The Organizer in Amphibians with Large Eggs: Problems and Perspectives
The amphibians are a diverse group that includes not only the frogs and toads (Anura), but also the newts and salamanders (Urodela), and the limbless and tailless amphibians (Gymnophiona). Development has been extensively investigated in the frog Xenopus
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The Organizer in Amphibians with Large Eggs: Problems and Perspectives Eugenia M. del Pino l and Richard P. Elinson2
21.1
Introduction
The amphibians are a diverse group that includes not only the frogs and toads (Anura), but also the newts and salamanders (Urodela), and the limbless and tailless amphibians (Gymnophiona). Development has been extensively investigated in the frog Xenopus laevis, but we have limited information for other amphibians, many of which have different reproductive modes (reviewed in Duellman and Trueb 1986). The study of the morphological variation and gene expression patterns among amphibians provides comparative data on early development. Such comparisons represent natural experiments that expand our understanding of development. The Spemann-Mangold organizer is a characteristic feature of development not only of the classically studied urodeles Triton cristatus and Triton taeniatus (Spemann and Mangold 1924), but also of other amphibians, including X. laevis (reviewed in Grunz 2001). Moreover, other vertebrates, such as zebrafish, chick and mouse, have an organizer homologue, detected by its ability to induce a secondary axis and by equivalent gene expression patterns with X. laevis (reviewed in Jouvin and Stern 2001). Therefore, the organizer is a conserved character of vertebrate development and should be present in all amphibians, including those with large eggs. Large egg size in amphibians, however, is often associated with altered geometry of the egg and embryo, and with changes in the schedule of developmental events. These modifications may alter the program of the organizer and may provide valuable insights into its formation and function. Unfortunately, not much is known about the organizer in amphibians with large eggs, and most of that information is at the level of morphology. Accordingly, we will discuss aspects of early development in relation to the organizer in the two most studied frogs with large eggs, the marsupial frog from Ecuador, Gastrotheca riobambae, and the Puerto Rican frog that does not have tadpoles, Eleutherodactylus coqui, with reference to other species when applicable. The characteristics of development in G. riobambae and E. coqui are reviewed in del Pino (1989), Elinson et al. (1990) and Callery et al. (2001). 1 Departamento de Ciencias Biol6gicas, Pontificia Universidad Cat61ica del Ecuador, Avenida 12 de Octubre y Patria, Apartado 17-01-2184, Quito, Ecuador 2 Department
of Biological Sciences, Duquesne University, Pittsburgh, Pennsylvania 15282, USA
H. Grunz (ed.), The Vertebrate Organizer © Springer-Verlag Berlin Heidelberg 2004
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E. M. del Pino and R. P. Elinson
21.2 Large and Small Amphibian Eggs
The eggs of amphibians vary greatly in size. According to Duellman and Trueb (1986), eggs range from 0.75-12mm in diameter among frogs. These extreme values occur respectively in the frog Hymenochirus boettgeri, a close relative of x. laevis, and in the marsupial frog Gastrotheca cornuta. The increase in egg diameter is frequently associated w
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