Cytogenetics of a Local Population of the Screwworm, Cochliomyia hominivorax,1 from Northeastern Mexico

Abstract

Larvae of the primary screwworm, Cochliomyia hominivorax (Coquerel), were collected near Aldama in Tamaulipas, Mexico. 3rd-instar, crawl-off larvae from 19 single-egg masses were obtained from cattle hosts, and from 8 other single-egg masses off sheep. Of the six chromosome pairs in this species, the sex, II, V, and VI pairs were polymorphic. The small X and Y sex chromosomes had five and four variants, respectively. F₁ and F₂ larval samples from cattle and sheep differed markedly. There were more fluorescently spotted X chromosomes in cattle screwworms than in sheep, and chromosome VI was polymorphic in cattle screwworms but monomorphic in sheep. Both cattle and sheep material showed significant heterozygote deficiency from expected values and a striking pattern of X chromosome-based homotypic mating.