Restricted Gene Flow and Incipient Speciation in Disjunct Pacific Ocean and Sea of Cortez Populations of a Reef Fish Species, Girella Nigricans

Abstract

Abstract. Population disjunctions have been proposed to play an important role in speciation processes. In this study, we have examined the possible role of the Pacific Ocean-Sea of Cortez disjunction as a contributing factor to cryptic speciation in a reef fish, the opaleye, Girella nigricans. Mitochondrial control region (D-loop) sequences (380 bp) of 117 individuals completely separated opaleye populations from the Pacific Ocean and the Sea of Cortez. Although opaleye exhibit pelagic larval stages that remain in the water column for several months, gene flow between the Pacific Ocean and the Sea of Cortez was found to be extremely limited (F ST ϭ 0.84, Nm ϭ 0.10). Whereas limited gene flow and reciprocal monophyly suggest that the observed physical and genetic disjunction are potentially contributing to the incipient speciation of Pacific and Sea of Cortez opaleye, moderate levels of D-loop sequence divergence (3.3%) and the absence of fixed allozyme markers challenge this idea. Pacific Coast populations also exhibited restricted gene flow levels (F ST ϭ 0.25, Nm ϭ 1.49) across Punta Eugenia, a recognized oceanographic boundary along the Baja California coast. Thus, opaleye individuals grouped into three clades: one clade in the Sea of Cortez, one Pacific clade south of Punta Eugenia, and one Pacific clade north of Punta Eugenia. Future work in this region will determine if our results can be generalized to other disjunct populations.

Chromosome-wide linkage disequilibrium caused by an inversion polymorphism in the white-throated sparrow (Zonotrichia albicollis)

“…Moreover, the very high F ST value (0.94) between the ZAL2 and ZAL2 m inside the inversion is also quite uncommon. Indeed, similar values of F ST in excess of > 0.8 have been associated with special circumstances, such as homologous loci in non-recombining segments of sex chromosomes (Ironside and Filatov 2005) or between cryptic species (Terry et al 2000). Thus, the degree of genetic differentiation observed between the ZAL2 and the ZAL2 m chromosomes is very high even for complex inversions, and is comparable to values observed in the most extreme cases of suppressed gene flow.…”

Phylogeography of the flag cabrilla Epinephelus labriformis (Serranidae): implications for the biogeography of the Tropical Eastern Pacific and the early stages of speciation in a marine shore fish

“…Avise, 2000 and references therein). Such restrictions in gene flow have long been thought to be most effective when coupled with geographical isolation or disjunction (Endler, 1977; Terry et al. , 2000; Bernardi et al.…”

Drift by drift: effective population size is limited by advection

“…Such a comparison can be made for species with pelagic larvae that are distributed on both the Pacific coast of the Baja California peninsula (strong advection) and in the semi-enclosed Gulf of California (weak advection). Our work would predict higher diversity in the weak advection Gulf of California region; given six appropriate studies, four support this hypothesis [35-38], one is equivocal but at least some population samples in the Gulf have higher diversity [39], and in one case the Gulf sister species is apparently less diverse [40]. …”