CHROMOSOME POLYMORPHISM IN THE BLACKFLY SIMULIUM VITTATUM (ZETT.)

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Abstract
Natural populations of the blackfly Simulium vittatum (Zett.) show a high degree of inversion polymorphism. Except for seven different two-step overlapping sequences in the short arm of chromosome I, all inversions are simple paracentrics. In all, 134 different arrangements have been found so far. A small inversion (IIIL-1) that is restricted to the Y chromosome is present in some populations from Ontario, New York, and Wisconsin.In the populations studied, there is no sibling speciation. Hardy-Weinberg tests of 226 inversions showed seven inversions that are not in equilibrium. Tests for intra-arm associations revealed that a few inversions in some chromosome arms are carried in the repulsion or coupled position to excess. It was demonstrated that non-random associations exist between certain inversions on different chromosomes. The inversion limits appear in clusters along chromosome arms IS, IL, IIL, and IIIL. The frequencies and distribution of all the inversions are presented in an appendix. The discussion compares these observations with the results from other dipteran inversion studies.