Triploidy causes sexual infertility in 'Cyrtanthus breviflorus' (Amaryllidaceae)

The balance between sexual and asexual reproduction can vary markedly in clonal plants. At one extreme, plants are sexually infertile and reproduction is solely clonal. Infertility can be caused by environmental and/or genetic factors, but the role of each is often unknown. Here we determine variation in sexual reproduction and explore the underlying factors causing sexual infertility in 'Cyrtanthus breviflorus' Harv. We examined open- and cross-pollinated fruit set, ploidy using flow cytometry, pollen viability, pollinator visits to flowers and pollen deposition onto stigmas. One population was sexually infertile; no plants produced fruit. Three populations were sexually fertile; >98% of plants produced fruit. Percent pollen viability differed between infertile (18%) and fertile (97%) populations. The most likely cause of infertility was unequal ploidy. Plants in the infertile population were triploid, whereas those in fertile populations were diploid. Pollination factors were not related to infertility. In infertile and fertile populations, pollen-collecting insects visited flowers frequently, depositing 4-fold more pollen grains onto stigmas than the number of ovules per flower. Our study is the first to demonstrate infertility and triploidy in 'C. breviflorus'. How triploidy became established despite high levels of pollinator activity remains a challenging question.