Chromocentre polymorphism in polytene chromosomes of Simulium costatum (Diptera: Simuliidae)
The polytene chromosomes of the black fly species Simulium (Nevermannia) costatum are joined at the centromeres in a strongly heterochromatic chromocentre. Examination of the larval salivary gland chromosomes revealed two populations with a unique polymorphism for attachment to the chromocentre involving all centromeres. All three homologous pairs of chromosomes are polymorphic for centromeres that do not join to the chromocentre. Samples from one of these populations were large enough for thorough study. In this population, the attachment polymorphism is in Hardy-Weinberg equilibrium for two of the centromeres and was in the same frequency for 2 successive years of sampling. The polymorphism could be either primary, retained from an ancestral nonchromocentric state, or secondary, evolving independently or introduced via hybrid introgression. The evolution of chromocentres is discussed in the context of species in the Simulium vernum group.Key words: black fly, polytene chromosome, chromocentre, polymorphism, evolution.