Speciation in marine systems: The case study of the sea urchin Arbacia incisa (Agassiz 1863)

It is generally believed that the formation of new species requires substantial accumulation of genetic differentiation across the genome and for this to occur, the range of the ancestral species should have been subdivided by a physical barrier to gene flow. However, in the ocean, despite the high dispersal potential of most marine species and the lack of obvious barriers to gene flow, the rate of speciation is high. To understand the speciation process in the sea, fertilization assays and allozyme data were used to examine the relationship between genetic differentiation and reproductive isolation in a broadcast spawning, high-dispersal species, the sea urchin Arbacia incisa. This species has a wide distribution range in the eastern Pacific that is subdivided by a land barrier, the Baja California peninsula. Fst and Nei's genetic distance estimates indicated a relatively low genetic structuring of populations despite geographic subdivision. The largest genetic distance between locations across the peninsula was D = 0.148, indicating that populations on opposite sides of the land barrier are not significantly more differentiated than local populations of reference species (0.031 ≤ D < 0.230). The average Fst value was 0.0619 and, except for a restricted exchange (Nm = 1.94) between the most distant sites across the peninsula, gene flow estimates were relatively high (average Nm = 3.8) along and between the two coasts of the Baja California peninsula and a site in Panama. Indications of metapopulation-type dynamics were found in the species, which could explain significant deficiencies of heterozygotes observed at most loci examined. Despite the low genetic structuring, high gene flow and relatively small genetic distance among populations of A. incisa, fertilization assays showed a pattern of incipient reproductive incompatibility between populations on opposite sides of the peninsula. The rate of decrease of fertilization with increasing sperm dilution was higher for between- than for within-population crosses for allopatric populations, while rates of fertilization decrease were not different between populations located along the same coastline. These results support recent findings, which indicate that the evolution of reproductive isolation in marine species with broadcast spawning might not require substantial genetic differentiation.