Cytogenetics of Simulium arcticum in Lewis and Clark and Powell Counties, Montana: A Test of the IIL-3 “montane” / IIL-7 “prairie” Hypothesis

Abstract

Conventional methods of collection, morphological classification, preparation ofpolytene chromosomes, and analysis were used to study the cytogenetics of the black fly, Simulium arcticum, at Canyon Creek (CC) and at the Little Blackfoot River (LBR). These sites were chosen for convenience and to represent eastern and western flowing streams, respectively, so that the “IIL-3- montane/IIL-7 prairie” hypothesis of Shields could be tested. Additionally, CC was studied for two summers to test the hypothesis that cytogenetic diversity would be similar from year to year and the LBR was studied at two locations to test the hypothesis that individual drainages have the same cytogenetic diversity in a single year. IIL-3 S. arcticum was the predominant sibling in both streams; the IIL-7 sibling of S. arcticum was not found in this study. Cytogenetic diversity at CC was similar for both years, except that Prosimulium onychodactylum appeared to be absent in 2003. Cytogenetic diversity at two sites ofthe LBR was also similar except that S. decorum, a rare species in Montana, was found in very low numbers at the downstream site. A new cytotype ofS. arcticum, IIL-18, was found in low numbers at CC both years, while it was found in relatively high frequency at the LBR in 2003. “Sex-exceptional” types (males having undifferentiated sex chromosomes) of S. arcticum were found in high (46.8%) frequency at the LBR; additional detailed analysis ofthese types is warranted.