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

Populations of the harpacticoid copepod Tigriopus californicus were sa mpled from five sites from San Diego, California, to Playa Altamira, B aja California, Mexico. Allozyme analyses revealed that all the study populations are sharply differentiated genetically. At the extreme, tw o populations, Punta Baja and Playa Altamira, have no alleles in commo n at the seven allozyme loci studied. All pairwise interpopulation cro sses successfully produced F-2 hybrids except those involving the Play a Altamira population. All crosses using Playa Altamira females failed to produce F-2 hybrids, while Playa Altamira males successfully produ ced F-1 progeny with females from all other sites. These F offspring, however, were completely sterile (with San Diego and Punta Banda femal es) or only occasionally produced F-2 offspring (with Punta Morro and Punta Baja females). These results suggest that allopatric differentia tion among Baja populations has resulted in exceptionally high levels of genetic divergence and nearly complete reproductive isolation of th e Playa Altamira population, which should now be recognized at the sem ispecies (or perhaps sibling species) level.