On the origin of B-chromosomes: neo XY systems and X-like supernumeraries in Orthoptera

Abstract

SUMMARYIn previous diversification mechanisms of B-chromosomes were advanced, and conditions for their origin were summarized (GREEN 1990). In the present work, some examples of B-chromosomes in statu nascendi in grasshoppers with neo XY systems are cited, including two cases described in the present work (Sinipta dalmani and Leptysma sp.). It was earlier proposed that increased tolerance to supernumerary Xs due to dosage compensation could contribute to the common occurrence of X-like B-chromosomes within the Orthoptera. This seems to be more frequent among newly arisen neo XY systems, possibly due to X-nondisjunction in heterozygous or hybrid females. Mechanisms of differentiation of X-like Bs are further exemplified in Lepty- sema argentina and Cylindrotettix obscurus. It is also pointed out that small Bs could originate from polysomy of the smallest chromosomes, tolerated because of the few genes involved (exemplified in Cylindrotettix obscurus). Finally, the minute-B category is attributed to residua...