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 Engelm is one of the most common widespread species in North American deserts. 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. The fruit set of the natural pollination, outcrossing, and supplementary pollen treatments were not statistically different. The seed set was higher in natural pollination and the pollen limitation index for the 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.

Floral visitors: We selected 32 individuals of O. macrocentra to study their floral visitors. During five consecutive days within the flowering peak, we checked 32 plants from anthesis to flower closure, covering three periods of the day, morning (09:00–11:00 h), mid-day (12:00–15:00 h), and late afternoon (18:00–20:00 h) (n=279 observations in 22 h). 

Pollen Ovule Ratio: We collected and fixed in 70% alcohol three flowers from each of 30 different individuals during different anthesis periods to estimate the number of filaments and ovules per flower (n=90 flowers). Each flower was divided into four equal parts. The number of filaments and ovules in one randomly chosen section was tallied and multiplied by four to estimate the total number of filaments and ovules per flower. Pollen grain number per flower was estimated from one flower of the same 30 individuals. A single closed anther per flower was collected and fixed in 70% alcohol. The pollen grains were counted by squashing the anthers in a Petri dish under a stereoscopic microscope 10× to 30×. Then, the total pollen per flower was estimated by multiplying the pollen counts by the mean number of filaments of its corresponding individual plant. 

Herkogamy: We collected 90 flowers from 30 different individual plants (three flowers per individual) throughout different times of the day and fixed them in 70% ethanol. The length of the style of each flower and one filament per flower were measured with a digital Vernier caliper to the nearest 0.01 mm. If, by means of a paired t-test, the length of both floral structures were significantly different, we confirmed herkogamy. 

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