Stabilizing selection and the evolution of genetic variance in multivariate traits in Drosophila serrata

Sztepanacz, Jacqueline Luise Pratt (2016). Stabilizing selection and the evolution of genetic variance in multivariate traits in Drosophila serrata PhD Thesis, School of Biological Sciences, The University of Queensland. doi:10.14264/uql.2016.1111

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Author Sztepanacz, Jacqueline Luise Pratt
Thesis Title Stabilizing selection and the evolution of genetic variance in multivariate traits in Drosophila serrata
Formatted title
Stabilizing selection and the evolution of genetic variance in multivariate traits in Drosophila serrata
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
DOI 10.14264/uql.2016.1111
Publication date 2016-11-18
Thesis type PhD Thesis
Supervisor Mark Blows
Total pages 202
Language eng
Subjects 0603 Evolutionary Biology
0604 Genetics
Formatted abstract
The presence of genetic variation in almost any individual trait and the prediction that many such traits are subject to stabilizing selection, comprise two basic tenets of evolutionary biology. However, despite the concerted efforts of theoreticians and empiricists we still understand very little about how much genetic variation is available to selection, how such variation is maintained in the presence of selection, and how prevalent or strong stabilizing selection is in natural populations. In my thesis I address the role of stabilizing selection in the evolution of genetic variance in multivariate traits, and their environmental and dominance variance.

Stabilizing selection is of central importance in evolutionary theories for the maintenance of genetic variance, and has been invoked as the key process determining macro-evolutionary patters of quantitative trait evolution. However, manipulative evidence for stabilizing selection, particularly on multivariate traits is lacking. Here I used artificial disruptive selection in Drosophila serrata to determine the relative strength of stabilizing selection acting on multivariate traits. Contrary to expectation, when disruptive selection was applied to the major axis of genetic variance, gmax, I observed a significant and repeatable decrease in phenotypic variance in replicate populations by an average of -0.135 phenotypic standard deviations. In contrast, the multivariate trait combination predicted to be under strong stabilizing selection showed a significant and repeatable increase in phenotypic variance in replicate populations by an average of 0.385 phenotypic standard deviations. Larger correlated responses in the predicted direction suggested that some traits were under weaker selection than, gmax. In addition, for other correlated traits I found that viability selection was operating on extreme phenotypes. My manipulation revealed that multivariate traits were subject to stabilizing selection in this population; however, the pleiotropic association among genetically correlated traits obscured a direct relationship between the strength of stabilizing selection acting on multivariate phenotypes and the levels of standing genetic variance in these phenotypes.

The extent to which an individual's phenotype is affected by stochastic variation that occurs within a given defined environment, and the consequences of such micro-environmental variance for fitness is poorly understood. Using a multigenerational breeding design in Drosophila serrata, I obtain an estimate of the additive genetic variance of the micro-environmental variance in a set of morphological wing traits in a randomly mating population. The micro-environmental variance of wing-shape had significant additive genetic variance in most single wing traits, and although heritability was low (< 1%), coefficients of additive genetic variance were of a magnitude typical of other morphological traits, indicating that the micro-environmental variance is an evolvable trait. The micro-environmental variance was genetically correlated among wing traits, suggesting that common mechanisms of environmental buffering exist for this functionally related set of traits. Through its association with fitness, I demonstrated that the major axes of micro-environmental variance were subject to variance reducing selection, although statistical support for the additive genetic association between fitness and the micro-environmental variance was weak. However, a positive covariance between the dominance genetic variance in fitness and micro-environmental variance indicated that the micro-environmental variance shares a genetic basis with fitness.

In contrast to our growing understanding for patterns of additive genetic variance in single and multi-trait combinations, the relative contribution of non-additive genetic variance, particularly dominance variance, to multivariate phenotypes is largely unknown. While mechanisms for the evolution of dominance genetic variance have been, and to some degree remain, subject to debate, the pervasiveness of dominance is widely recognized, and may play a key role in several evolutionary processes. Theoretical and empirical evidence suggests that the contribution of dominance variance to phenotypic variance may increase with the correlation between a trait and fitness; however, direct tests of this hypothesis are few. Using a multigenerational breeding design in an unmanipulated population of Drosophila serrata, I estimated additive and dominance genetic covariance matrices for multivariate wing shape phenotypes, together with a comprehensive measure of fitness, to determine whether there is an association between directional selection and dominance variance. Fitness, a trait unequivocally under directional selection, had no detectable additive genetic variance, but significant dominance genetic variance contributing 32% of the phenotypic variance. For single and multivariate morphological traits, however, no relationship was observed between trait-fitness correlations and dominance variance. A similar proportion of additive and dominance variance was found to contribute to phenotypic variance for single traits, and double the amount of additive compared to dominance variance was found for the multivariate trait combination under directional selection. These data suggest that for many fitness components a positive association between directional selection and dominance genetic variance may not be expected.
Keyword Stabilizing selection
Genetic variance
Environmental variance
Dominance variance
Artificial selection

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Created: Wed, 09 Nov 2016, 22:49:05 EST by Jacqueline Sztepanacz on behalf of Learning and Research Services (UQ Library)