DIFFERENTIATION AND INTEGRATION OF THE GENOME IN POPULATIONS OF THE MARINE COPEPOD TIGRIOPUS CALIFORNICUS.

Geographically separated populations of the intertidal copepod Tigriopus californicus are sharply differentiated at several enzyme-encoding gene loci. Two studies were performed to investigate the extent to which the gene pools of local populations are organized into harmoniously interacting (or "coadapted") gene complexes. In the first, the effects of interpopulation hybridization on development time were assessed. Results showed that while F1 hybrids did not differ from parental lines, mean F2 developmental times were as much as 50% longer. The second study used two unlinked enzyme polymorphisms as genetic markers to determine the genotypic specificity of F2 hybrid breakdown. For two sets of parental populations, the relative viabilities of the different two-locus genotypes were determined from segregation ratios among the F2 progeny. Sharp deviations from Mendelian ratios were observed; in the extreme, a block of genes marked by the MeF allozyme from the LJ (La Jolla) population was found to be nearly lethal when homozygous in the F2 of LJ × AB (Los Angeles) crosses. This same block of genes had a tenfold higher viability in crosses between LJ and SC (Santa Cruz). In the AB × LJ crosses, the two marker loci had independent (multiplicative) effects on viability. In the SC × LJ crosses, deviations from the multiplicative model were observed; the data indicate that parental homozygous genotypes have higher viability than predicted by independence, while nonparental homozygotes have lower than predicted viability. These results suggest that substantial integration of the genome has occurred within natural T. californicus populations. © 1987 The Society for the Study of Evolution.