Speciation and Population Genetic Structure in Tropical Pacific Sea Urchins

Abstract

Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene flow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in taxa with high dispersal potential. To understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene flow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects. These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene flow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.

New insight on population genetic connectivity of widespread amphidromous prawn Macrobrachium lar (Fabricius, 1798) (Crustacea: Decapoda: Palaemonidae)

“…Indeed, similar patterns of genetic differentiation were identified in two amphidromous species, the gastropod Neritina canalis (Crandall et al 2010) and the fish Sicyopterus lagocephalus (Lord et al 2012). Likewise, a sharp genetic break has been found between the Central and Western Pacific populations within the sea urchin species Echinometra mathaei and E. oblonga (Palumbi et al 1997), the coral reef fishes Acanthurus triostegus (Planes and Fauvelot 2002) and Scarus psittacus (Winters et al 2010), and the snapper fishes Lutjanus kasmira and L. fluvus (Gaither et al 2010). The biogeographic genetic break has commonly been explained by a combination of parameters, such as past changes in climate and water mass circulation and random long-distance dispersal events (e.g., Palumbi et al 1997;Planes and Fauvelot 2002;Lord et al 2012).…”

“…Therefore, Indo-Pacific species that occur up to the Central Pacific are generally thought to have colonized eastern remote islands through a stepping-stone pattern of dispersal. Accordingly, genetic studies of widely distributed marine and freshwater species have demonstrated genetic divergences between Western and Central Pacific populations associated with patterns of isolation by distances (e.g., Palumbi et al 1997;Planes and Fauvelot 2002;Crandall et al 2010;Gaither et al 2010;Winters et al 2010;Lord et al 2012).…”

Genetic structure of the high dispersal Atlanto-Mediterreanean sea star Astropecten aranciacus revealed by mitochondrial DNA sequences and microsatellite loci

“…Nevertheless, gene flow in marine species can be constrained by dispersal barriers, such as narrow water passages between land masses, sharp salinity gradients or different types of currents e.g., circular currents (eddies) or downward currents. As marine barriers are not always easily identified, they might lead to population structure even in high dispersal species (Quesada et al 1995;Palumbi et al 1997). …”

Absence of postzygotic isolating mechanisms: evidence from experimental hybridization between two species of tropical sea urchins

“…As in other Echinometra, Ec and Eo are sympatric but hybrids are rare or absent (Geyer and Palumbi, 2005). The only evidence of hybridization in the field is a single specimen out of 97 examined that had similar color and spicules characteristics of Em, but with the mtDNA of Eo (Palumbi et al, 1997). This specimen may have represented a backcross, perhaps of several generations (Rahman et al, 2004a).…”