Recurrent patterns of chromosome variation in a species group

Abstract

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.

Sex-specific SCAR markers in the dioecious plant Rumex nivalis (Polygonaceae) and implications for the evolution of sex chromosomes

“…Both studies characterised this Y chromosome-specific sequence of 180 bp to be tandemly arranged and highly repetitive. This fits well with the cytological results of Wilby and Parker (1988) on several Rumex taxa, in that Y chromosomes are highly heterochromatic except for a minute terminal euchromatic region corresponding to the pairing segment.…”

“…The straightforward alignment of Y-chromosomal sequences of R. nivalis with those of R. acetosa (despite the high rate of accumulation of mutations) supports the generally accepted hypothesis of a single origin of Y chromosomes and, hence, the heterogametic sex determination in Rumex (Löve 1969;Zuk 1970a, b;Wilby and Parker 1988;Shibata et al 2000). The higher divergence between the sequences of the Y-1 and Y-2 classes in R. nivalis and the existence of two comparatively diverse groups of Y chromosome-specific sequences in R. acetosa (with genetic distances about 40%; Fig.…”

“…2). The accumulation of mutations on Y chromosomes should coincide with Y chromosome degeneration (Charlesworth 2002) which is especially evident in all studied species of Rumex, in which Y chromosomes consist of a massive heterochromatic block except for a minute terminal euchromatic region corresponding to the pairing segment (Wilby and Parker 1988). Hence, our markers most likely represent sections of this tandemly repeated non-coding DNA, that are probably not under selection (Shibata et al 1999;Charlesworth 2002).…”

“…In the section Acetosa, including the well-studied Rumex acetosa, sex is determined by a balance between female determinants located on the X chromosomes and female and male determinants on the autosomes (Kihara and Ono 1923;Löve 1969;Zuk 1970a, b). Y chromosomes affect only male fertility but not sex determination (Wilby andParker 1988). Ruiz Rejón et al (1994) and Shibata et al (1999) independently detected a sequence of 180 bp located on both Y chromosomes of R. acetosa using two fundamentally different molecular methods.…”

Repeated DNA and heterochromatin structure in Rumex acetosa

“…There was no concentration of signal on any members of the chromosome complement. In order to test whether homologous sequences were present in related Rumex species, the seven probes isolated were then applied to three other species of the Rurnex acetosa complex, all of which carry the XX/ XY1Y2 sex determining system (Wilby & Parker, 1988). …”