Laura F. Galloway

Transgenerational Plasticity Is Adaptive in the Wild

Plants exhibit adaptive responses to light, but it is not known whether parental plants transmit environmental cues that elicit adaptive responses in offspring. We show that offspring life history (annual versus biennial) is influenced by the maternal light environment (understory versus light gap). This transgenerational plasticity is adaptive when offspring are grown in their maternal light environment, where seeds typically disperse. Projections of population growth show that plants that are appropriately cued for their light environment through maternal effects have 3.4 times greater fitness than otherwise. Transgenerational plasticity has evolved in response to natural variation in light and provides a flexible mechanism by which sedentary organisms cope with heterogeneous environments.

POPULATION DIFFERENTIATION IN AN ANNUAL LEGUME: LOCAL ADAPTATION

Abstract. Studies of many plants species have demonstrated adaptive genetic differentiation to local environmental conditions. Typically these studies are conducted to evaluate adaptation to contrasting environments. As a consequence, although local adaptation has been frequently demonstrated, we have little information as to the spatial scale of adaptive evolution. We evaluated adaptive differentiation between populations of the annual legume Chamaecrista fasciculata using a replicated common‐garden design. Study sites were established in three field locations that are home to native populations of C. fasciculata. Each location was planted for two years with seed from the population native to the study site (home population) and populations located six distances (0.1‐2000 km) from each site (transplanted populations). Seeds were planted into the study sites with minimum disturbance to determine the scale of local adaptation, as measured by a home‐site fitness advantage, for five fitness components: germination, survival, vegetative biomass, fruit production, and the number of fruit produced per seed planted (an estimate of cumulative fitness). For all characters there was little evidence for local adaptation, except at the furthest spatial scales. Patterns of adaptive differentiation were fairly consistent in two of the three sites, but varied between years. Little genetic variation was expressed at the third site. These results, combined with previous estimates of limited gene flow, suggest that metapopulation processes and temporal environmental variation act together to reduce local adaptation, except over long distances.

Population differentiation in an annual legume: genetic architecture.

Abstract. The presence or absence of epistasis, or gene interaction, is explicitly assumed in many evolutionary models. Although many empirical studies have documented a role of epistasis in population divergence under laboratory conditions, there have been very few attempts at quantifying epistasis in the native environment where natural selection is expected to act. In addition, we have little understanding of the frequency with which epistasis contributes to the evolution of natural populations. In this study we used a quantitative genetic design to quantify the contribution of epistasis to population divergence for fitness components of a native annual legume, Chamaecrista fasciculata. The design incorporated the contrast of performance of F2 and F3 segregating progeny of 18 interpopulation crosses with the F1 and their parents. Crosses were conducted between populations from 100 m to 2000 km apart. All generations were grown for two seasons in the natural environment of one of the parents. The F1 often outperformed the parents. This F1 heterosis reveals population structure and suggests that drift is a major contributor to population differentiation. The F2 generation demonstrated that combining genes from different populations can sometimes have unexpected positive effects. However, the F3 performance indicated that combining genes from different populations decreased vigor beyond that due to the expected loss of heterozygosity. Combined with previous data, our results suggest that both selection and drift contribute to population differentiation that is based on epistatic genetic divergence. Because only the F3 consistently expressed hybrid breakdown, we conclude that the epistasis documented in our study reflects interactions among linked loci.

