The chromosome characteristics of five closely-related species in Rumex subgenus Acetosa are considered. All are dioecious diploids with 2n = 12+XX in females and 2n = 12+XY1Y2 in males. The autosomes of all species are very similar, all highly acrocentric and displaying apparent homoeology, and only two chromosome changes have occurred in the evolution of the group -shift of an NOR and deletion of part of the long arm of chromosome 6. The heterochromatic V-chromosomes are more variable and only two species cannot be identified by examination of the Vs alone. Hypervariability in centromere location is characteristic of Vs in the whole subgenus although there is no quantitative variation in heterochromatin. On the autosomes, parallel polymorphisms for heterochromatic supernumerary segments on the short arms of pairs 1, 5 and 6 are found in 4 of the 5 species, and euchromatic B-chromosomes are common in three species. These recurrent patterns of chromosome variation emphasise the close relationships within the species group.
The dioecious angiosperm Rumex acetosa has an XXIXY1Y2 sex-chromosome system. Each V-chromosome is heterochromatic except for a minute terminal euchromatic pairing segment. The Vs are constant in size but have a variable centromere position. The centromeres can be located anywhere within the central 40 per cent of the chromosome but are excluded from the two distal 30 per cent regions. In a sample of 270 males from 18 different populations 68 distinct variants have been identified on the basis of V-morphology. All populations are highly polymorphic with a minimum of four variants in a sample of ten males. The origin and significance of this massive variability is considered in this paper. Increased mutation rate of the Ys may be implicated in maintenance of this variation.
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.
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