embryonic changes in head structures including craniofacial cartilages and in particular, the cement gland. We have then examined inductive interactions involved in cement gland formation by interspecific transplants and recombinants.E. coquiembryos can generate signaling that culminates in cement gland formation, butE. coquiectoderm appears to be incapable of a cement gland response. These results show here that inductive interactions in the anterior region of theE. coquiembryo have been modified during the evolution of direct development, and that changes in the competence of theE. coquiectoderm may be responsible for the loss of certain tadpole-specific structures, such as cement gland

Mesoderm Formation in Eleutherodactylus coqui: Body Patterning in a Frog with a Large Egg

AbstractThe direct developing frog, Eleutherodactylus coqui, develops from a large egg (diameter 3.5 mm). To investigate the effect of egg size on germ-layer formation, we studied mesoderm formation in E. coqui and compared it to that of Xenopus laevis (diameter 1.3 mm). First, we identified the position of prospective mesoderm in the 16-cell E. coqui embryo by cell-lineage tracing. Although the animal blastomeres are small, they form most of the blastocoel roof and make extensive contributions to some mesodermal tissues. Second, we performed recombinant analysis with X. laevis animal caps to define the distribution of mesoderm-inducing activity. Mesoderm-inducing activity in E. coqui was restricted around the marginal zone with strong activity in the superficial cells. Neither the vegetal pole nor the blastocoel floor had activity, although these same regions from X. laevis induced mesoderm. Third, we cloned Ecbra, a homologue of Xbra, an early mesoderm marker in X. laevis. Ecbra was expressed in the marginal ring close to the surface, similar to X. laevis, but E. coqui had weaker expression on the dorsal side. Our results suggest that mesoderm formation is shifted more animally and superficially in E. coqui compared to X. laevis

Germ plasm in Eleutherodactylus coqui, a direct developing frog with large eggs

<p>Abstract</p> <p>Background</p> <p>RNAs for embryo patterning and for germ cell specification are localized to the vegetal cortex of the oocyte of <it>Xenopus laevis</it>. In oocytes of the direct developing frog <it>Eleutherodactylus coqui</it>, orthologous RNAs for patterning are not localized, raising the question as to whether RNAs and other components of germ plasm are localized in this species.</p> <p>Methods</p> <p>To identify germ plasm, <it>E. coqui </it>embryos were stained with DiOC₆(3) or examined by <it>in situ </it>hybridization for <it>dazl </it>and <it>DEADSouth </it>RNAs. The cDNAs for the <it>E. coqui </it>orthologues were cloned by RT-PCR using degenerate primers. To examine activity of the <it>E. coqui </it>orthologues, RNAs, made from constructs of their 3'UTRs with <it>mCherry</it>, were injected into <it>X. laevis </it>embryos.</p> <p>Results</p> <p>Both DiOC₆(3) and <it>dazl </it>and <it>DEADSouth in situs </it>identified many small islands at the vegetal surface of cleaving <it>E. coqui </it>embryos, indicative of germ plasm. <it>Dazl </it>was also expressed in primordial germ cells in the genital ridge. The 3'UTRs of <it>E. coqui dazl </it>and <it>DEADSouth </it>directed primordial germ cell specific protein synthesis in <it>X. laevis</it>.</p> <p>Conclusions</p> <p><it>E. coqui </it>utilizes germ plasm with RNAs localized to the vegetal cortex to specify primordial germ cells. The large number of germ plasm islands suggests that an increase in the amount of germ plasm was important in the evolution of the large <it>E. coqui </it>egg.</p