The Population Genetics of the Colonization of the Western United States by the Apple Maggot, Rhagoletis Pomonella (Walsh) (Diptera: Tephritidae)
McPheron, Bruce Alan
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https://hdl.handle.net/2142/71104
Description
Title
The Population Genetics of the Colonization of the Western United States by the Apple Maggot, Rhagoletis Pomonella (Walsh) (Diptera: Tephritidae)
Author(s)
McPheron, Bruce Alan
Issue Date
1987
Doctoral Committee Chair(s)
Berlocher, Stewart H.
Department of Study
Entomology
Discipline
Entomology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Entomology
Abstract
The apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae), is native to eastern North America, but has recently been reported in various parts of the western United States. Larvae of the species infest a variety of fruits, but three hosts are of special interest. Hawthorns (Crataegus) are the native host, and the introduced apple and sour cherry are also attacked by the fly. This study defines levels of genetic variability, as assayed by starch gel electrophoresis, throughout the native range of the insect on different hosts and compares these levels to those exhibited by populations in the western United States.
Flies reared from individual hawthorn trees on a microgeographic scale do not represent a homogeneous population. There are a number of significant differences in allele frequency among flies from trees easily close enough for dispersal to be effective. The patterns of differentiation do not appear to be explained by genetic drift. Rather, variable selection is likely to be the cause of the heterogeneity.
The apple maggot has been proposed as an organism likely to fulfill the requirements to undergo sympatric speciation via host race formation. Fly populations infest sympatrically-growing hawthorns, apples, and sour cherries, each of which is available to the fly at a different time of year. However, direct evidence for any genetic differentiation among fly populations from these different host plants has been lacking. Apple maggots reared from sympatric hawthorn and apple trees do display consistent differences in allele frequencies at five sites in Urbana, Illinois.
Flies from western populations exhibit reduced levels of genetic variability compared to flies from the eastern (native) range. This reduction of variability is more pronounced in Utah and Colorado than in the Pacific Northwest. Also, populations in Utah-Colorado are much more different from one another than are populations across the eastern United States. Apple maggots in the Pacific Northwest appear to be hybridizing with a sibling species, R. zephyria, which is native to that region. These genetic observations are all consistent with the predicted effects of one or more colonization events from the eastern to the western United States.
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