Population structure of hypervariable Y-chromosomes in Rumex acetosa

Abstract

Males of the dioecious angiosperm Rumex acetosa have a single metacentric X and two heterochromatic V-chromosomes. The Vs are constant in size but hypervariable in morphology with centromeres positioned anywhere within the central 40 per cent of the chromosome. 280 males from 28 populations contained 129 variants determined by feulgen staining alone, and samples of 10 seed-grown males from single populations contain between 4 and 8 variants. Similar levels of variation characterise mature males, with variants occurring once or only a few times in a population. Each population has a distinctive spectrum of V-variants even when 40 males are examined, and closely adjacent populations are as dissimilar as distant populations. Small, isolated populations carry as many V-variants as large, continuous populations although they are homozygous for autosomal polymorphisms. In addition to centric shifts, populations usually have pairs of variants differing only in pairing segment location, suggesting that telomere switching may regularly occur. It is argued that this pattern of variation results from neutrality of the variants coupled with an extremely high rate of repositioning of the primary constriction.

Recurrent patterns of chromosome variation in a species group

“…The arm-ratios of Yl and Y2 vary from 061+ to 063-and 060+ to 065-respectively, in the notation of Wilby and Parker (1986). Centric disposition, therefore, appears to be continuous with no evidence of preferred sites.…”

“…Centric disposition, therefore, appears to be continuous with no evidence of preferred sites. As with R. acetosa, Yl variants tend to be less extreme than Y2 and there are approximately equal numbers of "+" and variants (Wilby and Parker, 1986).…”

“…The Y-chromosomes in R. acetosa are hypervariable with respect to centromere location (Wilby and Parker, 1986;1987). The centromeres can be located anywhere within the central 40 per cent of the chromosome without preferred locations.…”

“…At another level, however, the Ys are hypervariable. The centromere of each Y can be located anywhere within the central 40 per cent of the chromosome and there are no preferred sites (Wilby and Parker, 1986;1987). Centromere position, therefore, behaves as a continuous variable and, on average, every other male in natural populations is distinct on the basis of its Y-composition.…”

Mendelian and non-Mendelian inheritance of newly-arisen chromosome rearrangements

“…He points out that massive numbers of offspring would have to be scored to detect small amounts of drive although these could still be significant in leading to fixation. heterozygotes was about 063 rather than the expected 05 (Wilby and Parker, 1987 Most crosses were between the heterozygote and a standard plant, a backcross. Behaviour of two rearrangements was also examined in crosses involving individuals of the backcross generation.…”