Contrasting reproductive strategies and their functional consequences in two populations of Opuntia robusta (Cactaceae)

Hermaphroditism is the predominant sexual condition in angiosperms, but populations of a single species can diverge markedly in their reproductive allocation and in the expression of their mating system. Opuntia robusta provides a suitable system to examine this divergence, as its populations differ in floral morphology, sexual expression, and reproductive performance. We compared pollen–ovule ratios, early seed performance, and floral visitors in two populations from central Mexico that occur along an altitudinal gradient and differ in their mating‑system classification. Reproductive allocation varied between populations, with differences in ovule production and in the relative investment in male and female functions. The populations also differed in their mating system: the hermaphroditic population expressed functional autogamy, whereas the trioecious population exhibited facultative autogamy. Germination assays revealed consistent differences between populations, and pollination treatments confirmed that autonomous self‑pollination can produce viable seeds in both. Floral‑visitor communities were contrasting: one population was visited by a diverse assemblage of bees, whereas the other was dominated by a single genus. Statistical analyses revealed functional associations between visitors and germination, highlighting the consistent role of Macrotera sp. and the possible mechanical facilitation of autogamy by Nitidulidae in the hermaphroditic population. Taken together, these results demonstrate that O. robusta populations differ in their reproductive allocation, early offspring performance, and visitor assemblages, indicating functional divergence in mating‑system expression at short geographic scales. This population‑level variation may influence the stability of hermaphroditism and the expression or functional erosion of trioecy within the species.