Heterochrony describes the phylogenetic variation in the relative timing of major developmental events. Such heterochronic variation has been noted across phylogeny, including closely related species, suggesting 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 the nematode Caenorhabditis elegans reveal that control of temporal patterning is analogous to the dedicated genetic pathways that control the patterning of the spatial axes in Drosophila and other metazoans. These pathways generate graded or binary levels of regulatory factors that pattern particular axes of the developing animal. C. elegans heterochronic genes constitute a regulatory cascade that both generates a temporal decrease in the level of the LIN-14 and LIN-28 proteins and responds to the changes in these gene activities to coordinate the temporal sequence of many cell fates as the animal develops. The temporal regulation of lin-14 and lin-28 gene activities is posttranscriptional and mediated by the antisense RNA product of the lin-4 gene. Hormonal control of developmental timing is a common theme throughout phylogeny. Heterochronic genes that involve hormonal signaling have been identified in vertebrates as well as C. elegans.
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