Breakdown and restoration of chromosome stability following inbreeding in a locust.

Abstract

1. In a sample of 27 males and 15 females from an inbred culture of Pyrgomorpha kraussi four classes of individuals could be distinguished. Those: a) With a standard chromosome complement, 2n = 18 + XX/XY, b) Where the standard complement was supplemented by a single B chromosome, c) Presumably polysomic for structurally altered or unaltered members of the A complement, and d) As in c) but with a B chromosome. 2. Abnormal chromosome behaviour indicative of genotypic unbalance was found in twelve males. 3. Abnormality consists in the occurrence of: a) Polyploid cells, b) Aneuploid cells, c) Chromosome breakage, and d) Unstained regions. The last three anomalies affect only the A chromosomes. 4. There is considerably variation both within and between individuals in respect of these anomalies. But the number of stable males in the B chromosome class is significantly higher than in other classes and all the males with S + B are stable. 5. It is argued that metabolic DNA, and consequently heterochromatic B chromosomes, are important in relation to the mechanical aspects of chromosome behaviour. 6. The B chromosomes are of unknown origin but the extra A chromosomes in aneuploid cells rarely form multivalents and thus one of the conditions of independent evolution is partially fulfilled.