The B chromosomes of Pycnogaster cucullata (Charp.) and their effects on the A complement

Abstract

Pycnogaster cucullata (Charp.) is a polytypic species of Tettigonioidea having XO and XY chromosomal races. In all populations of these races examined, several individuals were found carrying B chromosomes that may be of two morphological types, namely St and d. Although during diplotene the Bs appear almost completely condensed (positively heteropycnotic), and meiotic and mitotic metaphases only two grey intercalary C-bands can be shown. The utilisation of a silver impregnation method allowed us to distinguish the B-chromatin from the facultative heterochromatin of the neo X chromosomes, because both types of heterochromatin present a different condensation cycle during meiotic prophase. The presence of two Bs in nine individuals from the PEG population can be correlated with the simultaneous occurrence of several types of abnormalities affecting the A-chromosomes: spontaneous breakage, depression in the amount of effective pairing, germ line polysomy, endopolyploidy and micronuclei. These effects have also been observed in the individuals of PEG and another population carrying only one B but at a low frequency. While germ line polysomy may be essentially related to the presence and number of Bs (additive effect), determinant factors present in the genetic background cannot be excluded as contributing to the control of effective pairing.