A cytological study of species in the Eusimulium aureum group (Diptera: Simuliidae)

Abstract

Earlier studies by R. W. Dunbar of the monophyletic (n = 2) Eusimulium aureum group were extended. Additional samples of his forms "A" and "B" (temperate North America), "C" and "D" (northern North America), "E" and "F" (Britain, northern Europe, Leningrad), and "G" (California to Guatemala) were analyzed. Two of Dunbar's forms, "I" (Gibraltar, Britain) and "J" (Mediterranean, Madeira, Azores, Leningrad), have been redefined. "K" (Gibraltar) and "L" and "M" (Canary Islands) are new forms. These are described in terms of fixed inversion differences, sex differential segments, and chromosomal polymorphisms. In a cytophylogeny, the Old World "J" forms the link to the New World "C." A preliminary polytene banding comparison of E. aureum and its putative n = 3 relatives reveals that the Old World forms are ancestral to those of the New World. All palaearctic forms have chiasmate meiosis in males, while all nearctic ones, with the remarkable exception of "G," are male achiasmate. Since male achiasmate meiosis is presumably a prerequisite for the reduction in chromosome number, the male-chiasmate condition of Old World forms is assumed to be due to secondary reversion, a postulate that is supported by the derivative male-chiasmate condition of "G."