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.
Duplication of chromosomes by some individuals of a species could result in substantial physiological and morphological alterations. This phenomenon, called polyploidy, is common in plants and recognized to have been important in plant evolution. Just as genetic mutations may give rise to novel diversity, polyploidy may in theory facilitate the exploitation of new habitats and influence pollinator and herbivore interactions with their host plants. However, populations differing in chromosome number are not usually recognized as different biological entities, reflecting uncertainty about their actual morphological, genetic, and ecological distinctness in the wild. The North American creosote bush (Larrea tridentata, Zygophyllaceae) is a shrub that occurs across the Chihuahuan, Sonoran, and Mojave Deserts, and it typifies the uncertainty regarding divergence among ploidy levels. This species is a dominant community member and important food resource for insects throughout its range, but is reported to have a different chromosome complement in each of the three deserts (these are often called "chromosome races"). The consequences of ploidy variation in creosote bush has not been tested rigorously. To investigate whether the chromosome races represent distinct units of biodiversity, we studied habitat, phenological, and morphological differences among the ploidy levels, as well as their reproductive interactions and genetic composition. Our research demonstrates that chromosome races of creosote bush diverged recently (late-Pliocene or Pleistocene) and spread rapidly throughout the North American deserts, following their arrival from South America. Extensive field sampling confirms that the ploidy levels occur in different geographic areas, only coming into contact at the boundaries between deserts. Furthermore, we find that the ploidy levels produce different numbers of flowers, and that they differ to some degree in flower and leaf size when naturally growing together. Although we encounter rare hybrids in areas where the chromosome races are in spatial contact, DNA analyses suggest that different ploidy levels are mostly distinct genetically. Overall, our findings suggest that the ploidy levels of creosote bush are distinct evolutionary lineages. For example, their geographical distributions coincide with strong environmental differences in temperature and preciptation. The ploidy levels differ in flower number and other traits that influence their reproductive output. Finally, the ploidy levels exhibit strong reproductive barriers that limit their potential for interbreeding. Although genetic incompatibilities probably play an important role in preventing hybridization, the infrequent spatial co-occurrence of ploidy levels (probably arising from different environmental affinities) is also important for separation of the chromosome races. We recommend recognition of the ploidy levels of L. tridentata at the subspecies level to reflect their unique ecological and genetic character, and to more accurately describe botanical diversity of the U.S. southwestern deserts.
