Restricted seed contribution and clonal dominance in a free-floating aquatic plant Utricularia australis R. Br. in southwestern Japan

Utricularia australis R. Br. is an aquatic angiosperm species common in natural and irrigation ponds in temperate regions. This species reproduces both sexually and vegetatively, but in southwestern Japan the occurrence of male-sterile populations, in which plants produce no pollen and propagate only vegetatively, has been recorded. We studied the reproductive contribution of seeds in normal pollen-producing populations using isozyme analyses, a pollination experiment under culture and field observations. Although seedlings obtained from controlled mating indicated segregation of isozyme, polymorphism of the isozyme genotype was detected mainly among populations, but rarely within each pond population. This suggested clonal dominance and rarity of seed or seedling survival in natural populations. In the pollination experiment, the mean seed set ratio in cross-pollination between plants of the same isozyme genotype (7.6%) did not differ significantly from self-pollination (7.6%), but was lower than cross-pollination between plants of different genotypes (45.7%). The low ratio in crossing between the same genotype plants was ascribed to the clonality of the parents. In general, these results corresponded with the low ratios in seed setting observed in natural populations (7.9-13.7%). All the male-sterile populations we surveyed showed the same genotype, thus male sterility in the study area was considered to have the same origin.