Chromosomal polymorphisms in the pitcher-plant mosquito, Wyeomyia smithii

Abstract

A local population of the pitcher-plant mosquito, Wyeomyia smithii (Diptera: Culicidae, Culicini) in western New York State contains naturally polymorphic salivary gland chromosomes. Maps depicting the proposed standard sequence of bands along each of the three chromosomes are presented. Structural conformations of heterokaryotypes reveal ten paracentric inversions. Four inversions occur in chromosome 1, and three in each of chromosomes 2 and 3. All the inversions are small, occupying 3–17% of the length of their respective chromosomes, and all are located in the terminal 1/5 to 1/2 of a chromosome arm. Three inversions are almost adjacent on chromosome 1. On chromosome 2 one inversion lies completely within another. Three inversions on chromosome 3 partially overlap. The remaining two inversions occur alone, one on chromosome 1 and the other on chromosome 2. Another polymorphism, with no visible alteration of band sequence, produces asynapsis of all but the tip of the left arm of chromosome 1, and shows a significant change in frequency between two successive years. No inversion exhibits a significant seasonal change in frequency, though some fluctuation in frequency was noted in two cases. The rarest inversion in the population exists at a frequency below 0.01, and the two most common inversions at frequencies of 0.44 and 0.52. The average inversion heterozygosity of an individual is between 1.7 and 2.4 based on two different estimation procedures. Heterokaryotypes for several of the inversions were significantly more frequent than they should have been, given binomial expectations. Cytogenetic analysis of W. smithii was deemed particularly desirable because this species has been and continues to be the subject of extensive biometrical-genetic, ecological, and evolutionary investigation.