Phylogeography of the spotted sand bass, Paralabrax maculatofasciatus: Divergence of Gulf of California and Pacific Coast populations

Have the warm tropical waters and currents of the southern Gulf of Californ ia, Mexico (also known as the Sea of Cortez), formed a barrier to gene flow , resulting in disjunct populations in the upper gulf that are isolated fro m the outer Pacific Coast? Phylogeographic and genetic divergences of the s potted sand bass, Paralabrax maculatofasciatus, from three Gulf of Californ ia and two outer Pacific coastal locations were tested using mitochondrial DNA (mtDNA) control region sequences. Sequence data from two congeners that are sympatrically distributed along the outer Pacific Coast, the barred sa nd bass, P. nebulifer, and the kelp bass, P. clathratus, were used to gauge the levels of genetic divergences. Differences among the three species and between the northern gulf and outer Pacific coastal populations of P. macu latofasciatus also were analyzed using 40 allozymic presumptive gene loci. Allozyme and mtDNA analyses each revealed many fixed differences among the species. Three significant allozymic frequency differences and two fixed mt DNA substitutions differentiated the gulf and outer Pacific coastal populat ions of P. maculatofasciatus. Three unique mtDNA haplotypes and three uniqu e allozyme alleles were identified from the outer Pacific coastal populatio n. The gulf sites contained four unique mtDNA haplotypes and six unique all ozyme alleles. Partitioning of the mtDNA variation revealed that 72% of the variance occurred between the gulf and outer Pacific Coast, 20% between sa mpling sites in the two regions, and 8% within the sites. There appears to be little gene flow across the waters of the southern Baja Penninsula, prod ucing divergence estimated as 120,000 to 600,000 years between the outer Pa cific coastal and the Gulf of California populations. This separation level may date to a hypothesized seaway closure near La Paz, Mexico, during the mid-Pleistocene, and characterizes other fish populations. A second pattern of deeper allopatric species-level divergences in some other fishes may da te to a Pliocene closure of a mid-Baja Penninsular seaway. Significant diff erences also were discerned in P. maculatofasciatus between the San Diego a nd central Baja California coastal sites and between the upper/central and the lower gulf locations. Variation between locations in the two regions ma y be indicative of larval retention and low adult migration, which needs to be tested further.