A UNIQUE CHROMOSOMAL DIMORPHISM IN SPECIES-A AND SPECIES-B OF THE ANOPHELES-QUADRIMACULATUS COMPLEX

An unusual example of naturally occurring overdominance associated with dimorphic forms (3L1 and 3L2) of the left arm of chromosome 3 was documented for species A and B of the Anopheles quadrimaculatus species complex. Polytene chromosomes from ovarian nurse cells were examined from mosquitoes of 20 populations, mostly from the southeastern United States but including one population from New York State. In every population surveyed, the heterokaryotype (3L1/3L2) was the predominant type at an unusually high frequency that implied selection for the hybrid condition. The heterokaryotype (3L1/3L2) was present at average frequencies of 72.3% +/- 1.16% for species A and of 73.3% +/- 1.15% for species B. The homokaryotypes, 3L1/3L1 and 3L2/3L2, were present at frequencies of 14.5% +/- 0.91% and 13.2% +/- 0.88%, respectively, in species A, and of 12.6% +/- 0.86% and 14.2% +/- 0.90%, respectively, in species B. Chi-square analyses showed that the relative frequencies of the three chromosome types were uniform across the various populations, thus indicating no significant geographical (clinal) effects for either species A or B. In the one population for which sufficient samples of species B were available (Octahatchee Lake, Florida), there were no temporal effects, and the frequencies of each type remained stable throughout the breeding season over a 2-year period.