DIRECT DEVELOPMENT IN THE LUNGLESS SALAMANDERS - WHAT ARE THE CONSEQUENCES FOR DEVELOPMENTAL BIOLOGY, EVOLUTION AND PHYLOGENESIS

Direct development is a widespread alternate reproductive mode in livi ng amphibians that is characterized by evolutionary loss of the free-l iving, aquatic larval stage. Courtship, mating, and oviposition occur on land, and the terrestrial egg hatches as a fully formed, miniature adult. While it is the most common reproductive mode in urodeles, deve lopment outside the reproductive tract of the female that proceeds dir ectly to a terrestrial hatchling occurs in only a single lineage, the lungless salamanders of the family Plethodontidae. Evolution of direct development in plethodontids has contributed importantly to the extra ordinary evolutionary success of this speciose, geographically widespr ead, and morphologically and ecologically diverse taxon. Developmental consequences and correlates include increased egg size and embryonic development time, loss of larval structures and ontogenetic repatterni ng, and altered pattern formation in organogenesis. Evolutionary and p hylogenetic consequences and correlates include the loss of larval con straints and origin of morphological novelty, and frequent homoplasy. Analysis of direct development in an evolutionary context illustrates the complex interplay between processes of phylogenetic divergence and developmental biology, and substantiates the prominent role of develo pmental processes in both constraining phenotypic variation and promot ing phenotypic diversity. Despite the proven suitability of direct-dev eloping plethodontid salamanders for laboratory and field study, knowl edge of basic features of their developmental biology remains far belo w that available for many other urodeles. Examination of such features of these ''non-model'' organisms is an appropriate and deserving goal of future research.