Spatially structured genetic variation in a broadcast spawning bivalve: quantitative vs. molecular traits

Abstract

Understanding the origin, maintenance and significance of phenotypic variation is one of the central issues in evolutionary biology. An ongoing discussion focuses on the relative roles of isolation and selection as being at the heart of genetically based spatial variation. We address this issue in a representative of a taxon group in which isolation is unlikely: a marine broadcast spawning invertebrate. During the free-swimming larval phase, dispersal is potentially very large. For such taxa, small-scale population genetic structuring in neutral molecular markers tends to be limited, conform expectations. Small-scale differentiation of selective traits is expected to be hindered by the putatively high gene flow. We determined the geographical distribution of molecular markers and of variation in a shell shape measure, globosity, for the bivalve Macoma balthica (L.) in the western Dutch Wadden Sea and adjacent North Sea in three subsequent years, and found that shells of this clam are more globose in the Wadden Sea. By rearing clams in a common garden in the laboratory starting from the gamete phase, we show that the ecotypes are genetically different; heritability is estimated at 23%. The proportion of total genetic variation that is between sites is much larger for the morphological additive genetic variation (Q ST ¼ 0.416) than for allozyme (F ST ¼ 0.000-0.022) and mitochondrial DNA cytochrome-c-oxidase-1 sequence variation (U ST ¼ 0.017). Divergent selection must be involved and intraspecific spatial genetic differentiation in marine broadcast spawners is apparently not constrained by low levels of isolation.

Influence of the North Equatorial Current on the population genetic structure of Tridacna crocea (Mollusca: Tridacnidae) along the eastern Philippine seaboard

“…This result is consistent with previous reports of conspecific populations from the South China Sea and Sulu Sea reefs (Juinio-Meñez et al 2003), and contrasts with earlier population genetic studies of other tridacnid species, which revealed little genetic differentiation over broader geographic scales (reviewed by Juinio-Meñez et al 2003). Locus-specific heterozygote deficiencies (in this case GPI-1*) is a pattern frequently observed in bivalves (Luttikhuizen et al 2003 and references therein), and may be attributed to genotype scoring errors, as well as to biological/reproductive factors e.g. genetic differences between cohorts, differential selection, and subpopulation structure (Wahlund effect).…”

Common garden experiments in the genomic era: new perspectives and opportunities

“…In addition, common garden experiments are also used to study the consequences of local adaptation for conservation (McKay et al, 2001) or even for ecosystem functioning (Bassar et al, 2010). Despite its name, and although it has been used extensively with plants (Linhart and Grant, 1996), this experimental approach can also be applied to a large variety of organisms including fish (Bassar et al, 2010;DeFaveri and Merilä, 2014), invertebrates (Spitze, 1993;Luttikhuizen et al, 2003) and small mammals (Bozinovic et al, 2009). The main limitations to this experimental design are the ability to breed the species and to grow the produced offspring in laboratory or seminatural conditions.…”

Distinguishing adaptive from nonadaptive genetic differentiation: comparison of QST and FST at two spatial scales

“…This led to the idea of testing selective divergence in quantitative traits by comparison of their population differentiation (Q ST ) with the population differentiation obtained from neutral molecular markers (F ST ) (Spitze, 1993). Three outcomes, each having a unique interpretation, are possible (Merila and Crnokrak, 2001): (i) Q ST 4F ST : this is commonly interpreted as evidence of divergent selection and adaptation to local environments (Podolsky and Holtsford, 1995;Bonnin et al, 1996;Latta and Mitton, 1997;Jaramillo-Correa et al, 2001;Steinger et al, 2002;Storz, 2002;Chan and Arcese, 2003;Luttikhuizen et al, 2003); (ii) Q ST and F ST do not differ: genetic drift alone is sufficient to explain the pattern of differentiation (Yang et al, 1996), although effects of natural selection and drift in certain cases can be indistinguishable in certain cases (Sokal and Wartenburg, 1983); and (iii) Q ST oF ST : convergent selection favoring the same phenotype in different environments is assumed (Kuittinen et al, 1997;Petit et al, 2001;Edmands and Harrison, 2003). However, interpretation of any discrepancy between Q ST and F ST as evidence of selection on a particular quantitative trait may not always hold.…”