Date of Degree
MS (Master of Science)
Andrew A. Forbes
Ecological interactions can play a major role in driving the process of speciation when they lead to a decrease in gene flow between diverging lineages. Various pre- and post-zygotic ecological barriers to gene flow are known to be important in speciation, but the specific barriers that cause the initiation of speciation are often unknown. Phytophagous (plant feeding) insects are powerful systems for evaluating ecologically based reproductive barriers because these organisms generally have a history of traits such as host shifting and host mediated sexual selection associated with speciation. Previous work on the sunflower maggot fly (Strauzia longipennis) indicates that three genetically distinct, diverging varieties co-occur on the host plant Helianthus tuberosus. In this work, I 1) confirm the existence of three diverging varieties by genotyping microsatellite loci, 2) evaluate the presence and strength of three pre-zygotic barriers to reproduction - habitat isolation, pre-mating sexual isolation, and allochronic isolation - between the varieties, and 3) measure the impacts of allochronic isolation on resource partitioning by evaluating host preference, oviposition location, and larval location between diverging Strauzia varieties sharing the same host plant species. I find evidence of pre-mating sexual isolation and allochronic isolation between the three varieties, indicating that these may be reproductive barriers that arise during early stages of divergence. These barriers may have occurred without (or before) the host-shift that is typical of many other diverging phytophagous insect systems. I also find evidence that allochronic isolation leads to resource partitioning of the single host plant resource, which may help the three varieties share the same host plant.
New biological diversity is the result of speciation – when one species becomes two or more new species. Because it can take millions of years for speciation to be completed, the overall process of speciation can be difficult to study. Many different factors may contribute to speciation, including genetics, behavior, and ecology of the organisms involved. Though all of these factors are important, ecological interactions - how organisms interact with their local habitats - may be especially important in the beginning stages of speciation, especially for specialist insects. Small differences in the ways that different populations interact with their habitats can reduce their contact with one another. Such a decrease in contact between groups of organisms can initiate the formation of new species.
In this work, I study interactions between three varieties of the sunflower maggot fly (genus Strauzia) that are in the early stages of speciation. I measure the importance of three ecological interactions, habitat isolation via habitat choice, sexual isolation (i.e. mate choice), and allochronic isolation (i.e. differences in life cycle timing) in reducing mating between the three varieties. I find that sexual isolation and allochronic isolation both decrease the amount of contact between the varieties. I also evaluate how these three varieties are able share the same host plant habitat without displacing or outcompeting one another. This work provides us with new and valuable information about the early stages of speciation.
Copyright 2016 Alaine Hippee