Californian hybrid zone between Culex pipiens pipiens and Cx. p. quinquefasciatus revisited (Diptera: Culicidae)

Phallosome morphology (DV/D ratio) and allozyme variation were used to reexamine the transition from Culex pipiens pipiens L. to Cx. p. quinquefasciatus Say, detected in California from the northern Central Valley to the Mexican border of the United States of America. Significant deficiency of heterozygotes was observed at the diagnostic locus Mdhp-2 in populations from the central part of the hybrid zone. Long tails of introgression were detected: populations from both north and south ends of the transect were not genetically pure Cx. p. pipiens or Cx. p. quinquefasciatus, respectively, as previously considered, but included approximately 5% introgressed individuals. A narrow reversed cline from the Delta area into the Sacramento Valley, characterized by increasing frequencies of Cx. p. quinquefasciatus alleles proceeding to the north, was confirmed. Both these clines appear to be related mainly to temperature gradients. Over the last 50 yr, an increase in the proportion of Cx. p. pipiens DV/D phenotypes was detected proceeding north to south along the main latitudinal cline, as well as in the narrow reversed cline. Accordingly, the center of the main latitudinal hybrid zone has apparently moved approximately 100 km to the south. This phenomenon is only partially paralleled by the differentiated locus Pgm of the 3 for which comparison was possible. Similarities to and differences from previous studies are discussed, also in relation with comparable data from another hybrid zone between Cx. p. pipiens and Cx. p. quinquefasciatus recently detected in Madagascar. Hybrid index scores based on differentiated allozymes and the diagnostic locus Mdhp-2 prove to be better descriptors than the DV/D ratio of hybridization and introgression occurring between Cx. p. pipiens and Cx. p. quinquefasciatus. This seems to be caused mainly by the influence of temperature on male genitalia development, and the weaker association found between genetic markers and DV/D phenotypes in hybrid populations.