Pre-mating reproductive isolation between three sympatric varieties of a specialist insect

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.