Herkogamy in the Plains Pricklypear Cactus Opuntia polyacantha Haw. (Cactaceae: Opuntioideae)

The spatial separation of anthers and stigmas, also known as herkogamy, is a widespread reproductive strategy in flowering plants. Herkogamy has evolved alongside, or in lieu of, self-incompatibility systems, and functions to limit the rate of self-pollination and autogamy. The prevalence of herkogamy in the cactus family (Cactaceae) is a matter of debate, with no known examples in the subfamily Opuntioideae. In this study, I investigated whether the plains pricklypear cactus Opuntia polyacantha, a member of this subfamily, exhibits herkogamy, and whether it is part of a previously described syndrome. I collected data for 23 floral morphometric traits from 142 hermaphroditic flowers, sampled near Saskatoon, Saskatchewan. I quantified herkogamy in two ways: as a conventional, unidimensional variable (basal herkogamy) and as a novel, two-dimensional variable (triangular herkogamy). Secondary data were collected to document pollen morphology, self-pollination rates, ploidy levels and floral visitor diversity and behaviours. Morphometric analyses showed that triangular herkogamy is more representative than basal herkogamy in O. polyacantha, revealing a continuously variable herkogamous syndrome. Regressions, ANOVAs and principal component analysis revealed few trends between herkogamy and the 23 morphometric traits, demonstrating that the syndrome lacks distinctive floral morphs. The secondary data concurred with the literature, showing uniform pollen morphology, moderate rates of self-incompatibility, variable ploidy levels, and typical floral visitor assemblages. I conclude that O. polyacantha exhibits an unordered, non-categorical form of herkogamy, which has not previously been described, and that a two-dimensional quantification can enhance the study of this floral syndrome in other cacti