Naturally occurring triploid birch hybrids from woodlands in Iceland are partially fertile

Woodlands in Iceland are characterized by shrub-like birch, a result of introgressive hybridization between diploid dwarf birch, Betula nana, and tetraploid downy birch, B. pubescens. Introgression occurs via triploid hybrids. We evaluated the fertility of triploid birch by examining pollen viability and seed germination. Pollen samples were collected from woodlands throughout Iceland from 99 plants, including 22 triploid hybrids. The germination of pollen from the triploid hybrids was about one third that of the diploid or tetraploid species (mean values: 9% vs. 26% and 29%). Most triploid plants had a pollen germination of less than 2%. However, some showed exceptionally high pollen fertility (six plants: 11–79%). Microscopic examination revealed normal pollen tube growth and elongation in hybrids with low germination, while other hybrids showed abnormal tube growth. Fluorescence microscopy indicated pollen viability in all ploidy groups, with green autofluorescence from the cytoplasmic contents and red fluorescence from the exine wall. The Aniline test of pollen viability showed that grains with three pores (normal, triporate grains) stained positively, but non-triporate grains were unstained and empty. Germination tests were performed on seeds collected from 246 plants, including 21 triploid hybrids. The germination percentage of seeds from triploid hybrid plants was about 20 times lower than that of the diploid or the tetraploid species (population means: 0–4% vs. 3–41% and 6–54%). The present study shows that triploid birch hybrids are not sterile. Both the paternal and maternal fertility of the hybrids should be sufficient to facilitate gene flow via backcrossing with the parental species.