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Chromosomes As Isolating Mechanisms in Trimerotropine Grasshoppers

David D. Shaw, David C. Lightfoot and David B. Weissman
Journal of Orthoptera Research
No. 7, Proceedings: 7th International Meeting, Orthopterists' Society, Cairns, Australia, 1998 (Dec., 1998), pp. 157-163
Published by: Orthopterists' Society
DOI: 10.2307/3503513
Stable URL: http://www.jstor.org/stable/3503513
Page Count: 7
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Chromosomes As Isolating Mechanisms in Trimerotropine Grasshoppers
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Abstract

Trimerotropine grasshoppers are characterized by high levels of chromosome variation - both within and between species - that has prompted a subdivision of the genus into 4 cytologically distinct groups. It has been suggested that some of the chromosomal variants may have played a direct role during species formation because of their negative impact upon meiosis, fertility, and viability. Consequently, such rearrangements have the potential to act as efficient isolating mechanisms that can initiate divergent evolution and species formation by severely reducing the fitness of any hybrids. We have analyzed an area of overlap between Trimerotropis cyaneipennis Bruner and T. fratercula McNeill in New Mexico. Here, the blue/yellow wing and hind femora color differences used to separate the species in other areas are no longer diagnostic. However, the two species do differ in chromosome number due to a chromosome fusion difference involving autosomes 4 and 5. We identified 7 individuals from 3 localities that are heterozygous for the fusion and analyzed both the behavior of their chromosomes during meiosis and the production of aneuploid gametes after spermatogenesis. A comparison of these data, obtained from hybrids between two closely related species with previous analyses involving more distantly related hybrids but involving the same fusion, has allowed us to make an assessment of the role of this chromosomal rearrangement in inhibiting gene flow between taxa that still hybridize in nature. The analysis clearly indicates that, like the other chromosomes within the hybrid genome, the behavior of the chromosome rearrangement depends to a large extent upon the relatedness of the two parents. Consequently, we conclude that the chromosome fusion that distinguishes these two species has played no direct role as an isolating mechanism during the speciation process.

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