Current issues concerning the nature of ancestry and homology are discussed with reference to the evolutionary origin of the tetrapod limb. Homologies are argued to be complex conjectural inferences dependant upon a pre-existing phylogenetic analysisand a theoretical model of the evolutionary development of ontogenetic information. Ancestral conditions are inferred primarily from character (synapomorphy/homology) distributions within phylogeny, because of the deficiencies of palaeontological data. Recent analyses of tetrapod limb ontogeny, and the diverse, earliest morphologies known from the fossil record, are inconsistent (...) with typological concepts such as fixed ancestral patterns or bauplans, emphasising the incompatibility of these with evolutionary continuity. The evolutionary origin of the tetrapod limb is also examined in the light of its recent discussion in developmental genetics. While this field promises to reveal more of the fundamental ontogenetic content of homology (identity), at present it is concerned mostly with the abstraction of a new set of types, rather than investigating diversity and change. (shrink)

The precocious induction in vivo and in culture of insect and amphibian metamorphosis by exogenous ecdysteroids and thyroid hormones, and its retardation or inhibition by juvenile hormone and prolactin, respectively, has allowed the analysis of such diverse processes of post‐embryonic development as morphogenesis, tissue remodelling, functional reorganization, and programmed cell death. Metamorphosis in vertebrates also shares many similarities with mammalian development in the late foetal and perinatal period. This review describes the regulation of expression of some of the ‘adult’ gene (...) products during metamorphosis in invertebrates and vertebrates. Recent studies on metamorphosis have revealed the important role played by auto‐induction of hormone receptor genes, based on which a model will be presented to explain the activation of ‘downstream’ genes which give rise to the adult phenotype. It will also be argued that metamorphosis is an ideal model for analyzing some of the major mechanisms governing post‐embryonic development. (shrink)

Recent work has shown that segmentation underlies the patterning of the vertebrate hindbrain and its neural crest derivatives. Several genes have been identified with segment‐restricted expression, and evidence is now emerging regarding their function and regulatory relationships. The expression patterns of Hox genes and the phenotype of null mutants indicate roles in specifying segment identity. A zinc finger gene Krox‐20 is a segment‐specific regulator of Hox expression, and it seems probable that retinoic acid receptors also regulate Hox genes in the (...) hindbrain. The receptor tyrosine kinase gene Sek may mediate cell‐cell interactions that lead to segmentation. These studies provide a starting point for understanding the molecular basis of segmental patterning in the hindbrain. (shrink)