NATURAL SELECTION IN LEPIDOPTERA ACROSS BIOLOGICAL SCALES

Abstract

Here, I survey the influence of selection on evolutionary change across multiple biological scales in Lepidoptera (butterflies and moths). First, I examine the evolution of the Na+K+-ATPase protein in milkweed butterflies, revealing that these butterflies have an amino acid substitution at the 111 position from the Glutamine found in most insects to either Leucine or Valine. I additionally find that monarch butterflies have a second substitution at the 122 position from Asparagine to Histidine.

A second species of milkweed butterfly, the soldier, has this same amino acid substitution, making its origins within these butterflies of interest. I therefore examined the relationship of these two butterflies with the closely allied queen butterfly, which does not have this mutation. In contrast to previous studies, I find that monarchs and soldiers are sister taxa, making shared common ancestry the most parsimonious explanation for the N122H mutation in both species.

Next, I measured the efficacy of selection acting on protein coding regions in 34 Lepidopteran species to measure relationships between effective population size (Ne) and protein evolution. I found that evolutionary constraint is similar across all species, independent of Ne. This means that most negatively selected mutations have a large selection coefficient and are purged from these populations. In contrast, the amount of adaptive divergence observed between species is positively correlated with standing diversity levels, suggesting that advantageous mutations generally have small selection coefficients and can fix more easily in populations with large Ne.

Lastly, I investigated relationships between ecological divergence and genetic exchange. I did this by examining patterns of gene flow across a hybrid zone between two divergent butterflies, the eastern tiger swallowtail and the Canadian tiger swallowtail. I find that genes located on autosomes exhibit a clinal pattern across a latitudinal gradient, whereas sex-linked genes are sharply delineated at the hybrid zone. This indicates that genetic regions responsible for maintaining these distinct species may be located on the sex chromosomes, and do not introgress due to selection.

Overall, this work highlights the ways in which selection can affect genetic diversity across biological scales, from genes and genomes, to populations and species.