TEMPORAL PATTERN-FORMATION BY HETEROCHRONIC GENES

Heterochrony describes the phylogenetic variation in the relative timi ng of major developmental events. Such heterochronic variation has bee n noted across phylogeny, including closely related species, suggestin g that particular genetic loci control global aspects of developmental timing, and that variation at those loci may play important roles in evolutionary change. Genetic analyses of heterochronic mutations in th e nematode Caenorhabditis elegans reveal that control of temporal patt erning is analogous to the dedicated genetic pathways that control the patterning of the spatial axes in Drosophila and other metazoans. The se pathways generate graded or binary levels of regulatory factors tha t pattern particular axes of the developing animal. C. elegans heteroc hronic genes constitute a regulatory cascade that both generates a tem poral decrease in the level of the LIN-14 and LIN-28 proteins and resp onds to the changes in these gene activities to coordinate the tempora l sequence of many cell fates as the animal develops. The temporal reg ulation of lin-14 and lin-28 gene activities is posttranscriptional an d mediated by the antisense RNA product of the lin-4 gene. Hormonal co ntrol of developmental timing is a common theme throughout phylogeny. Heterochronic genes that involve hormonal signaling have been identifi ed in vertebrates as well as C. elegans.