Disparity in floral traits and breeding systems in the iconic columnar cactus Pachycereus pringlei (Cactaceae)

Abstract

Studies on breeding systems and flower morphology are valuable to infer how environmental factors impose evolutionary change in plants. This study focused on the characterization of floral morphs and reproductive systems in Pachycereus pringlei and how this iconic columnar cactus might be a useful genus to understand the evolution of these highly variable structures. We determined breeding systems, characterized floral morphs in five genetic populations, and used pollen:ovule ratios and stamen-stigma distance to verify the sexual system in bisexual flowers. Our inquiries provide insights into the factors driving intra-specific disparity in flower attributes and the reproductive versatility in P. pringlei. Foremost, the lability of breeding systems is expressed primarily as gynodioecy in the North, trioecy in the South, mainly dioecy in CBS, and hermaphroditic in Catalana and Cerralvo Islands. A latitudinal trend in ovule production, dimensions in gynoecium, androecium, and floral display characters is consistent with a northward increase in vegetative traits suggesting physiological responses to the environmental variation characterizing the Baja California Peninsula (BCP). Also, nectary size decreases northwards in staminate flower, but these flowers are larger in the CBS and South populations suggesting sex-specific selection, e.g., pollinator-driven, acting in different magnitudes on sexual attributes of floral morphs and populations. The presence of rudimentary structures of the dysfunctional sex support the hypothesis of the evolution of unisexuality from an early bisexual ancestor. In conclusion, this investigation provides insights into the factors driving intra-specific disparity in flower attributes and the reproductive versatility in P. pringlei to replace ancestral conditions, specifically the substitution of the hermaphrodite phase with dioecious, gynodioecious, and trioecious breeding systems throughout mainland BCP and Sonora. We posit that the biogeographic patterns of breeding systems and floral traits of this emblematic cactus resulted from the interaction of past factors (northwards range expansion) and contemporary biotic and abiotic factors.