2015 field study finds divergent reproduction in two Opuntia robusta populations

A field study of two central Mexican populations of the cactus Opuntia robusta found differences in ovule production, seed germination, floral visitors and the functional expression of mating systems during the 2015 flowering season. Controlled treatments also confirmed that autonomous self-pollination produced viable seeds in both populations, although their reproductive performance and visitor–germination associations differed.

The researchers compared the hermaphroditic Matías Rodríguez population with the San Nicolás Tecoaco population, which had previously been classified as trioecious. The sites were about 6 km apart in Hidalgo, Mexico: Matías Rodríguez was located in pine–oak forest, while San Nicolás Tecoaco occupied xerophytic scrub. The observational and experimental study combined floral measurements, pollen and ovule counts, five controlled pollination treatments, fruit and seed measurements, germination assays and opportunistic observations of floral visitors.

All flowering plants observed in 2015 at both sites were hermaphroditic, including plants at the trioecious site. Male and female floral morphs had been documented there in other years, but none flowered during the study period. Consequently, the reproductive analyses for both populations were restricted to hermaphroditic flowers, and sex could not be included as a factor in the statistical models. Researchers sectioned 68 floral buds from Matías Rodríguez and 20 from San Nicolás Tecoaco for reproductive measurements, with each bud collected from a different plant.

Flowers at Matías Rodríguez had a modeled mean of 670 ovules per flower, with a 95% confidence interval of 611–736, compared with 448 ovules at San Nicolás Tecoaco, with a 95% confidence interval of 383–514. A negative-binomial model estimated an approximately 33% reduction in ovule production at the trioecious site. Pollen production did not differ detectably between the populations. The mean pollen–ovule ratio was 349 at Matías Rodríguez, with a 95% confidence interval of 296–412, and 479 at San Nicolás Tecoaco, with a wider interval of 360–645. The population effect on this ratio was marginal rather than conclusive.

The study tested automatic self-pollination, manual self-pollination, cross-pollination, supplemental pollination and open pollination. Fruit set was highest under open pollination in both populations, accounting for approximately 62–65% of flowers at Matías Rodríguez and 57–80% at San Nicolás Tecoaco. The trioecious population generally produced more seeds per fruit, with treatment means ranging from 128 ± 157 to 286 ± 188, compared with 83 ± 130 to 178 ± 148 in the hermaphroditic population. Estimated fruit volume also tended to be greater at San Nicolás Tecoaco, although treatment-specific comparisons varied between sites.

A beta-binomial mixed model detected higher germination among seeds from the trioecious population than from the hermaphroditic population. Estimated marginal germination proportions were 0.302 at San Nicolás Tecoaco, with a 95% confidence interval of 0.245–0.366, and 0.213 at Matías Rodríguez, with a 95% confidence interval of 0.169–0.265. The model estimated a 59% higher germination probability at the trioecious site, with an odds ratio of 1.59, and indicated substantial variation among plants. In contrast, a non-parametric comparison of medians, 0.29 versus 0.20, did not reach statistical significance.

Pollination treatment had no detectable effect on germination in the hermaphroditic population, with estimated probabilities ranging from 0.19 to 0.26. In the trioecious population, autonomous self-pollination had an estimated germination proportion of 0.44, with a 95% confidence interval of 0.29–0.60, compared with 0.24 for open pollination, with a 95% confidence interval of 0.15–0.35. The mixed model estimated that autonomous self-pollination increased germination relative to the control. Manual self-pollination had an estimated proportion of 0.37, while cross-pollination had a proportion of 0.22; neither was reported as significantly different from the control in the mixed-model comparisons. Overall, the treatments showed that autonomous selfing was functionally viable in both populations and that no inbreeding depression was detected in the evaluated early fitness traits.

Observed visitors included solitary bees in the genera Anthophora, Macrotera and Tetraloniella, as well as nitidulid beetles. Within Matías Rodríguez, the number of Macrotera visits was positively associated with germination, while Tetraloniella and Anthophora visits were negatively associated with it. Nitidulidae showed a marginal positive association. In San Nicolás Tecoaco, Macrotera showed only a marginal positive association, and the other visitor groups had no detectable associations with germination. These are statistical associations, not measurements of pollination effectiveness, and they do not establish that any visitor caused the differences in germination.

Visitor observations were opportunistic, sampling effort differed slightly between sites, and researchers did not quantify pollen deposition or standardized visitation rates. The study was also restricted to a single flowering season, with only hermaphroditic flowers analyzed in 2015 despite the trioecious population’s known male and female morphs. The sites differed in environment, sampling duration and number of observed flowers. The results therefore document population-level differences in reproductive allocation, early seed performance and visitor assemblages, but do not establish pollinator effectiveness, long-term evolutionary change or causes of the site differences.

Subject of Research: Comparative reproductive ecology of two Mexican Opuntia robusta populations

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

Article References: Mendoza-Reyes, C. H. S., Miranda-Pérez, A., Núñez-Farfán, J., & Janczur, M. K. (2026). Contrasting reproductive strategies and their functional consequences in two populations of Opuntia robusta (Cactaceae). BMC Plant Biology, 26(1), Article 1629. https://doi.org/10.1186/s12870-026-09625-0

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09625-0

Keywords: Opuntia robusta, mating systems, autonomous self-pollination, pollen–ovule ratio, seed germination, floral visitors, reproductive allocation, trioecious population, hermaphroditic population, Hidalgo Mexico, Contrasting, reproductive