UC Riverside

Hybridization is accountable for a considerable number of plant species of which some are remarkably good invaders. Interspecific hybridization can promote plant invasiveness by leading to (1) higher reproductive output, (2) higher genetic diversity, and (3) novel traits or genotypes resulting in range expansion. Through descriptive and experimental analyses, the aim of this study is to understand if an invasive hybrid-derived lineage, California wild radish, varies in novel traits such as fruit structure and within-fruit seed characteristics, relative to its two progenitors, the cultivar Raphanus sativus and the wild R. raphanistrum. I first asked if changes in fruit structural and material properties have important effects on pre-dispersal granivory. Data included fruit morphometrics, biomechanical measurements of the strength necessary to break open the fruit wall, and records of removal after experimentally exposing the fruits to granivores. Combining these data revealed that hybrid fruits differ structurally from fruits of both progenitors, thereby lowering the damage rates by granivores. Second, I asked if pre-dispersal seed predation differs among the three Raphanus lineages and if there are fitness consequences resulting from this. Plants from all three lineages were sown in common gardens, either protected or unprotected from avian granivory experiments. Relative to both progenitors, the hybrid lineage suffered the highest rates of granivory at the unprotected treatments but its seed and fruit production was not depressed by granivory. Finally, I explored how within-fruit seed characteristics may be a feature explaining the competitive superiority of the hybrid-derived lineage. I addressed this topic by directly comparing patterns of within-fruit seed size, paternity, and fitness variables including fecundity and phenology. Within-fruit seed paternity revealed that all three lineages have multiple paternity and that in natural conditions the identity of the pollen donor affects seed weight and final reproductive output. The hybrid-derived lineage had larger peduncular seeds. Taken together, these results indicate that the strength of the fruit wall is a key defense mechanism underlying invasion success in the hybrid-derived lineage.