Chromosomal polymorphism in Nucella lapillus

Abstract

A technique for the examination of chromosomes in testicular material in Nucella lapillus is described. The diploid chromosome number varies between 26 and 36. Eight pairs of meatcentric chromosomes are always present and range in size from 9mu to 4mu. Additionally, 10 pairs of acrocentric chromosomes (4mu to 1mu) are present in the 2n equal 36 form and correspond to five pairs of metacentric chromosomes in the 2n equal 26 form. Trivalents occur in individuals heterozygous for chromosome arrangements; this is consistent with the interpretation that the polymorphism is Robertsonian in nature. The southern British distribution of the chromosome polymorphism in Nucella is described. The polymorphism occurs regularly only in bay's in the south-west; elsewhere the populations are monomorphic 2n equal 26. Two areas where the polymorphism occurs have been examined in detail. In each of these the frequency of acrocentrics is negatively associated with increasing wave force, but there is some variation suggesting that other factors relating to water movement (tidal currents and range) are important also. It is pointed out that the overall distribution of the polymorphism can be explained on the assumption that a high chromosome frequency is favoured where total water movement is least.