The population structure of Simulium vittatum (Zett.): the IIIL-1 and IS-7 sibling species

Abstract

This paper describes two sibling species in Simulium vittatum (Zett.) on the basis of their salivary gland chromosomes. The IIIL-1 sibling is characterized by a Y chromosome carrying the IIIL-1 inversion; the IS-7 sibling is characterized by the IS-7 X chromosome. The basic banding sequences are identical in the two sibling species. The siblings differ however in the relative frequencies of standard and inverted sequences for a number of autosomal polymorphisms that they share. Standard sequences are predominant in the IIIL-1 sibling and inverted sequences in the IS-7 type. The IIIL-1 sibling is distributed from the Atlantic, south to the Gulf of Mexico and west to the Saskatchewan–Alberta border. The IS-7 sibling ranges from the Atlantic to Alaska, but so far has not been found south of Pennsylvania. The sibling species are sympatric over a wide zone extending through parts of Quebec, Ontario, and northeastern United States via Michigan, Wisconsin, and Minnesota to Saskatchewan and Alberta. It is argued that the two sibling species originated in the region of the present sympatry. Evidence for this conclusion derives from the geographic pattern of sex chromosome and autosome polymorphism in North America and from cytological characteristics of certain extraterritorial populations (Iceland).

Chromosomal comparisons among and within populations of Simulium (Chirostilbia) pertinax (Diptera, Simuliidae)

“…These populations showed similar banding patterns on their polytene chromosomes and could be differentiated only on the basis of inversion polymorphisms (Kiknadze et al 1996). A tendency to monomorphism is often found in isolated populations (Carson 1959, Heed & Russell 1971, Rothfels & Featherston 1981 but S. pertinax populations are not isolated. The use of insecticides during more than two decades may have contributed to the homogeneity of these populations by reducing their chromosomal variability.…”

“…In the present study, analysis of 299 larvae demonstrated chromosomal homosequentiality among six populations from different sites, indicating that S. pertinax is a monotypic species, with little variation in its polytene chromosomes. Chromosomal homosequentiality may also occur among morphospecies (Bedo 1979a, Rothfels & Featherston 1981, Henderson 1986), although this is an exception in Simuliidae (White 1977.…”

Molecular differentiation of two sibling species of the black fly Simulium vittatum (Diptera: Simuliidae) based on random amplified polymorphic DNA

“…Some species are medically and economically important, such as the vectors of filarial worms that cause onchocerciasis. Simulium vittatum, which is found throughout North America as well as in Greenland, Iceland, and the Faroe Islands, is a pest of domesticated and wild large mammals (Rothfels and Featherston 1981).…”

“…Simulium vittatum has been well characterized cytogenetically (Rothfels and Dunbar 1953;Pasternak 1964;Rothfels and Featherston 1981). Pasternak (1964) provided polytene chromosome maps of S. vittatum and an analysis of inversion polymorphisms in natural populations, noting significant mutual exclusion of the IIIL-1 and IS-7 inversions.…”

“…To explain this disequilibrium, he proposed a lethal interaction between the two inversions and argued the existence of a single species. Rothfels and Featherston (1981), however, believed S. vittatum to be composed of two cytospecies (= sibling species), IIIL-1 and IS-7 (named for their diagnostic sex-linked inversions), which arose out of sympatric speciation, probably in northeastern North America. Morphological characters have not been found that can distinguish the two cytospecies.…”