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Philip Barnes

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This study involves an analysis of the genetic changes that have occurred in mating preference between a DDT-resistant population (91R) and its unselected, wild type control population (91C) in Drosophila melanogaster. Although the resistant strain and its control were both founded from the same natural population (Merrell and Underhill, 1956), significant behavioral isolation has been detected between these two populations (Boake et al., 2003; Samboy, 2008). The 91R is now highly resistant to DDT, and it has remained separate from the 91C (control) strain for the last 61 years in the lab.

Three hypotheses can be proposed for how genetic change could have occurred to cause the observed behavioral isolation between these two strains: 1) the behavioral and genetic changes occurred in both strains simultaneously, 2) the behavioral and genetic changes occurred in just the 91C strain, and 3) the behavioral and genetic changes occurred in just the 91R strain. To test these hypotheses, both the 91C and 91R strains were compared to two laboratory wild type “tester” strains (4-88 and ORC) by multiple-choice, simultaneous mass-matings. Significant differences from random mating in the form of within-group preferences demonstrate significant partial behavioral isolation between two strains, while random mating indicates no significant behavioral differences.

The results of the study showed no behavioral difference between the wild type ORC strain and either the 91C or 91R strains. When 91C and 91R were compared to the wild type 4-88 stock, however, the 91C strain showed random mating, while the 91R strain showed significant behavioral isolation. These results suggest that changes leading to the behavioral isolation between 91C and 91R may have occurred primarily in the 91R strain.

Differences in results depending on which tester stock was used (ORC vs. 4-88) can be explained by natural variation in wild type stocks of D. melanogaster (Nanda et al., 2011). The ORC population has been in the lab longer than 4-88 and may have been subjected to more inbreeding and random genetic drift as a result of small population sizes as well as population crashes (bottlenecks). Also, the less variable environmental conditions in the lab may have resulted in natural selection favoring a “laboratory phenotype” more strongly in ORC, because of its longer time in the lab, than in 4-88. Because the 4-88 population was more recently introduced to the lab, it may still retain more natural genetic variation. These differences may also contribute to some genetic and behavioral differences between ORC and 4-88.

Genetic and behavioral changes primarily in the 91R strain, in contrast to the 91C strain, make evolutionary sense, since evolving resistance to DDT probably involves very strong selective pressures. These strong selective pressures may also have effects on other genes via linkage, related metabolic pathways, pleiotropy, and/or epistasis. Tests of 91R and 91C against additional wild type tester stocks may help to examine the validity of this conclusion.



The views expressed in this paper are solely those of the author.