The Salivary Gland Chromosomes of Six Species in the Iiis-1 Group of Prosimulium Roub. (Diptera: Simuliidae)

The morphology of the salivary gland chromosomes of six North American species of the genus Prosimulium was studied. Five of these species, P. caudatum Shewell, P. dicum Dyar and Shannon, P. "Y", P. magnum D. and S., and P. multidentatum Twinn, share inversion IIIS-1; P. "X" has standard IIIS, but is related to P. caudatum through inversion IIS-10.From an analysis of interspecific rearrangements and intraspecific chromosomal polymorphism, a phylogeny was constructed for this IIIS-1 group of species. The more extensive population data for P. magnum revealed an incipient split of this species into two types of populations, differing in details of the sex chromosomes, and in distribution. Hybrid populations are rare. Sex chromosome polytypism was also observed in P. dicum, P. "Y", and P. multidentatum.