The movement of transgenes into populations for which they are not intended remains a primary concern for genetically engineered crops. Such gene flow in itself is not a risk. However, we know that the transfer of genes from traditionally improved crops into wild populations has already resulted, on occasion, in the evolution of weeds more difficult to control, as well as an increased extinction risk for rare species. Just like traditional crops, genetically engineered crops could occasionally create the same problems. Currently in California, the movement of transgenes from most commercialized transgenic crops into wild plant populations is unlikely — the exception being canola. However, other transgenic plants have been field-tested in California, and if these become commercialized, in certain cases, transgenes are likely to move into the wild or into other crops of the same species. Such gene flow could result in various problems. The best containment for transgenes may involve risk assessment decisions by scientists embarking on projects to determine whether the proposed combination of organism and trait will pose any problems and if so, to determine how to create a safe product.

Invasive species are a major component to global change. Recent work suggests that evolutionary processes are often involved in the formation of invasive species. Hybridization and whole genome duplication are evolutionary processes that can contribute to the evolution of invasive species; multiple invasive species are the result of hybridization and whole genome duplication in the same lineage (allopolyploid species). Through this work we evaluate the potential invasiveness of a newly formed allopolyploid species, Salsola ryanii, which evolved by hybridization between S. tragus (an invasive species native to Russia and China) and S. australis (an invasive species native to Australia and/or South Africa). We investigate the potential invasiveness of this newly formed through three studies 1) a study of how the range of this newly formed species has shifted in the decade following initial discovery; 2) a common garden comparison of plant size, mass and phenology between S. ryanii and its progenitors; 3) a study of the number of independent evolutionary origins of S. ryanii as determined through an analysis of genetic structure.

Through a systematic collection of Salsola sp in California we documented a rapid range expansion from three to fifteen populations within the decade following initial discovery. This range expansion included expansion from two floristic provinces to six different floristic provinces. We compare the range expansion in S. ryanii to other systems for

which similar data is available and demonstrate that S. ryanii has experienced the most rapid range expansion of any newly formed allopolyploid invasive for which data is available. A dramatic range expansion provides support for the hypothesis that S. ryanii has the potential to become a problematic weedy and invasive species.

In the common garden comparison of S. ryanii and its progenitors we document a significant increase in mass and plant size in the newly formed allopolyploid species. Which of the progenitor species had the larger mass and size was dependent on year. The high fitness of S. ryanii regardless of year suggests a strong potential for S. ryanii to replace one or both of its progenitors in all or part of their range. Salsola ryanii’s large plant size makes it a more problematic weed and the high seed production associated with a large plant size increases the potential for spread.

Number of independent evolutionary origins of potentially invasive newly formed allopolyploid species is linked to evolutionary potential; species resulting from multiple origins which admix with each other have potentially greater genetic diversity and evolutionary potential than species resulting from a single origin. We use an analysis of genetic structure to determine whether S. ryanii results from a single or multiple origins. From the analysis of genetic structure it appears that S. ryanii is the result of at least three independent origin events followed by wide dispersal and admixture. A species resulting from multiple origins followed by admixture theoretically has a stronger potential to become an invasive species than a species resulting from a single origin.

Together these studies provide strong empirical support for the hypothesis that allopolypolyploidization can lead to the evolution of increased invasiveness; specifically this work provides strong evidence that S. ryanii is likely to become more invasive than its invasive progenitors. Documentation of range extension of S. ryanii provides land managers with information about new areas where this species is present. Land managers can utilize this information to control the spread of this newly formed invasive species. It additionally provides a needed first common garden comparison between a newly formed allopolyploid species and its progenitors and adds to our understanding of allopolyploid origins.

Distant Citrus relatives often possess genetically-based traits that can be useful for the improvement of standard Citrus cultivars, such as natural resistance to pathogens. Thus, sexual compatibility data between Citrus and its relatives has potential value to Citrus improvement programs as it can be used to predict the introgressability of these genes. This study reviews the current knowledge of cross compatibilities among Citrus and related genera and identifies gaps in the data. Although many gaps are identified, compatibility data within the Hard-Shelled Citroid subtribe, Balsamocitrinae, is largely lacking. To address this deficiency, a selection of diverse germplasm from the Aurantioideae subfamily was cross pollinated onto Aegle marmelos, the most economically important species in the Balsamocitrinae. Pollinations from the Clauseneae and the Triphasiinae were unsuccessful. Pollinations from the Citrinae subtribe resulted in fruit set in Aegle marmelos, but all of the nearly 1800 seeds produced from these unions were inviable. When male parents from the Balsamocitrinae were utilized, fruit set and germination rates were high. Additionally, geitonogamy in Aegle marmelos caused flower abortion indicating self-incompatibility. The author concludes that Aegle marmelos is obligately outcrossing with the requirement for pollen to be received from other closely related taxa in the Balsamocitrinae. These findings provide an important update in what is known regarding cross compatibilities within the Aurantioideae.

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.

Studying transgene flow will enhance our understanding of the fate of crop alleles in nearby populations. In addition to the potential for weediness, gene flow from transgenic crops into neighboring fields could have severe economic consequences if those fields are destined for markets with strict thresholds for transgene detection. This study can serve as a model system for understanding transgene introgression in free-living populations.Transgene escape has been documented for at least eight different species. I chose to work with Alfalfa (Medicago sativa) due to its proximity and because there was already previous data from a 2011 survey to allow for a greater time span analysis of results. Almost half of the populations at each of the three sites surveyed were still remaining in 2019, indicating that feral alfalfa populations are self-perpetuating. About half of the populations surveyed at each site were still remaining from 2019 to 2021. The presence of the transgene increased at all three sites from 2011 to 2019, likely due to unilateral gene flow from nearby transgenic alfalfa production. It is of concern that feral populations may act as bridges to facilitate transgene movement, so the increase in transgene presence is not desirable. Mowing, tilling, and spraying are sometimes used to manage feral roadside populations. Tilling was the most efficient method to eradicate feral alfalfa populations. No differences in effectiveness were observed between other management types. This is the longest timespan data has been collected for transgene flow into feral populations. Having data that spans across a decade will enable a more complete understanding of the dynamics between population presence, transgene presence, management type, and other site characteristics.