H. D. Bradshaw

Poplar is increasingly recognized as an excellent model tree for the study of tree growth and its underlying physiology and genetics. By studying trees of the genus Populus (poplars, cottonwoods, aspens), which in their native ecosystems play a major role in the re-colonization of sites after disturbances, new insights have been gained into plantation culture and the development of improved cultivars. Of the 20 chapters in this publication, authored by an international group of researchers, one section deals with systematics, genetics, genetic manipulation and biotic interactions of Populus, while the other deals with stress response and the physiology of growth and productivity.

A paradigm of evolutionary biology is that adaptation and reproductive isolation are caused by a nearly infinite number of mutations of individually small effect. Here, we test this hypothesis by investigating the genetic basis of pollinator discrimination in two closely related species of monkeyflowers that differ in their major pollinators. This system provides a unique opportunity to investigate the genetic architecture of adaptation and speciation because floral traits that confer pollinator specificity also contribute to premating reproductive isolation. We asked: (i) What floral traits cause pollinator discrimination among plant species? and (ii) What is the genetic basis of these traits? We examined these questions by using data obtained from a large-scale field experiment where genetic markers were employed to determine the genetic basis of pollinator visitation. Observations of F2 hybrids produced by crossing bee-pollinated Mimulus lewisii with hummingbird-pollinated Mimulus cardinalis revealed that bees preferred large flowers low in anthocyanin and carotenoid pigments, whereas hummingbirds favored nectar-rich flowers high in anthocyanins. An allele that increases petal carotenoid concentration reduced bee visitation by 80%, whereas an allele that increases nectar production doubled hummingbird visitation. These results suggest that genes of large effect on pollinator preference have contributed to floral evolution and premating reproductive isolation in these monkeyflowers. This work contributes to growing evidence that adaptation and reproductive isolation may often involve major genes.

▪ Abstract Plant adaptation to serpentine soil has been a topic of study for many decades, yet investigation of the genetic component of this adaptation has only recently begun. We review the defining properties of serpentine soil and the pioneering work leading to three established physiological and evolutionary mechanisms hypothesized to be responsible for serpentine tolerance: tolerance of a low calcium-to-magnesium ratio, avoidance of Mg toxicity, or a high Mg requirement. In addition, we review recent work in serpentine ecology documenting the high proportion of endemic species present, the adaptive morphologies of serpentine-tolerant plants, and the distinctive structure of serpentine communities. Studies of the physiological mechanisms proposed to confer serpentine tolerance have shown that uptake of particular ions and heavy metals varies between serpentine-tolerant and -intolerant species. Recent studies examining the genetic basis of serpentine adaptation have shown serpentine-adaptive quantitative trait loci (QTL) to have large phenotypic effects, drought tolerance to be as important as metal tolerance, and serpentine adaptation to have evolved independently multiple times within species. Investigations of plant races and species adapted to contrasting soil types have shown disparate flowering times, divergent floral morphologies, and pollen incompatibility to contribute to reproductive isolation. Finally, we propose that future studies involving serpentine systems should merge the fields of ecology, evolution, physiology, and genetics.

Evolutionists have long recognized the role of reproductive isolation in speciation, but the relative contributions of different reproductive barriers are poorly understood. We examined the nature of isolation between Mimulus lewisii and M. cardinalis, sister species of monkeyflowers. Studied reproductive barriers include: ecogeographic isolation; pollinator isolation (pollinator fidelity in a natural mixed population); pollen competition (seed set and hybrid production from experimental interspecific, intraspecific, and mixed pollinations in the greenhouse); and relative hybrid fitness (germination, survivorship, percent flowering, biomass, pollen viability, and seed mass in the greenhouse). Additionally, the rate of hybridization in nature was estimated from seed collections in a sympatric population. We found substantial reproductive barriers at multiple stages in the life history of M. lewisii and M. cardinalis. Using range maps constructed from herbarium collections, we estimated that the different ecogeographic distributions of the species result in 58.7% reproductive isolation. Mimulus lewisii and M. cardinalis are visited by different pollinators, and in a region of sympatry 97.6% of pollinator foraging bouts were specific to one species or the other. In the greenhouse, interspecific pollinations generated nearly 50% fewer seeds than intraspecific controls. Mixed pollinations of M. cardinalis flowers yielded >75% parentals even when only one-quarter of the pollen treatment consisted of M. cardinalis pollen. In contrast, both species had similar siring success on M. lewisii flowers. The observed 99.915% occurrence of parental M. lewisii and M. cardinalis in seeds collected from a sympatric population is nearly identical to that expected, based upon our field observations of pollinator behavior and our laboratory experiments of pollen competition. F1 hybrids exhibited reduced germination rates, high survivorship and reproduction, and low pollen and ovule fertility. In aggregate, the studied reproductive barriers prevent, on average, 99.87% of gene flow, with most reproductive isolation occurring prior to hybrid formation. Our results suggest that ecological factors resulting from adaptive divergence are the primary isolating barriers in this system. Additional studies of taxa at varying degrees of evolutionary divergence are needed to identify the relative importance of pre- and postzygotic isolating mechanisms in speciation.