The supernumerary segment systems of Rumex acetosa

Abstract

The dioecious flowering plant Rumex acetosa is polymorphic for heterochromatic supernumerary segments carried on the short arms of chromosomes 1 and 6 in British populations. SS1 is found in the majority of populations but at a very low frequency, averaging only 4 per cent of chromosome 1. There is no obvious pattern to this distribution. SS6 was found in half the populations examined, all from the warmer southern and western parts of Britain. Overall it accounts for 6 per cent of chromosome 6 and averages 12 per cent in polymorphic populations. In heterozygotes SS1 is transmitted through the egg at a frequency of 0·63, a highly significant deviation from expectation. Pollen transmission shows a slight but nonsignificant deficit of SS1 chromosomes. Mendelian expectations were fulfilled for seven crosses involving SS6. One cross, however, showed a highly significant excess transmission through the pollen. It is argued that meiotic drive plays a role in the maintenance of the ubiquitous SS1 low-frequency polymorphism but not the SS6 polymorphism which shows clear environmental correlations.