NINOMIYA,H.*; ELINSON,R.P.: The geometry of mesoderm formation in the direct developer Eleutherodactylus coqui, a frog with big eggs

E. coqui is a direct developing frog, which hatches as a froglet without going through a tadpole stage. It develops from a big egg (diameter 3.5mm), much of which forms large yolky cells, which are assumed to have primarily a nutritive role. The large fraction of the egg devoted to yolk may affect the geometric disposition of germ layer formation in this embryo. To address this question, we investigated mesoderm formation in E. coqui and compared it to that of Xenopus laevis (egg diameter 1.3mm). First, we examined the position of prospective mesoderm along the animal-vegetal axis of the egg. Animal and vegetal regions are initially divided by the fourth cleavage furrow, close to the animal pole, unlike in X. laevis, where the animal cells are formed by the third cleavage in a supra-equatorial position. We followed cell lineage of 16-cell E. coqui embryos by injecting them with the lineage tracer Rhodamine dextran amine. Although animal cells are small, they form most of the blastocoel roof and make extensive contributions to differentiated mesodermal tissues. Second, we performed recombinant analysis to examine which regions have mesoderm inducing activity. We combined parts of E. coqui blastula or early gastrula with X. laevis animal cap and examined mesoderm differentiation of X. laevis cells. Mesoderm inducing activity was restricted to the marginal or sub- marginal regions in E. coqui. Neither the vegetal pole nor the inner vegetal core, including the blastocoel floor, had activity, although these same regions from X. laevis blastulae induced mesoderm. Our results suggest that mesoderm formation is shifted to more animal and superficial regions in E. coqui compared to X. laevis.