Sexual dimorphism in flower size

Sexual dimorphism in the secondary characters of unisexual flowers is often observed. Most prior reports indicate that male flowers have larger petals than female flowers. In this article we examine hypotheses regarding patterns of perianth (sepals and petals) size dimorphism. (1) Developmental associations between the corolla and the stamens constrain the independent evolution of these characters. (2) The role of the perianth in enclosing the reproductive structures in the bud results in a correlation between the size of the enclosed structures and the perianth parts. (3) In animal-pollinated species, the perianth serves to attract pollinators; fitness gains achieved through allocation to attractive structures differ between the sexes. To test these hypotheses, we compared 919 species with unisexual flowers; quantitative measurements of floral parts were made for 84 of these species. Unlike most previous studies, this study found that the petals of male flowers were larger than those of females in less than one-half of the cases, demonstrating that developmental associations are not strictly responsible for patterns of sexual dimorphism. Relative perianth size appears to be evolutionarily labile, even within genera. Patterns of perianth-size dimorphism differed between temperate and tropical species and between animal- and wind-pollinated species. Perianth size was strongly associated with the size of the reproductive structures enclosed in each flower, suggesting that the function of petals and sepals to protect developing reproductive structures may determine their size. However, the size of the reproductive structures was a better predictor of perianth size in wind-than in animal-pollinated species, which supports the attractive function hypothesis of the perianth. We conclude that the two functional roles of the perianth, protection and attraction, are the primary determinants of perianth size in unisexual flowers.

Epistasis and its consequences for the evolution of natural populations

Throughout the neodarwinian synthesis, theorists have debated the role of gene interactions, or epistasis, in the evolutionary process. Unfortunately, empirical measurement of the role of epistasis in the evolution of natural populations has, until now, been difficult. Two developments in empirical approaches have occurred: (1) application of theory to the evolution of natural populations, and (2) the concurrent development of molecular marker-assisted techniques to understand the architecture of quantitative genetic variation. Thus, exciting developments in both theory and empirical data collection provide the stimulus for documenting the role of epistasis in the evolutionary process.

Assessing available carbon: Comparison of techniques across selected forest soils

Abstract Available carbon (C) is defined as soil organic C that heterotrophic microorganisms can readily utilize as an energy and C source. Several techniques for assessing available C have been described in the literature, but none has become standard. Four of these methods, (i) mlneralizable C (Min‐C), (ii) cold water soluble C (CWS‐C), (iii) boiling water extractable C (BWE‐C), (iv) and total C (Tot‐C), were compared to each other and to a denitrification potential (DP) bioassay of C availability. These comparisons were made across selected forest soils which exhibited wide ranges of textural composition and organic C content and which ranged in acidity from pH 3.1 to 5.4. Carbon mineralized to CO2 during a 7 day aerobic incubation of field‐moist soil (Min‐C) provided the best prediction of DP. Min‐C also appeared to distinguish between available C and refractory C in a soil with high Tot‐C. Air drying increased CWS‐C levels 2‐ to 10‐fold. The C made soluble by drying and rewetting was strongly correla...

Response to natural environmental heterogeneity : maternal effects and selection on life-history characters and plasticities in Mimulus guttatus

Recent studies in plant populations have found that environmental heterogeneity and phenotypic selection vary at local spatial scales. In this study, I ask if there is evolutionary change in response to environmental heterogeneity and, if so, whether the response occurs for characters or character plasticities. I used vegetative clones of Mimulus guttatus to create replicate populations of 75 genotypes. These populations were planted into the natural habitat where they differed in mean growth, flowering phenology, and life span. This phenotypic variation was used to define selective environments. There was variation in fitness (flower production) among genotypes across all planting sites and in genotype response to the selective environment. Offspring from each site were grown in the greenhouse in two water treatments. Because each population initially had the same genetic composition, variation in the progeny between selective environments reveals either evolutionary change in response to environmental heterogeneity or environmental maternal effects. Plants from experimental sites that flowered earlier, had shorter life spans and were less productive, produced offspring that had more flowers, on average, and were less plastic in vegetative allocation than offspring of longer‐lived plants from high‐productivity areas. However, environmental maternal effects masked phenotypic differences in flower production. Therefore, although there was evidence of genetic differentiation in both life‐history characters and their plasticities in response to small‐scale environmental heterogeneity, environmental maternal effects may slow evolutionary change. Response to local‐scale selective regimes suggests that environmental heterogeneity and local variation in phenotypic selection may act to maintain genetic variation.

