Genetic Analysis of Culex pipiens Populations in the Central Valley of California

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Abstract
In the Central Valley of California two subspecies of the mosquito Culex pipiens exist—C. p. quinquefasciatus in the warm southern regions and C. p. pipiens in cooler northern regions. The criterion used to distinguish subspecies is a measurement of the male genitalia, the DV/D ratio. The geography of the Central Valley causes a unique temperature pattern which is not a simple south-north gradient. Based on DV/D measurements, the subspecies of C. pipiens track this unique temperature gradient. Three of 10 allozyme loci had electrophoretically detectable alleles which paralleled the morphological cline. One of the three (Pgm) exhibited a cline which reversed at the same point where the temperature and DV/D reverse, i.e., just north of Sacramento. Further analysis revealed that the genes controlling DV/D are independent of Pgm. It seems likely that natural selection directly or indirectly affects Pgm allele frequencies. Some populations along the cline contained males which ranged in their DV/D from C. p. quinquefasciatus to C. p. pipiens with up to 50% intermediate between the usual criterion separating the subspecies. We concluded that these intermediate populations, possibly of hybrid origin, are stable, freely interbreeding units. This last conclusion has important implications for the systematic status of intermediate groups of the C. pipiens complex found elsewhere in the world.