Genetic evidence for species cohesion, substructure and hybrids in spruce.

The origin and history of species are shaped by various evolutionary dynamics, including their persistence in the face of potential gene flow from related taxa. In this study, we use broad geographical and taxonomic sampling (2,219 individuals) to establish the distribution of species, hybrids and cryptic genetic variation within the conifer genus Picea (spruce) across western North America. We demonstrate that the six species of spruce in this region are distinguishable based on their genetic composition, and that the more closely related Engelmann spruce (P. engelmannii) and white spruce (P. glauca) have generated numerous and widespread hybrids. These hybrids occur in the central Rocky Mountains, well to the south of the well-established region of admixture in Canada. Additionally, we provide evidence for subdivision within Engelmann spruce, manifested as a southern Rocky Mountains form, and a northern Rocky Mountain and Cascade mountains (western) form. In the intervening central Rocky Mountains region (forests in Wyoming and adjacent states) we found primarily individuals with admixed ancestry. Following their origin, these species of spruce have interacted repeatedly and in different geographical contexts. Multiple pairs of species have been shown to hybridize, yet the species persist and retain distinguishable compositions. At the same time, large geographical areas exist where hybrids are pervasive. Consequently, spruce provide a case study for the maintenance of species boundaries, particularly for how widespread hybridization need not lead to the collapse and loss of species.

Genetic architecture of life history traits and environment-specific trade-offs.

Life history theory predicts the evolution of trait combinations that enhance fitness, and the occurrence of trade-offs depends in part on the magnitude of variation in growth rate or acquisition. Using recombinant inbred lines, we examined the genetic architecture of age and size at reproduction across abiotic conditions encountered by cultivars and naturalized populations of Brassica rapa. We found that genotypes are plastic to seasonal setting, such that reproduction was accelerated under conditions encountered by summer annual populations and genetic variances for age at reproduction varied across simulated seasonal settings. Using an acquisition-allocation model, we predicted the likelihood of trade-offs. Consistent with predicted relationships, we observed a trade-off where early maturity is associated with small size at maturity under simulated summer and fall annual conditions but not under winter annual conditions. The trade-off in the summer annual setting was observed despite significant genotypic variation in growth rate, which is often expected to decouple age and size at reproduction because rapidly growing genotypes could mature early and attain a larger size relative to slowly growing genotypes that mature later. The absence of a trade-off in the winter setting is presumably attributable to the absence of genotypic differences in age at reproduction. We observed QTL for age at reproduction that jointly regulated size at reproduction in both the summer and fall annual settings, but these QTL were environment-specific (i.e. different QTL contributed to the trade-off in the fall vs. summer annual settings). Thus, at least some of the genetic mechanisms underlying observed trade-offs differed across environments.

Genotypes of Brassica rapa respond differently to plant-induced variation in air CO2 concentration in growth chambers with standard and enhanced venting.

Growth chambers allow measurement of phenotypic differences among genotypes under controlled environment conditions. However, unintended variation in growth chamber air CO2 concentration ([CO2]) may affect the expression of diverse phenotypic traits, and genotypes may differ in their response to variation in [CO2]. We monitored [CO2] and quantified phenotypic responses of 22 Brassica rapa genotypes in growth chambers with either standard or enhanced venting. [CO2] in chambers with standard venting dropped to 280 micromol mol(-1) during the period of maximum canopy development, approximately 80 micromol mol(-1) lower than in chambers with enhanced venting. The stable carbon isotope ratio of CO2 in chamber air (delta13C(air)) was negatively correlated with [CO2], suggesting that photosynthesis caused observed [CO2] decreases. Significant genotype x chamber-venting interactions were detected for 12 of 20 traits, likely due to differences in the extent to which [CO2] changed in relation to genotypes' phenology or differential sensitivity of genotypes to low [CO2]. One trait, 13C discrimination (delta13C), was particularly influenced by unaccounted-for fluctuations in delta13C(air) and [CO2]. Observed responses to [CO2] suggest that genetic variance components estimated in poorly vented growth chambers may be influenced by the expression of genes involved in CO2 stress responses; genotypic values estimated in these chambers may likewise be misleading such that some mapped quantitative trait loci may regulate responses to CO2 stress rather than a response to the environmental factor of interest. These results underscore the importance of monitoring, and where possible, controlling [CO2].