Date of Degree
PhD (Doctor of Philosophy)
First Committee Member
Second Committee Member
Third Committee Member
Fourth Committee Member
A primary goal of evolutionary biology is to elucidate the factors necessary for a single interbreeding species to become two independent species. Observations and data collected and recorded since the 6th century B.C. have added to our comprehension of the “the origin of species—that mystery of mysteries” (DARWIN 1859). To continue to add to our knowledge of how speciation occurs and how species interact, it is crucial to determine 1) how different categories of genes evolve as species diverge, 2) what happens to hybrids of two species, and 3) if genetic exchange is allowed between species, where it is located.
In the first research aim of my dissertation, I look for population genetic trends and signatures of gene flow in a minimally studied set of Drosophila sister species using sequences of 26 nuclear and mitochondrial regions in 29 isofemale lines of D. subobscura and D. madierensis. Standard population genetic tests revealed that the X chromosome evolves faster than the autosomes in these species. We also find evidence of genetic exchange for some autosomal genes while both the sex chromosomes and mitochondrial genomes remain distinct between species.
In the second research aim of my dissertation, I assess the rates of gene expression evolution for sex-biased genes located on the X chromosome and autosomes. We find that gene expression evolves faster in males than females and find evidence of faster-X evolution that is exclusive to genes expressed at higher levels in males. The X chromosome has previously been shown to have a disproportionately large influence on hybrid male sterility compared to autosomes. I investigate this trend and find that the sex chromosomes have a large influence on autosomal expression levels in hybrid males and hybrid females. Specifically, uniparental inheritance of the X chromosome results in greater differences between reciprocal hybrids and higher levels of hybrid misexpression.
One way to determine how species separate and stay separate is to study genomes. Genomes are manuals for cells. Many species have two copies of this manual in each cell. When the manual is copied to make new cells, copying mistakes are made, making each manual slightly different. While each genome is unique, genomes of the same species are more alike to each other than to those of other species. Over time, genomes of different species become more distinct, particularly the X chromosome and genes expressed higher in one sex. Parts of genomes that have evolved in one species can sometimes be ‘copied and pasted’ into a second species through fertile hybrids. In many species, hybrid females are fertile while hybrid males are sterile; a difference that may be due to incompatibilities between copies of the genomes.
The ability of multiple fly species to hybridize in nature and in the laboratory makes them an amendable system for studying evolutionary patterns. In our fly species, we uncovered signals that some specific parts of the genome have been ‘copied and pasted’ from one species into the other. In studying why male hybrids are sterile, we find that the amount of product made from genomes changes more rapidly in males compared to females. Our findings also reveal that hybrids with only one type of the X chromosome (e.g., males) have more genes expressed at inappropriate levels compared with hybrids that have 2 copies of the X chromosome section of the genome (e.g., females).
xiv, 174 pages
Includes bibliographical references (pages 172-174).
Copyright © 2016 Danielle Kay Herrig
Herrig, Danielle Kay. "Evaluating gene flow, gene expression divergence, and hybrid expression in Drosophila sister species." PhD (Doctor of Philosophy) thesis, University of Iowa, 2016.