Rapid speciation and mitogenomic evolution in lacustrine cyclic parthenogens
Costanzo, Katie S.
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The ancient evolutionary history and diversity of crustaceans make them an ideal group to address both macro- and micro-evolutionary processes. They can provide insight to both historic events that shaped early biological diversity and ongoing or recent mechanisms that facilitate diversity. The aim of my dissertation is to use the model system of branchiopod crustaceans to address historic evolutionary relationships within pancrustacean arthropods and also test a model of sympatric ecological speciation in very recently-derived taxa. First (Chapter 2), I describe the complete mitochondrial genomes of three new branchiopod crustaceans, Chydorus brevilabris, Eubosmina coregoni , and Eubosmina tanaki . I provide a detailed comparative analysis of genomic traits and used the sequences of the protein-coding genes of the three branchiopod species along with those of 73 other pancrustaceans to further analyze the evolutionary relationship among crustaceans and hexapods. The results indicate much variation among the mitochondrial genomes of the three species, all exhibiting a unique gene order different from the ancestral pancrustacean state. Concurrently, there is variation in genome length suggesting a different mutation-selection balance within the mitochondrial genomes between mutations that results in duplications and selection for smaller genomes. Phylogenetically, the branchiopods form a monophyletic group and I find further support for the paraphyletic relationship among hexapods and crustaceans with evidence for two major transitions from crustaceans to hexapods. In the next two parts (Chapter 3 and 4), through ecological and molecular studies, I examine the role of ecological factors in facilitating habitat isolation, divergence, and ultimately speciation in two-putative sister-species of Daphnia, Daphnia parvula and Daphnia retrocurva . I establish the degree of habitat isolation and describe what ecological factors differ between standing freshwaters inhabited by either D. parvula or D. retrocurva . In the laboratory, I test the relative vulnerabilities of D . parvula and D. retrocurva to both a vertebrate and invertebrate predators and measure morphological traits of both species that are suggested to pose as anti-predator traits (Chapter 3). Finally, DNA sequences from molecular markers from multiple sympatric populations of D. parvula and D. retrocurva are used to examine the evolutionary relationship between the two species and specifically test a model of allopatric speciation that previously has been proposed (Chapter 4). The results indicate that despite their young age and mixed breeding system, D. parvula and D. retrocurva exhibit significant ecological and genetic divergence. Daphnia parvula and D. retrocurva show strong habitat isolation with D. parvula habitats smaller in size than habitats of D. retrocurva , which contains greater densities of invertebrate predators. In the laboratory, D. retrocurva is less vulnerable to invertebrate predation, whereas D. parvula is less vulnerable to vertebrate predation and is smaller and more transparent than D. retrocurva , consistent with adaptations to avoid visual predators. Genetically, they exhibit low sequence divergence and haplotype sharing suggesting introgression or parallel evolution. Phylogenetically, individual gene trees indicate a polyphyletic pattern with D. parvula and D. retrocurva , yet when considering data combined from all four markers analyzed the two species tend to segregate more into separate groups. The genealogical sorting index indicates they are indeed two genetically distinct groups with a relatively small proportion of the genetic variation found between the two species and shared haplotypes among them. Due to the ecological and evolutionary importance of predation in aquatic communities I suggest predation may have facilitated the rapid divergence between D. parvula and D. retrocurva in a sympatric model of ecological speciation with ongoing gene flow.