Genetic structure in aquatic bladderworts: clonal propagation and hybrid perpetuation

The free-floating aquatic bladderwort Utricularia australis f. australis is a sterile F(1) hybrid of U. australis f. tenuicaulis and U. macrorhiza. However, co-existence of the hybrids and parental species has not been observed. In the present study, the following questions are addressed. (a) Does the capacity of the two parental species to reproduce sexually contribute to higher genotypic diversity than that of sterile F(1) hybrid? (b) Are there any populations where two parental species and their hybrid co-exist? (c) If not, where and how do hybrids originate?

The presence and absence of Utricularia was thoroughly investigated in two regions in Japan. An amplified fragment length polymorphism (AFLP) analysis was conducted for 397 individuals collected from all populations (33 in total) where Utricularia was observed.

The mean number of genotypes per population (G) and genotypic diversity (D) were extremely low irrespective of the capacity to reproduce sexually: G was 1.1-1.2 and D was 0.02-0.04. The hybrid rarely co-existed with either parental species, and the co-existence of two parental species was not observed. Several AFLP bands observed in the hybrid are absent in both parental genotypes, and parent and hybrid genotypes in the same region do not show greater genetic similarity than those in distant regions.

The capacity to reproduce sexually in parental species plays no role in increasing genotypic diversity within populations. The observed genotypes of the hybrid could not have originated from hybridization between the extant parental genotypes within the study regions. Considering the distribution ranges of three investigated taxa, it is clear that the hybrid originated in the past, and hybrid populations have been maintained exclusively by clonal propagation, which may be ensured by both hybrid vigor and long-distance dispersal of clonal offspring.