Genetic Architecture of Physiological Phenotypes: Empirical Evidence for Coadapted Gene Complexes

Synopsis. Physiological phenotypes are the result of the coordinated function of many genes, some of which may be differentiated between conspecific populations. Within any one population, natural selection will favor evolution of a coadapted set of alleles which optimizes physiological performance and reproductive success. The existence of such coadapted gene complexes may be assessed by assaying phe? notypes of interpopulation hybrids: inferior performance of hybrids suggests that the allelic combinations present in the parental populations are coadapted. This approach has been used to examine the genetic architecture of physiological traits in the copepod Tigriopus californicus, a species characterized by sharp genetic differentiation of populations. Developmental time and response to osmotic stress both show pronounced F2 hybrid breakdown, a result consistent with genetic coadaptation within populations. To better understand the biochemical and molec? ular mechanisms underlying hybrid breakdown, we are investigating a speciific biochemical phenotype, the activity of the enzyme cytochrome c oxidase (COX). COX (encoded by multiple nuclear and mitochondrial genes) catalyzes the oxidation of cytochrome c (encoded by a nuclear gene). Two approaches are being used to address the extent of coadaptation (both among nuclear genes and between nuclear and mitochondrial genes) underlying COX function: (1) studies of the DNA (and inferred amino acid) sequences of component genes among populations in search of coordinate patterns of amino acid substitution across loci, and (2) direct studies of COX function in interpopulation hybrids and backcrosses. These ap? proaches provide evidence for the existence of nuclear/nuclear and/or nuclear/mitochondrial coadaptation within natural populations of T. californicus.