GENETIC DIFFERENTIATION AND REPRODUCTIVE INCOMPATIBILITY AMONG BAJA-CALIFORNIA POPULATIONS OF THE COPEPOD TIGRIOPUS-CALIFORNICUS

Populations of the harpacticoid copepod Tigriopus californicus were sampled from five sites from San Diego, California, to Playa Altamira, Baja California, Mexico. Allozyme analyses revealed that all the study populations are sharply differentiated genetically. At the extreme, two populations, Punta Baja and Playa Altamira, have no alleles in common at the seven allozyme loci studied. All pairwise interpopulation crosses successfully produced F-2 hybrids except those involving the Playa Altamira population. All crosses using Playa Altamira females failed to produce F-2 hybrids, while Playa Altamira males successfully produced F-1 progeny with females from all other sites. These F offspring, however, were completely sterile (with San Diego and Punta Banda females) or only occasionally produced F-2 offspring (with Punta Morro and Punta Baja females). These results suggest that allopatric differentiation among Baja populations has resulted in exceptionally high levels of genetic divergence and nearly complete reproductive isolation of the Playa Altamira population, which should now be recognized at the semispecies (or perhaps sibling species) level.