Chromatid autonomy in Banasa (Hemiptera: Pentatomidae)

Abstract

1. Of the ten species ofBanasa investigated, one has the typical chromosome complement of the TribePentatomini (2n=12+2 sex chromosomes); three have an extra pair of autosomes (2n=14+2 sex chromosomes); and six have doubled the number of autosomes (2n=24+2 sex chromosomes). 2. The autosomes in the 26-chromosome species ofBanasa are distinctly smaller than in either the 14- or the 16-chromosome species, but the size of the sex chromosomes has undergone no such decrease. 3. In five of the species in which the number of autosomes has been doubled, the DNA value remains close to that found in the presumably more primitive species with 14 chromosomes. No DNA data are available for the sixth species. 4. It is concluded that the evidence argues against both polyploidy and an accumulation of random changes in chromosomal structure as primary factors in the evolution of the 26-chromosome species. It is proposed instead that the doubling of the autosome number is due to a separation and individuation of the two chromatids that compose each autosome. 5. The suggestion is made that both chromatid autonomy and polyteny have played important roles in the chromosomal evolution of many other species also. But it is emphasized that only rarely can these two factors be solely responsible for evolutionary changes; fortuitous alterations in chromosome structure as well as true polyploidy (especially in plants) have also been involved in arriving at the conditions as we see them today.