MECHANISTIC BASIS OF LIFE-HISTORY EVOLUTION IN ANURAN AMPHIBIANS - DIRECT DEVELOPMENT

The primitive, or ancestral reproductive mode for Recent amphibians in volves a complex, biphasic life history, Yet evolutionarily derived, a lternate modes are seen in all three living orders and predominate in some clades, Analysis of the consequences and mechanistic bases of one such mode-direct development-can provide insights into the evolutiona ry opportunities and constraints conferred by the ancestral metamorphi c ontogeny, Direct development in the anuran genus Eleutherodactylus i nvolves fundamental alterations to many features of embryonic and post hatching development, At hatching, young emerge as fully formed, albei t tiny versions of the adult; most larval features are absent, Pervasi ve changes in ontogenetic timing, in particular the precocious (embryo nic) formation of many adult structures, appear to be correlated with early development of the thyroid axis, although responsiveness to exog enous thyroid hormone is diminished or even lacking in at least some p eripheral tissues, Changes in cranial patterning are likely mediated b y the embryonic neural crest, although many gross features of crest bi ology are highly conserved, Laboratory-based analyses of direct develo pment and other derived reproductive modes in amphibians, using contem porary methods developed for more standard, ''model'' organisms, may c ontribute important insights into life-history evolution that compleme nt those derived from analyses of morphology, ecology and phylogeny.