Flowering plants have a remarkable diversity in sexual systems. These sexual sys- tems have ecological and evolutionary implications, yet comprehensive studies are hindered by the scarcity of species-level sexual system data. In this dissertation, we will focus on the genus Cylindropuntia (family Cactaceae), which although has a variety of sexual systems, has been poorly studied. My first goal is to accurately determine the sexual system of C. wolfii reported anecdotally as gynodioecious. To achieve this, we carried out experimental crosses and histological analysis of putative female and bisexual flowers. We found that C. wolfii is functionally dioecious where the putative bisexual flowers were actually function- ally male. Dioecious species often exhibit sexual dimorphism, which can have significant ecological effects, particularly in terms of plant-pollinator interactions. My second goal is to identify whether C. wolfii has sexual dimorphism and if that influences pollinator at- traction. We conducted a comprehensive analysis of quantitative and qualitative traits in C. wolfii flowers and compared them statistically. Our findings indicate that male flowers of C. wolfii are larger and more brightly colored, leading to a higher attraction of potential pollinators compared to female flowers. In this study, we utilized fluorescent dyes as pollen analogues, which are available in various colors. However, previous research has not investigated whether the color of these dyes influences pollinator attraction. Therefore, my third goal was to assess whether there is any bias in pollinator attraction due to the color of the dye. We conducted fluorescent dye assays on: Oscularia deltoides, which exhibits consistent flower coloration, and C. wolfii, which displays flower color polymorphism. We discovered that the presence of green dye influences the attraction of bees. Dioecy is hy- pothesized to lead to low genetic diversity and so my fourth goal was to determine whether the dioecious Cylindropuntia species have lower genetic diversity than hermaphrodites. We found that dioecious Cylindropuntia has comparable diversity to that of hermaphrodites. But, the overall genetic diversity of all Cylindropuntia were found to be low signaling the need for protection measures. Our study sheds light on sexual separation in plants and its influence on ecological and genetic factors.

Syngameons are complex, multispecies hybridization networks made up of three or more species. The consequences of gene flow within a syngameon are largely unstudied, but participation may confer synergistic benefits that surpass those experienced by hybrid pairs. Climate change and habitat perturbation will likely increase the amount of interspecies interactions and could result in more hybridization events. The pinyon pines that occur throughout the southwestern United States and Baja California, Mexico are thought to hybridize due to their overlapping distributions, long-distance wind-mediated pollen dispersal mechanism, similar pollen dispersal times, observed intermediate morphology, and demonstrated lack of genetic incompatibility barriers. While the taxonomy of some species remains unresolved, drought tolerant traits have been observed within at least three taxa. If hybridization occurs in this complex, these traits could be introgressed across species barriers, which could be essential to the future survival of these species as high pinyon mortality has been observed after prolonged periods of drought. Using morphology, chloroplast haplotypes, next-generation sequencing, and climatic data, we address the presence of species barriers and hybridization, exploring how hybridization influenced the creation of the syngameon and how climate change may impact its future. Five taxa were genetically confirmed to be in this syngameon (Pinus edulis, P. monophylla, P. quadrifolia, P. californiarum, and P. x fallax), with extensive admixture among species. Species identities appear to be maintained at range cores with introgression only occurring where species distributions overlap, making this system a range-edge syngameon. Notably, sequential hybridization was detected where two species (P. edulis and either P. monophylla or P. quadrifolia) hybridized to create P. californiarum, which is currently hybridizing with P. edulis to produce the P. x fallax lineage. Populations composed exclusively of or mostly of admixed individuals were found in areas of tri-species sympatry, suggesting that genetic swamping could be occurring in these areas. Future niche models predict the loss of suitable habitat in all climate scenarios modeled, with minor expansions northwards and up in elevation. Populations of P. californiarum showed low structure and genetic diversity, with high levels of inbreeding, indicating that this rarely recognized species should be of conservation concern.