Large effect quantitative trait loci for salicinoid phenolic glycosides in Populus: Implications for gene discovery.

Genomic studies have been used to identify genes underlying many important plant secondary metabolic pathways. However, genes for salicinoid phenolic glycosides (SPGs)-ecologically important compounds with significant commercial, cultural, and medicinal applications-remain largely undescribed. We used a linkage map derived from a full-sib population of hybrid cottonwoods (Populus spp.) to search for quantitative trait loci (QTL) for the SPGs salicortin and HCH-salicortin. SSR markers and primer sequences were used to anchor the map to the V3.0 P. trichocarpa genome. We discovered 21 QTL for the two traits, including a major QTL for HCH-salicortin (R2 = .52) that colocated with a QTL for salicortin on chr12. Using the V3.0 Populus genome sequence, we identified 2,983 annotated genes and 1,480 genes of unknown function within our QTL intervals. We note ten candidate genes of interest, including a BAHD-type acyltransferase that has been potentially linked to Populus SPGs. Our results complement other recent studies in Populus with implications for gene discovery and the evolution of defensive chemistry in a model genus. To our knowledge, this is the first study to use a full-sib mapping population to identify QTL intervals and gene lists associated with SPGs.

Developmental trajectories in cottonwood phytochemistry.

We examined the hypothesis that ecologically important phytochemical traits differ predictably among various developmental zones of trees (i.e., mature and juvenile zones of individual trees and juvenile ramets that sprout from roots) and that the slope of this phytochemical gradient represents a "developmental trajectory." We focused on Populus fremontii (Fremont cottonwood), P. angustifolia (narrowleaf cottonwood), and their natural hybrids. Two major patterns emerged. First, within narrowleaf and hybrids, concentrations of important phytochemicals (condensed tannins and phenolic glycosides) differ greatly and predictably between developmental zones. Second, developmental trajectories differ greatly among these cottonwood species and their hybrids: Fremont exhibits a flat trajectory, narrowleaf a steep trajectory, and hybrids an intermediate trajectory, suggesting an additive genetic component and an ontogenetic basis to this phytochemical variation. Because diverse herbivorous species respond to the phytochemistry of their host plants, we predict that the developmental trajectories of plants play a major role in mediating ecological interactions and structuring communities, and that biodiversity in a stand of trees is determined by both interplant genetic diversity and intraplant ontogenetic diversity.

Plant genetic determinants of arthropod community structure and diversity.

To test the hypothesis that genes have extended phenotypes on the community, we quantified how genetic differences among cottonwoods affect the diversity, abundance, and composition of the dependent arthropod community. Over two years, five major patterns were observed in both field and common-garden studies that focused on two species of cottonwoods and their naturally occurring F1 and backcross hybrids (collectively referred to as four different cross types). We did not find overall significant differences in arthropod species richness or abundance among cottonwood cross types. We found significant differences in arthropod community composition among all cross types except backcross and narrowleaf cottonwoods. Thus, even though we found similar richness among cross types, the species that composed the community were significantly different. Using vector analysis, we found that the shift in arthropod community composition was correlated with percent Fremont alleles in the host plant, which suggests that the arthropod community responds to the underlying genetic differences among trees. We found 13 arthropod species representing different trophic levels that were significant indicators of the four different cross types. Even though arthropod communities changed in species composition from one year to the next, the overall patterns of community differences remained remarkably stable, suggesting that the genetic differences among cross types exert a strong organizing influence on the arthropod community. Together, these results support the extended phenotype concept. Few studies have observationally and experimentally shown that entire arthropod communities can be structured by genetic differences in their host plants. These findings contribute to the developing field of community genetics and suggest a strategy for conserving arthropod diversity by promoting genetic diversity in their host plants.

Cottonwood hybrids gain fitness traits of both parents: a mechanism for theirlong-term persistence?

Using surveys of natural populations, experimental crosses, and common garden trials, we tested the hypothesis that hybrid cottonwoods (Populus fremontii × P. angustifolia) from the Weber River in northern Utah would produce as many viable offspring as their parental species. We found that both F(1) generations and backcross generations can be just as fit as the parent taxa. First, F(1) hybrids produced as many viable seed as P. angustifolia (but less than P. fremontii), and backcross genotypes produced as many viable seeds as both parent taxa. Second, hybrids produced nearly two times as many ramets from root sprouts as P. angustifolia and four times as many ramets as P. fremontii. Third, the high mortality of germinated seedlings of all tree types (i.e., >90%) and very low mortality of asexually derived ramets provide hybrids with equal sexual reproduction and enhanced asexual reproduction, especially since backcross hybrids exhibit transgressive segregation in ramet production. Our findings suggest that the introgression of P. fremontii seed traits into the hybrid genome is responsible for their equivalent performance (at least to one parent) in sexual reproduction, while the contributions of asexual traits from P. angustifola results in hybrids having equal or greater fitness.

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community.

We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.

Density- and size-dependent spacing of ant nests: evidence for intraspecific competition.

We present three lines of evidence which each suggest that intraspecific competition has significantly influenced the spacing patterns of Formica altipetens colonies. First, nearest-neighbor analysis of nest spacing patterns detected significant uniformity in six of eight plots. Second, there was a signifcant increase in the distance separating nearest neighbors as ant nest diameters increased. Third, ant nest density predicted substantial variation in the colony dispersion index, indicating the existence of a dispersion continuum at our study site.