Independent axes of genetic variation and parallel evolutionary divergence of opercle bone shape in threespine stickleback

Abstract

Evolution of similar phenotypes in independent populations is often taken as evidence of adaptation to the same fitness optimum. However, the genetic architecture of traits might cause evolution to proceed more often toward particular phenotypes, and less often toward others, independently of the adaptive value of the traits. Freshwater populations of Alaskan threespine stickleback have repeatedly evolved the same distinctive opercle shape after divergence from an oceanic ancestor. Here we demonstrate that this pattern of parallel evolution is widespread, distinguishing oceanic and freshwater populations across the Pacific Coast of North America and Iceland. We test whether this parallel evolution reflects genetic bias by estimating the additive genetic variance-covariance matrix (G) of opercle shape in an Alaskan oceanic (putative ancestral) population. We find significant additive genetic variance for opercle shape and that G has the potential to be biasing, because of the existence of regions of phenotypic space with low additive genetic variation. However, evolution did not occur along major eigenvectors of G, rather it occurred repeatedly in the same directions of high evolvability. We conclude that the parallel opercle evolution is most likely due to selection during adaptation to freshwater habitats, rather than due to biasing effects of opercle genetic architecture.

Figure 1

Opercle (OP) morphology has evolved rapidly (46 years) and in parallel in threespine sticklebacks. Alizarin Red stained Middleton Island fish imaged by epifluorescence. The opercle is prominent among the many skull bones distinguishable (Bowne 1994) in these side views of the stickleback head. The opercles of the two freshwater lacustrine fish (B, C) differ from the elongated bone of local oceanic fish (A; representing the putative ancestral morphology). All three sites were marine habitats along the shore of the island before the 3 m uplift accompanying a 1964 earthquake (Gelmond et al. 2009). Points along the edge of the opercle in A show the positions of the nine landmarks digitized for morphometric analyses, and the wire-frame diagrams to the right compare the morphologies in the manner used in following figures. In (B), we indicate the prominent dilation (double headed arrow)—diminution (single headed arrow) shape change characterizing opercle evolution between oceanic and freshwater populations globally. Scale bar: 2 mm.

Figure 2

Opercle shape configurations reveal a common pattern of divergence between oceanic (gray) and freshwater (black) populations across regions. Figures represent total Procrustes deformations from the consensus configurations for all of the oceanic and lake populations for each region (A–E), and globally for all of the regions collectively (F). The arrows in A show the dilation–diminution shape change.

Figure 3

(A). PC1 largely captures evolutionary divergence from oceanic to freshwater populations across disparate geographical regions. Open circles indicate consensus scores for oceanic populations, whereas filled circles indicate consensus scores for freshwater populations. Abbreviations for the populations are listed in Table 1. (B). Wire-frame diagrams showing how the opercle shape configuration changes along the PC1 and PC2 axes. The deformation from high to low PC1 (from ancestral oceanic, to descendent freshwater populations) prominently includes dilation–diminution (arrows). The deformation from high to low PC2 (from the region where the PB freshwater population maps to the region where the SK freshwater population maps) prominently includes a widening of the entire configuration.

Figure 4

Freshwater opercle shapes can show extreme (A) or milder (B) divergence along PC1 from local oceanic populations, and in some cases show marked divergence along a second axis, PC2 (C). Points represent PC scores for individual fish. Keys for the symbols are given on each panel. Abbreviations for the populations are listed in Table 1.

Figure 5

Leading eigenvectors of G are not aligned with the vector of evolutionary divergence. Laboratory-reared individuals from Boot Lake (filled circles) and Rabbit Slough (open circles) plotted in the previously described PC1 and PC2 phenotypic space. These individuals are from single-pair families (crosses within Boot Lake and Rabbit Slough lines), and grown to the same body size as the half-sibling families of stickleback used for the G matrix analysis. The first two axes of the G matrix (i.e., g_max and g₂;estimated from the Rabbit Slough half-sibling families, not shown in this figure) are superimposed (bold oval) on these points and centered on the mean of the laboratory reared single pair family of Rabbit Slough stickleback. The dashed arrow shows the vector of divergence in this space from the mean of Rabbit Slough two-dimensional G to the mean of the Boot Lake family. The dotted line shows the perturbation in the evolutionary trajectory of the population predicted by the influence of G, with the trajectory calculated under the assumption of Gaussian selection operating independently on the two traits and an intensity of selection strong enough to generate the necessary response within 1,000 to 10,000 generations.