Prickly Problems: Cylindropuntia's Low Genetic Diversity Despite Inbreeding Avoidance

Dioecy, the separation of sexes, is found in 6% of flowering plants. One widely known hypothesis suggests that it is an adaptation to mitigate inbreeding. A contrary hypothesis suggests that dioecy is an evolutionary dead end. However, contrasting patterns emerged from population genetic studies that compared the genetic diversity between dioecy versus hermaphroditic species. Specifically, in Silene, it was shown that dioecious species possess higher genetic diversity than hermaphroditic species, challenging the dead end hypothesis. To evaluate whether dioecy is indeed advantageous, further studies are needed in systems with diverse sexual systems such as the genus Cylindropuntia (Cactaceae). It encompasses species with sexual separation observed solely in polyploids. Notably, these polyploids (C. wolfii and C. chuckwallensis) share similar ploidy, flower colors, and geographic proximity, raising speculation about their shared ancestry. Moreover, C. wolfii has been reported to have a low seed production, highlighting the need to assess the reproductive strategies of the species. Our first goal was to compare the genetic diversity patterns among species with different sexual systems within the genus Cylindropuntia (Cactaceae). Our second goal was to investigate genetic shared ancestry among the polyploid species. As C. wolfii is struggling to sexually reproduce, our third objective was to investigate whether it is dominated by clonal reproduction, diversity parameters, and population structure. The clonality of C. wolfii was assessed using a combination of field survey and genetic analysis. The pattern of genetic diversity in species with diverse sexual systems did not support the dead end hypothesis. The field survey of C. wolfii revealed no seed recruitment, but the genetic analysis on the current adult plants showed low signs of clonality, suggesting that this species has recently shifted to clonal reproduction. Results showed that overall, this genus had low genetic diversity and high differentiation, implying that it is vulnerable to environmental threats.