Heterochromatin and sibling species of Simulium praelargum s.l. (Diptera: Simuliidae).

Cytological descriptions are given for two sympatric sibling species, Simulium praelargum IIIL-st and Simulium praelargum IIIL-1.2, from Darjeeling, West Bengal, India. Sibling IIIL-1.2 differs from IIIL-st by a two step fixed included inversion (IIIL-1.2) found on the long arm of chromosome III. Both siblings possess heterochromatinized IIIS polytene chromosome ends, while IIIL-1.2 is unique within the Simuliidae in that it exhibits a very large enhanced chromocentre that persists in normal somatic tissue, contrary to other chromocentre-containing taxa. In IIIL-1.2, the chromocentre occurs as a positively allocyclic heteropycnotic body in normal mitotic interphase cells of neuroblast ganglia, oogonia, and spermatogonia. In mitotic metaphase chromosomes, the chromocentre forms large pronounced primary (centromere) constrictions and appears to be associated with nonhomologous pairing. Initial cytological studies on taxa within the feuerborni group have revealed heterochromatic chromosomal polymorphisms. Progressive fixation of these polymorphisms in different taxa within the group opens up the possibility of using comparative molecular or genomic approaches to begin to define the functional and structural aspects of the epigenome and to further characterize mitosis and meiosis in S. praelargum IIIL-1.2.