Abstract
Chironomus stigmaterus Say (Diptera : Chironomidae) exhibits genetic divergence between a chromosomally polymorphic form in Texas and New Mexico and a virtually monomorphic form restricted to California. Twenty-eight rearrangements have been recorded in the species but only eight were observed in California while 21 occurred in Texas and New Mexico. The majority of these rearrangements were found only in single heterozygotes but 10 inversions formed polymorphisms sufficiently stable to become established in populations throughout Texas and New Mexico. Inversion frequencies were high in populations of western Texas and eastern New Mexico but became lower in peripheral populations, suggesting a clinal reduction to monomorphism in California. However, no populations were found in Arizona to prove distribution continuity. When these data are considered with those of a companion electrophoretic study (Hilburn, 1979), it appears the observed dichotomy in rearrangement heterozygosities is a manifestation of divergence in the total genetic compositions of the two taxa. The data support the hypothesis that, in the southwestern United States, C. stigmaterus is represented by two subspecies having different levels of chromosomal heterozygosity, different enzyme alleles, and, possibly, partial reproductive isolation.
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