Study of invasive species from an evolutionary genetics perspective can both offer insights into the history of these problematic organisms and help anticipate and address their future behavior. Here, we conduct experiments to understand the introduction history and contemporary evolution of a model invasive species, California wild radish. In the mid-1800's, two species of Raphanus were introduced to California; R. sativus was introduced intentionally as a crop and R. raphanistrum was accidentally introduced as a weed. Within 150 years, these two species hybridized into a coalescent lineage, termed the California wild radish, that was able to invade the majority of naturally and human-disturbed areas of the state, from north to south and from beaches to inland valleys. We first asked, what were the introduction sources of each of the progenitor parents involved in the creation of the hybrid-derived lineage? Using chloroplast DNA markers, we found that at least two cultivars and two European R. raphanistrum populations contributed to the genetic diversity within California Raphanus populations. We also determined that hybridization occurred reciprocally, with both progenitor species acting as seed parents in the past. The second question we posed was, has hybridization directly contributed to the ability of California wild radish to invade? We compared the survival and reproduction of multiple populations of California wild radish to that of several populations of each pure progenitor species across years and across sites. California wild radish tends to survive and reproduce better than either pure progenitor, in one environment producing an average of three times as many seeds as cultivated radish and R. raphanistrum. The overwhelming propagule pressure generated by California wild radish, together with its historically documented ability to spread, indicates that hybridization can precipitate the evolution of invasiveness. Finally, we asked, have populations of California wild radish locally adapted since their invasion of the state, and what does the outcome mean for the management of invasive species? A reciprocal transplant of northern, coastal and southern, inland populations of California Raphanus revealed rapid local adaptation that has resulted in divergence of morphological and life history characteristics at a regional level. As a consequence of divergence, targeting different life history stages for control in these two regions would be the most efficient management strategy.
|Advisor:||Ellstrand, Norman C.|
|School:||University of California, Riverside|
|School Location:||United States -- California|
|Source:||DAI-B 69/10, Dissertation Abstracts International|
|Subjects:||Plant biology, Plant sciences|
|Keywords:||California wild radish, Hybridization, Invasive species, Raphanus sativus|
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