The B-chromosome system of Tettigidea lateralis (Say)

Abstract

Additional samples from a discrete population of the pygmy grasshopper Tettigidea lateralis, at the periphery of the species range, have shown that the frequency of the B-chromosomes has remained stable over a two year period (30–35%), and that there is no significant difference for this metric in the two sexes. The intensity of the preferential movement of the B with the X at male first meiotic division has also remained constant in time and homogenous in different individuals. Hence it is possible that this distortional effect plays a role in the equilibrium frequency of the B in the population. The B's may possess special adaptive properties under ecologically marginal conditions, since in a number of more ‘central’ demes they occur at much lower frequencies (7–9%). — Unique morphological and/or behavioural variants of the standard B were encountered in addition to distinct cases of spontaneous fragmentation of A elements. The meiotic behaviour and chromatic expression of these centric fragments provide evidence on the possible origin and evolution of supernumeraries. A modified Giemsa staining technique has been used to identify regions of centromeric (C-) heterochromatin in mitotic and meiotic chromosomes. The C-banding pattern of the X and the allocyclically similar B is compared. It suggested that the B may have originated from the X by deletion of centromeric heterochromatin. This may have affected the centromere “strength” of the nascent B leading to its preferential movement with the X at anaphase I.