Rumex acetosa (Polygonaceae) is a perennial dioecious weed and one of the few plant species which possess well-differentiated sex chromosomes (2n=12+XX in females and 2n=12+XY1Y2 in males). Such a chromosomal sex determination system constrains the average primary sex ratios to a 1:1 ratio. Yet, the operational sex ratios (the number of males per female at sexual maturity) may become biased due to differential mortality, longevity or vegetative vigor between the sexes. Although time-consuming cytological analyses would allow studies on sex ratios among seedlings in dioecious species that have distinguishable sex chromosomes, investigations on sex ratios have mostly relied on information obtained at sexual maturity. A genetic method has now been applied to resolve gender among nonflowering plants of R. acetosa. The method involves amplification of a Y chromosome specific DNA sequence by PCR. To detect the mechanisms responsible for the observed sex ratio variation, a combination of methods is used to examine the patterns of seasonal sex ratio variation observed in populations of R. acetosa living in different habitats.

Key words: Polygonaceae, Rumex acetosa, sex ratio variation, sex specific DNA marker