Heterochrony and neotenic salamanders: possible clues for understanding the animal development and evolution.

A synthesis of developmental genetics with evolutionary genetics is now making possible to understand significant evolutionary changes in multicellular organisms. The key concept for unifying the two must be heterochrony. Heterochrony causes evolutionary modifications due to changes in timing and/or rate of development. The heterochrony is conventionally categorized into three patterns as neoteny (retardation in somatic development), progenesis (acceleration in gonadal development), and direct development (acceleration in somatic development, resulting in lack of larval or tadpole stages). A lot of species showing neoteny are known in urodeles, but not in anurans. Neotenic urodeles are also divided into three categories; permanent or obligate, "inducible" obligate and facultative neotenies. Hynobius retardatus, a specific population of which had been reported to show neoteny but is believed to be extinct at present, has become to be used for experimental analysis of heterochronic expression of several adult characters during its ontogeny. Gonadal maturation and a transition of globin subunits from larval to adult types have been shown to occur independently on the morphological metamorphosis in H. retardatus. Mechanisms underlying the heterochrony, including morphogenetic clock, heterochronic genes in Drosophila and C. elegans, temporal colinearity in Hox gene complex in mice, and atavistic transformation induced by altered expression of Hox genes are discussed in terms of current molecular biology.