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

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).