DISSERTATION RESEARCH: Polyploidy and Reproductive Isolation in the North American Creosote Bush (Larrea tridentata, Zygophyllaceae)

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Many plant species have both diploid individuals (possessing two sets of chromosomes) and polyploid individuals (possessing four or more sets). Related diploid and polyploid plants are rarely named as different species, in part due to uncertainty about their potential for interbreeding in nature. Creosote bush, a dominant shrub of North American warm deserts, has both diploid and polyploid populations. The PIs will evaluate the ability of these populations to interbreed. Creosote bushes are excellent for such studies because the diploid and polyploid populations come in contact at many different locations, thus allowing replicated studies of the level of interbreeding. The PIs will use field studies and molecular laboratory techniques to evaluate the following possible influences on interbreeding: a) the tendency of diploids and polyploids to occur in different environments, b) the tendency of diploids and polyploids to reproduce at different times of the year, and c) the reproductive success of naturally-occurring triploid and pentaploid hybrids. The multiplicative effects of these reproductive barriers will be calculated. If the barriers to interbreeding are strong, a case can be made for diploids and polyploids being distinct species.If there is strong evidence that diploid and polyploid populations are unable to interbreed, the many other species that have both diploids and polyploids will need to be re-evaluated; these results could potentially lead to a conclusion that plant species diversity in temperate regions is far greater than currently believed. Undergraduates will be involved in all parts of the proposed research in the field and laboratory. The PIs expect to give two undergraduates the opportunity to complete senior theses based on their involvement in the research.