The Simulium arcticum Complex: A Cytogenetic Description of Taxa at Wise River and Reproductive Status and Continuity of Taxa at the Clearwater River (2007-2008)

Abstract

This study had two major objectives: 1) to determine if IIL-73-74 larvae of the 5. arcticum complex at Wise River were a cytotype new to science and to determine the reproductive status of this taxon, and 2) to determine the reproductive status of .S'. arcticum types at the Clearwater River by subjecting the distributions of the autosomal polymorphism, IS-1, to equilibrium analysis. In both studies I used cytogenetic analysis to: compare the banding sequences of polytene chromosomes of salivary glands of larvae within the 5. arcticum compl ex, confidently identified larvae to sibling and cytotype, and scored autosomal polymorphisms such that reproductive status could be determined. For the study at Wise River, analysis of a possible new cytotype, IIL-73-74, was expanded. Although the lack of autosomal polymorphisms present within the samples prevented the analysis of reproductive status at Wise River, I concluded that the IIL-73-74 inversion characterized larvae that were a cytotype new to science. For the study of reproductive status and continuity of taxa, the Clearwater River in Missoula County, Montana was chosen. Previous analyses at the Clearwater River site indicated: 1) that at least two taxa of the S. arcticum, IIL-3 and IIL-22, were abundantly present, and 2) that heterozygosity of the wide-spread autosomal inversion polymorphism, IS-1, was nearly 50% so that reproductive status between the two taxa could be determined. This ongoing study also tests the Geographic Distribution/Taxon Age Continuum hypothesis (Shields, 2006) which suggests that taxa with broad, geographic distributions may be evolutionarily old and therefore are expected to be reproductively isolated in sympatry than presumably younger taxa which have more restricted distributions. The circumstance at the Clearwater River presents an “in between” situation in that a taxon with a broad geographic distribution, IIL-3, is sympatric with one having a very limited distribution, IIL-22. The data suggest that these two taxa are not reproductively isolated. However, since the two taxa: 1) have inversion break points that differ by only one band, and 2) share identical Y chromosome inversions, it is possible that the IIL-22 types are derived from an ancestral IIL-3 type and that the IIL-3 types at the Clearwater River are indeed themselves derived. Finally, when the data from 2008 are compared with those of 2007, a remarkable similarity in types and their frequencies is apparent, which lends credence to the assumption that taxa persist at the same location from year to year. Some of the data from this thesis have been published (Shields et al., 2009a; Shields et al., 2009b).