Testing the Cytotype Concept in Black Flies (Diptera: Simuliidae)

Abstract

Paracentric inversions have been linked to maleness in 17 taxa of the Simulium arcticum complex (Diptera: Simuliidae). Linkage to maleness not only defines taxa but also suggests that the sex determining gene or genes lie within the inversion or close to it. Determination of whether an inversion is sex-linked or alternatively is autosomal essentially depends on the analysis of many individuals. This research may thus determine whether linkage is absolute or nearly so. Previous research on nearly 15,000 larvae from 234 sites indicated that inversion linkage to maleness was nearly complete for specific inversions. These include the inversions IIL-3 s. s., IIL-9, IIL-10, IIL-15, IIL-17, IIL-18, IIL-19, IIL-21, IIL-22, IIL-38, IIL-51, IIL-68, IIL-73•74, IIL-79, IIS-12, and IL-3•4. IIL-15 has been observed in 80 male larvae and has not been observed in any females at 12 sites throughout western Montana. The present study tested the linkage of the IIL-15 inversion to maleness on the Y chromosome of larvae in the Simulium arcticum complex found at Prickly Pear Creek, Jefferson County, Montana. This site had already presented a decent sample of the IIL-15 inversion, with 14 male larvae. Inversion variation in the IIL arm was determined for 201 larvae. Four males had the IIL-15 inversion while none of the 80 females had it. Thus, to date, all larvae of the S. arcticum complex having the IIL-15 inversion have been males, suggesting that this rearrangement is Y-linked. The previously described sibling species, S. brevicercum and S. arcticum sensu stricto were observed most commonly in my sample, while eight new inversions were found. Of these, seven were discovered in males and one of them was discovered in a female, suggesting that some of these inversions may be autosomal.