Heterozygosity and asymmetry: Ectodysplasin as a form of genetic stress in marine threespine stickleback

Abstract

Genome‐wide heterozygosity has long been hypothesized to play a role in buffering organisms against developmental perturbations, potentially resulting in increased symmetry. If true, this could in part explain the maintenance of standing genetic variation in wild populations. Marine threespine sticklebacks (Gasterosteus aculeatus) were sampled across their eastern Pacific coastal distribution from Alaska to California and variations in asymmetry for both structural and nonstructural armor traits (lateral plates) were assessed. Structural plates consistently showed less asymmetry than nonstructural plates, but standardized measures of heterozygosity were not correlated with the extent of asymmetry expressed by a fish. Fish that were heterozygous for the major‐effect gene controlling lateral plate variation (Ectodysplasin) had higher occurrences of asymmetry, even when the individuals were phenotypically fully plated. Collectively, this suggests that heterozygosity at a major‐effect locus can have a greater impact on asymmetry than heterozygosity sampled across the genome.

Genetic and environmental canalization are not associated among altitudinally varying populations of Drosophila melanogaster

“…This is reminiscent of the evolution of insecticide resistance in blowflies, which showed a pleiotropic increase in fluctuating asymmetry due to the resistance locus or in sticklebacks with the effects of the Eda locus on both the expression of armor plates and fluctuating asymmetry of plates (McKenzie and Clarke 1988; Morris et al. 2019). Consistent with Waddington's model for the evolution of canalization, it could be that modifiers that increase the mutational robustness of wing morphology in Drosophila have not yet risen to appreciable frequency in the high‐altitude population, as has occurred with the modifier variants influencing asymmetry in the blowflies (Davies et al.…”

“…This approach should be coupled with studies of adaptively diverged natural populations that are likely to share the appropriate evolutionary history to address these questions (i.e., McKenzie and Clarke 1988; Morris et al. 2019). Finally, this work suggests that trying to clearly delineate between selection for “environmental” and “genetic” canalization may be difficult given the interplay between genotypic and environmental effects in terms of trait expression and variation.…”

Genetic and environmental canalization are not associated among altitudinally varying populations ofDrosophila melanogaster

“…While the work of Lack et al (2016a,b) and Groth et al (2018) clearly demonstrate evolutionary changes in genetic canalization associated with adaptive trait evolution (body size and wing morphology), the most likely evolutionary scenario is one of the loss of genetic canalization associated with the pleiotropic effects of variants that have contributed to the evolution of mean trait expression at high altitudes. This is reminiscent of the evolution of insecticide resistance in blowflies which showed a pleiotropic increase in fluctuating asymmetry due to the resistance locus or in sticklebacks with the effects of the Eda locus on both the expression of armour plates, and on fluctuating asymmetry of plates (McKenzie and Clarke, 1988; Morris et al, 2019). Consistent with Waddington’s model for the evolution of canalization, it could be that modifiers that increase the mutational robustness of wing morphology in Drosophila have not yet risen to appreciable frequency in the high altitudinal populations, as it occured with the modifier variants influencing asymmetry in the blowflies (Davies et al, 1996).…”

“…This approach should be coupled with studies of adaptively diverged natural populations that are likely to share the appropriate evolutionary history to address these questions (i.e. (Morris et al, 2019; McKenzie and Clarke, 1988)). Finally, this work suggests that trying to clearly delineate between selection for ‘environmental’ and ‘genetic’ canalization may be difficult given the interplay between genotypic and environmental effects in terms of trait expression and variation.…”

Climate‐driven habitat change causes evolution in Threespine Stickleback

“…We found Eda to explain 80% of the variation in plate number and 85% of the variation in plate morph (using ANOVA analysis as described in Colosimo et al, ; Kitano et al, ). Though the degree of Eda dominance may vary by population (Cresko et al, ), other studies in California show a similarly high degree of matching between genotype and plate morph (Morris, Kaufman, & Rogers, ) indicating that Eda C may show weaker dominance effects in Californian populations. To test this assumption, we used the R package ‘SNPassoc’ (Gonzalez et al, ) and Akaike information criteria to compare dominance, recessive, codominance, and additive genotype–phenotype association models using our sample of stickleback with known genotypes.…”