Heterokairic Genes and the Eco-Evo-Devo of Timing

Concepts of developmental timing have traditionally been framed under heterochrony as evolved (genetically based) differences in timing, while environmentally induced shifts in timing within genotypes have been treated more loosely. In this article, heterokairy is presented as plasticity in the timing of developmental events, and the term "heterokairic genes" is proposed for environmentally modulated heterochronic genes that underlie this plasticity. Evidence from nematodes, insects, plants, and vertebrates is assembled, with emphasis placed on systems where environmental cues are relayed through endocrine or metabolic pathways to known timing modules/genes. On this basis, a distinction is drawn between validated heterokairic genes, supported by direct mechanistic data, and a broader set of candidates inferred from gene-environment interactions in developmental timing. The eco-evolutionary consequences of such genes are considered, and experimental and genomic strategies for their identification are outlined. It is argued that heterokairic genes provide a useful bridge between environmental variation, developmental mechanisms, and evolutionary change in timing.