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

The Corn Snake Yolk Sac Becomes a Solid Tissue Filled With Blood Vessels and Yolk-Rich Endodermal Cells

The amniote egg was a key innovation in vertebrate evolution because it supports an independent existence in terrestrial environments. The egg is provisioned with yolk, and development depends on the yolk sac for the mobilization of nutrients. We have examined the yolk sac of the corn snake Pantherophis guttatus by the dissection of living eggs. In contrast to the familiar fluid-filled sac of birds, the corn snake yolk sac invades the yolk mass to become a solid tissue. There is extensive proliferation of yolk-filled endodermal cells, which associate with a meshwork of blood vessels. These novel attributes of the yolk sac of corn snakes compared with birds suggest new pathways for the evolution of the amniote egg

Expression of cyclin D1, cyclin D2, and N-myc in embryos of the direct developing frog Eleutherodactylus coqui, with a focus on limbs

Species of frogs that develop directly have removed the tadpole from their ontogeny and form adult structures precociously. To see whether cell cycle regulators could be involved in this altered embryogenesis, we examined the expression of ccnd1, ccnd2, and mycn in embryos of the direct developing frog, Eleutherodactylus coqui. Notable differences compared to embryos of Xenopus laevis, a species with a tadpole, included prominent expression of ccnd2 in the midbrain and ccnd1 in the mandibular neural crest. The former may contribute to the precocious appearance of the adult-type visual system and the latter to the adult-type jaw. Large domains of ccnd2 and mycn presage the early appearance of limb buds, and ccnd1 and mycn are implicated in digit development. © 2013 Elsevier B.V. All rights reserved