The current understanding of Xenopus laevis development provides a comparative background for the analysis of frog developmental modes. Our analysis of development in various frogs reveals that the mode of gastrulation is associated with developmental rate and is unrelated to egg size. In the gastrula of the rapidly developing embryos of the foam-nesting frogs Engystomops coloradorum and Engystomops randi, archenteron and notochord elongation overlapped with involution at the blastopore lip, as in X. laevis embryos. In embryos of dendrobatid frogs and in the frog without tadpoles Eleutherodactylus coqui, which develop somewhat more slowly than X. laevis, involution and archenteron elongation concomitantly occurred during gastrulation; whereas elongation of the notochord and, therefore, dorsal convergence and extension, occurred in the postgastrula. In contrast, in the slow developing embryos of the marsupial frog Gastrotheca riobambae, only involution occurred during gastrulation. The processes of archenteron and notochord elongation and convergence and extension were postgastrulation events. We produced an Ab against the homeodomain protein Lim1 from X. laevis as a tool for the comparative analysis of development. By the expression of Lim1, we were able to identify the dorsal side of the G. riobambae early gastrula, which otherwise was difficult to detect. Moreover, the Lim1 expression in the dorsal lip of the blastopore and notochord differed among the studied frogs, indicating variation in the timing of developmental events. The variation encountered gives evidence of the modular character of frog gastrulation.Brachyury ͉ Gastrotheca ͉ Lim1

The vast majority of embryological research on amphibians focuses on just a single genus of frogs, Xenopus. To attain a more comprehensive understanding of amphibian development, experimentation on non-model frogs will be essential. Here, we report on the early development, rearing, and embryological analysis of tú ngara frogs (genus Engystomops, also called Physalaemus). The frogs Engystomops pustulosus, Engystomops coloradorum, and Engystomops randi construct floating foam-nests with small eggs. We define a table of 23 stages for the developmental period in the foam-nest. Embryos were immunostained against Lim1, neural, and somite-specific proteins and the expression pattern of RetinoBlastoma Binding Protein 6 (RBBP6) was analyzed by in situ hybridization. Due to their brief life-cycle, frogs belonging to the genus Engystomops are attractive for comparative and genetic studies of development.

A polyclonal antibody was used to detect the expression of the homeodomain protein Lim1 (Lhx1) in embryos of Xenopus laevis, Engystomops randi, Colostethus machalilla and Gastrotheca riobambae. These frogs belong to four separate families, and have differences in their modes of reproduction and developmental rates. The expression of Lim1 in embryos of these frogs resembled the X. laevis expression pattern. Thus, the dorsal blastopore lip, axial mesoderm, pronephros and certain cells of the central nervous system were Lim1-positive in embryos of all frogs. There were, however, time differences; thus, in the mid-gastrula of the rapidly developing embryos of X. laevis and E. randi, the Lim1 protein was simultaneously detected in the prechordal plate (head organizer) and notochord (trunk organizer). In contrast, only the prechordal plate was Lim1-positive during gastrulation in the slow developing embryos of C. machalilla. The notochord elongated and became Lim1-positive after closure of the blastopore in C. machalilla and G. riobambae embryos. The prechordal plate of G. riobambae embryos could not be clearly detected, as the Lim1-signal remained around the blastopore during gastrulation. These observations indicate that the timing of gene expression at the dorsal blastopore lip in embryos of slow developing frogs differs from that of X. laevis. Moreover, the comparison shows that the developmental processes of the head and trunk organizers are basically separable and become dissociated in embryos of the slow developing frog, C. machalilla.

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