Genetic architecture of physiological phenotypes: Empirical evidence for coadapted gene complexes

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 co adapted set of alleles which optimizes physiological performance and reprod uctive success, The existence of such coadapted gene complexes may be asses sed by assaying phenotypes 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 genet ic architecture of physiological traits in the copepod Tigriopus californic us, a species characterized by sharp genetic differentiation of populations . Developmental time and response to osmotic stress both show pronounced F- 2 hybrid breakdown, a result consistent with genetic coadaptation within po pulations. To better understand the biochemical and molecular mechanisms un derlying hybrid breakdown, we are investigating a specific biochemical phen otype, the activity of the enzyme cytochrome c oxidase (COX), COX (encoded by multiple nuclear and mitochondrial genes) catalyzes the oxidation of cyt ochrome c (encoded by a nuclear gene). Two approaches are being used to add ress the extent of coadaptation (both among nuclear genes and between nucle ar and mitochondrial genes) underlying COX function: (1) studies of the DNA (and inferred amino acid) sequences of component genes among populations i n search of coordinate patterns of amino acid substitution across loci, and (2) direct studies of COX function in interpopulation hybrids and backcros ses. These approaches provide evidence for the existence of nuclear/nuclear and/or nuclear/mitochondrial coadaptation within natural populations of T, californicus.