Morphological and Genetic Examination of Host-Associated Divergence in the Parasitoid Wasp, Torymus umbilicatus (Gahan)





Paper Type

Master's Thesis


College of Arts and Sciences

Degree Name

Master of Arts in Biology (MA)



College Dean

Dr. Barbara A. Hetrick


Sympatric speciation has been debated since the days of Darwin, but with the advent of relatively new molecular technology, this age-old dispute may soon be settled. In the current study, the parasitoid wasp, Torymus umbilicatus (Gahan), was used to investigate both morphological and genetic evidence for indications of host-associated divergence (HAD) in sympatry. Female parasitoids from midge-induced galls found on three different native coastal host plants in two non-contiguous locations were dissected. Statistical analysis of dissection data indicated significant (p < 0.05) size differences between parasitoid populations at host plant genus level as well as morphological effects from both site and host-plant. Molecular support was provided by alignment of parasitoid mtDNA nucleotides which indicated genetic variation among and between female parasitoid populations, a strong indication of divergence. Subsequent phylogenetic analysis showed iterated patterns of relatedness supporting the ancestral host plant hypothesis and suggesting that, though parasitoid populations from secondary and tertiary host plants appear to be diverging from ancestral populations, they do not appear to be diverging from each other. This may indicate enough gene exchange between them to prevent divergence. A recent molecular examination of the gall midge from the second trophic level of this tri-trophic system located genetic differences among host-associated populations of Asphondylia gall midges at the host plant genus level and divergence appears to have occurred sympatrically. Few studies have investigated whether sympatric divergence cascades across multiple trophic levels; the genetic examination of the gall midge of this system suggested HAD in sympatry and, in the case of the parasitoid T. umbilicatus, molecular evidence now suggests that parasitoids exhibit similar patterns of genetic differentiation among and between populations. This study provided both morphological and genetic data to support the hypothesis that host associated sympatric divergence may be occurring in parasitoids at the level of plant genus. Results of this examination may also indicate a tri-trophic cascade of HAD is occurring in this native coastal system.

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