Partner limitation and restoration of sexual reproduction in the clonal dwarf shrubLinnaea borealis L. (Caprifoliaceae)

Summary

Reports suggest that there is widespread reproductive failure inLinnaea borealis in Britain, Scandinavia, and North America. Our investigations of Scottish populations of this clonal dwarf shrub indicate that, although visited by a number of different insects, pollen transfer in this species is highly effective and principally occurs by small flies (Muscidae). However, natural levels of fruit set varied between populations (from 0% to 25.1%) and reproductive failure was most severe in populations which were composed of single clones. Microscopic examination of stigmas showed that there is no barrier to pollen flow since at least 85% of stigmas have sufficient germinating pollen to effect seed set. Fluorescence microscopy of germinating pollen grains indicates high levels of pollen rejection in the style and only a small proportion of the pollen tubes were able to reach the ovary. It was concluded that lack of xenogamous pollination limits fruit formation in populations ofL. borealis. Reproductive success in an isolated population with extreme reproductive failure was restored by experimental field pollination with viable pollen imported from plants from another population. In Scotland,L. borealis occurs in small, isolated populations and restoration of reproduction can be achieved by the reintroduction of compatible mating partners. This is an important consideration for the conservation management of isolated populations since their long-term recovery may only be possible by translocation of different genotypes from elsewhere into the population. Habitat fragmentation in any part of the species range may impose a potential reproductive bottleneck by causing loss of population diversity and this could explain the low levels of seed set recorded for this species in other parts of its range.