Variability in mating strategies in a widespread cactus in the Chihuahuan Desert

Opuntia is the most diverse and widespread genus in the Cactaceae; several species have broad distribution ranges and have successfully adapted to distinct habitats. One explanation for their success is the diversity of their reproductive systems. Opuntia macrocentra is one of the most common widespread species in North American deserts. A previous work on a northern population documented that this species has a mixed-mating system and receives a low frequency of pollinators. In a southern population of O. macrocentra, we described its floral visitors, anthesis schedules and flower morphometry. Through controlled pollination experiments (selfing, outcrossing, supplementary pollen and natural pollination), we assessed the breeding and mating systems and measured pollen limitation. Eight different bee species visited the flowers. Based on their conduct, Diadasia rinconis and Lasioglossum sp. were the main pollinators. Flowers were homogamous and herkogamous. Floral traits and pollination experiments showed that O. macrocentra is xenogamous and self-incompatible. Fruit set of the natural pollination, outcrossing and supplementary pollen treatments were not statistically different. Seed set was higher in natural pollination and the pollen limitation index for seed set was negative. Based on these results, we conclude that O. macrocentra is not pollen limited in the studied population. These results contrast with those of the previously studied population; therefore, we caution about making generalizations about the reproductive system based solely on studies in one population. The diversity of mating systems displayed by O. macrocentra may explain its broad distribution and adaptation to different environmental factors, one of which may be pollinator availability. Our results highlight the need to describe the reproductive ecology of populations instead of species, because reproductive systems and ecological factors such as plant–animal interactions may differ across their distribution range.