Hybridization betweenPyrola grandifloraandPyrola minorin West Greenland: a tension zone maintained by clonality

Abstract

Interspecific hybridization may lead to novel hybrid lineages and transfer of genetic material, possibly enabling species to expand into new habitats. Alternatively, hybrids may be inviable or sterile, constraining movement of parental species and genes into each others range. Pyrola minor (L.) and Pyrola grandiflora (Rad.) hybridize on Disko Island in Greenland. A recent study found most hybrids to be F1plants produced by P. minor mothers. Here, we test if later generation hybrids are indeed absent in a larger sample, using morphological and molecular markers. We further test if unidirectional hybridization is due to reproductive barriers. Intermediate plants were all F1hybrids from P. minor mother plants. Our analyses indicate that hybrids originated from few hybridization events and have been maintained by clonal reproduction since then. Barriers to pollen flow between the species result from a combination of few pollinators, different flower morphology and phenology, high proportions of abnormal pollen in hybrids, and reduced pollen tube growth of heterospecific and hybrid pollen. In general, pollen from P. grandiflora grew better than pollen from P. minor, which may explain the unidirectional hybridization. Hybridization between P. grandiflora and P. minor therefore seems to operate like a tension zone in which F1hybrids are maintained by clonal propagation.