XVI International Botanical Congess
Although theory predicts an equal sex ratio, most Rumex acetosa populations are female biased. The causes of female excess were investigated on two adjacent but contrasting Icelandic islands. On Lundey island, where R. acetosa is dominant, the ratio was skewed twice. Seed populations were slightly (1.6:1) but adult populations highly female biased (3.9:1). Progeny sex ratios of individual inflorescences were not correlated with degree of seed abortion as expected if the bias is postzygotically induced. Seed set was not pollen limited and the female biased primary ratio is consistent with certation being the principal cause of skewness at the extremely high pollen densities prevailing on Lundey. On Therney island, where R. acetosa is a minor vegetation component, the 1.1:1 primary ratio did not deviate significantly from that predicted. There, a 4 fold difference in survivorship between males and females is needed to explain the extreme female bias (19:1) of the adult population. Six hypotheses, that can explain the extreme female bias, have been tested and will be discussed.