The effect of maternal and paternal environments on seed characters in the herbaceous plant Campanula Americana (Campanulaceae)

Maternal environments typically influence the phenotype of their offspring. However, the effect of the paternal environment or the potential for joint effects of both parental environments on offspring characters is poorly understood. Two populations of Campanula americana, a woodland herb with a variable life history, were used to determine the influence of maternal and paternal light and nutrient environments on offspring seed characters. Families were grown in the greenhouse in three levels of light or three levels of nutrients. Crosses were conducted within each environmental gradient to produce seeds with all combinations of maternal and paternal environments. On average, increasing maternal nutrient and light levels increased seed mass and decreased percentage germination. The paternal environment affected seed mass, germination time, and percentage germination. However, the influence of the paternal environment varied across maternal environments, suggesting that paternal environmental effects should be evaluated in the context of maternal environments. Significant interactions between family and the parental environments for offspring characters suggest that parental environmental effects are genetically variable. In C. americana, the timing of germination determines life history. Therefore parental environmental effects on germination timing, and genetic variation in those parental effects, suggest that parental environments may influence life history evolution in this system.

PARENTAL ENVIRONMENTAL EFFECTS ON LIFE HISTORY IN THE HERBACEOUS PLANT CAMPANULA AMERICANA

Although environmental parental effects, especially maternal effects, are well known in plants we have almost no information about their expression in nature. This study explores the influence of maternal and paternal light and nutrient environments on germination characters under natural conditions in the herbaceous plant Campanula americana. Families were grown in a greenhouse under three levels of light or three levels of nutrients, and crosses were conducted within each environmental gradient to produce seeds with all combinations of maternal and paternal environments. Seeds produced under the controlled environments were planted into the home site of the population and another local site, and germination was monitored over the fall and spring germinating seasons. The paternal light environment influenced percentage germination, demonstrating that the offspring phenotype may depend on the environment a set of pollen donors is grown in. The effect of maternal nutrient level on percentage germination depended on the offspring environment. Percentage germination in response to maternal nutrient and paternal light environments varied among families, suggesting that these parental environmental effects are genetically variable. Both maternal light and nutrient environments influenced season of germination. Germination season determines life history in C. americana: fall-germinating individuals are annuals while spring-germinating seeds are biennials. Maternal plants grown under low and high light and low nutrient conditions produced more biennial offspring while the remaining maternal environments had an equal frequency of annual and biennial offspring. In C. americana maternal environments influence life history and therefore fitness through their effects on season of germination.

Outcrossing rate and inbreeding depression in the herbaceous autotetraploid, Campanula americana

Polyploidy in angiosperms is frequently associated with an increase in self-compatibility. Self-fertilization can enhance polyploid establishment, and theory predicts reduced inbreeding depression in polyploids relative to diploids. Therefore, we may expect mating systems that promote self-fertilization or mixed-mating in polyploid species. However, few studies have measured polyploid mating systems and inbreeding depression. We report the outcrossing rate and inbreeding depression for Campanula americana, a self-compatible protandrous herb. Allozyme genotypes suggest that C. americana is an autotetraploid with tetrasomic inheritance. We found that the multilocus outcrossing rate, tm=0.938, did not differ from unity. This result was unexpected since previous work demonstrated that pollinators frequently move from male- to female-phase flowers on the same plant, that is, geitonogamy. Self and outcross pollinations were conducted for three populations. Offspring were germinated in controlled conditions and grown to maturity in pots in nature. Inbreeding depression was not significant for most seed and germination characters. However, all later life traits except flowering date differed between inbred and outcrossed individuals resulting in a 26% reduction in cumulative fitness for inbred plants. Limited early- and moderate later-life inbreeding depression suggest that it is buffered by the higher levels of heterozygosity found in an autotetraploid. C. americana appears to have a flexible mating system where within flower protandry and/or cryptic self-incompatibility result in a high outcrossing rate when pollinators are abundant, but self-compatibility and limited inbreeding depression maintain reproductive success when mates are limited.