A cytological study of natural hybrids between Prosimulium multidentatum and P. magnum

In a limited area of sympatry in New York State, hybridization occurs between Prosimulium multidentatum (Twinn) and “forms” 2 and 3 of P. magnum Dyar and Shannon. Salivary gland chromosomes of the interspecific hybrids are the composite of those of the parental species with respect to fixed inversion differences, sex chromosomes and chromocenter attachment of centromeres. Backcross hybrids with P. multidentatum occur in the same area. The preferred direction of cross (P. multidentatum ♀ x P. magnum ♂) is rationalized by the earlier emergence of P. multidentatum, and the earlier emergence of males and greater longevity of females of both species. Chromosome pairing, both in F1 and backcross, is loose in hybrid segments and tight in species homozygous parts. Likewise chiasma frequency is low in hybrid as compared to parentally homozygous chromosomes. In the face of locally recurrent hybridization species integrity is maintained by meiotic irregularities in the hybrids and especially by disturbances in the sex chromosome balance of backcross hybrids, the sex chromosomes of P. multidentatum and P. magnum being non-homologous.