18 Heterochronic Genes

Abstract

I. INTRODUCTION
Caenorhabditis elegans is a convenient animal for the genetic and molecular analysis of developmental timing, since the worm’s developmental pattern is simple and well-characterized (Sulston and Horvitz 1977; Sulston et al. 1983). Changes in the relative timing of developmental events are termed “heterochrony.” Genes have been identified in C. elegans that appear to act almost exclusively in the control of the relative timing of stage-specific events (Chalfie et al. 1981; Ambros and Horvitz 1984). Mutations in these so-called “heterochronic genes,” which include lin-4, lin-14, lin-28, and lin-29, cause precocious or retarded development of certain cell lineages, leading to, for example, larvae with adult tissues or adults with larval tissues. The genetic and molecular analysis of heterochronic genes affords an opportunity to examine in detail the regulatory principles underlying an organized developmental schedule. Because heterochrony may be a common mechanism for evolutionary variation (Gould 1977), the heterochronic genes of C. elegans are of potential evolutionary and developmental interest.

Genetic analysis of the heterochronic genes has revealed that they act in regulatory pathways that specify the proper sequence of developmental events through the larval stages to the adult (Ambros and Horvitz 1984; Ambros 1989; Liu and Ambros 1989; Euling and Ambros 1996a). The heterochronic genes so far identified affect only postembryonic development, indicating that separate mechanisms control the steps of embryonic development. These genes also do not appreciably affect development of the gonad, suggesting that separate genetic circuitries control the timing of gonadal and nongonadal development in the larva. This...