Collinsia sparsiflora in serpentine and nonserpentine habitats: using F2 hybrids to detect the potential role of selection in ecotypic differentiation

Microbe-mediated adaptation in plants

Interactions with microbial symbionts have yielded great macroevolutionary innovations across the tree of life, like the origins of chloroplasts and the mitochondrial powerhouses of eukaryotic cells. There is also increasing evidence that host-associated microbiomes influence patterns of microevolutionary adaptation in plants and animals. Here we describe how microbes can facilitate adaptation in plants and how to test for and differentiate between the two main mechanisms by which microbes can produce adaptive responses in higher organisms: microbe-mediated local adaptation and microbe-mediated adaptive plasticity. Microbe-mediated local adaptation is when local plant genotypes have higher fitness than foreign genotypes because of a genotype-specific affiliation with locally beneficial microbes. Microbe-mediated adaptive plasticity occurs when local plant phenotypes, elicited by either the microbial community or the non-microbial environment, have higher fitness than foreign phenotypes as a result of interactions with locally beneficial microbes. These microbial effects on adaptation can be difficult to differentiate from traditional modes of adaptation but may be prevalent. Ignoring microbial effects may lead to erroneous conclusions about the traits and mechanisms underlying adaptation, hindering management decisions in conservation, restoration, and agriculture.

Reproductive isolation and the maintenance of species boundaries in two serpentine endemic Jewelflowers

Speciation occurs when reproductive barriers substantially reduce gene flow between lineages. Understanding how specific barriers contribute to reproductive isolation offers insight into the initial forces driving divergence and the evolutionary and ecological processes responsible for maintaining diversity. Here, we quantified multiple pre- and post-pollination isolating barriers in a pair of closely related California Jewelflowers (Streptanthus, Brassicaceae) living in an area of sympatry. S. breweri and S. hesperidis are restricted to similar serpentine habitats; however, populations are spatially isolated at fine-scales and rarely co-occur in intermixed stands. Several intrinsic postzygotic barriers were among the strongest we quantified, yet, postzygotic barriers currently contribute little to overall reproductive isolation due to the cumulative strength of earlier-acting extrinsic barriers, including spatial isolation, and flowering time and pollinator differences. Data from multiple years suggest that pre-pollination barriers may have different strengths depending on annual environmental conditions. Similarly, crossing data suggest that the strength of intrinsic isolation may vary among different population pairs. Estimates of total reproductive isolation in S. breweri and S. hesperidis are robust to uncertainty and variability in individual barrier strength estimates, demonstrating how multiple barriers can act redundantly to prevent gene flow between close relatives living in sympatry.

Exposure to predators does not lead to the evolution of larger brains in experimental populations of threespine stickleback

Natural selection is often invoked to explain differences in brain size among vertebrates. However, the particular agents of selection that shape brain size variation remain obscure. Recent studies suggest that predators may select for larger brains because increased cognitive and sensory abilities allow prey to better elude predators. Yet, there is little direct evidence that exposure to predators causes the evolution of larger brains in prey species. We experimentally tested this prediction by exposing families of 1000-2000 F2 hybrid benthic-limnetic threespine stickleback to predators under naturalistic conditions, along with matched controls. After two generations of selection, we found that fish from the predator addition treatment had significantly smaller brains (specifically smaller telencephalons and optic lobes) than fish from the control treatment. After an additional generation of selection, we reared experimental fish in a common environment and found that this difference in brain size was maintained in the offspring of fish from the predator addition treatment. Our results provide direct experimental evidence that (a) predators can indeed drive the evolution of brain size--but not in the fashion commonly expected and (b) that the tools of experimental evolution can be used to the study the evolution of the vertebrate brain.

Strong pollinator-mediated selection for increased flower brightness and contrast in a deceptive orchid

Contrasting flower color patterns that putatively attract or direct pollinators toward a reward are common among angiosperms. In the deceptive orchid Anacamptis morio, the lower petal, which makes up most of the floral display, has a light central patch with dark markings. Within populations, there is pronounced variation in petal brightness, patch size, amount of dark markings, and contrast between patch and petal margin. We tested whether pollinators mediate selection on these color traits and on morphology (plant height, number of flowers, corolla size, spur length), and whether selection is consistent with facilitated or negative frequency-dependent pollination. Pollinators mediated strong selection for increased petal brightness (Δβpoll = 0.42) and contrast (Δβpoll = 0.51). Pollinators also tended to mediate stabilizing selection on brightness (Δγpoll = -0.27, n.s.) favoring the most common phenotype in the population. Selection for reduced petal brightness among hand-pollinated plants indicated a fitness cost associated with brightness. The results demonstrate that flower color traits influence pollination success and seed production in A. morio, indicating that they affect attractiveness to pollinators, efficiency of pollen transfer, or both. The documented selection is consistent with facilitated pollination and selection for color convergence toward cooccurring rewarding species.

The remodeling of seedling development in response to long-term magnesium toxicity and regulation by ABA-DELLA signaling in Arabidopsis

Little information is available about signaling response to magnesium toxicity (MgT) in plants. This study presents the first evidence that abscisic acid (ABA) and DELLA proteins participate in signaling response to long-term MgT in Arabidopsis thaliana (Landsberg erecta). Morphological, physiological, and molecular characteristics of a wild-type and two Arabidopsis mutants, ABA-insensitive mutant abi1-1 and constitutive elevated GA response mutant quadruple-DELLA (DELLA-Q: gai-t6 rga-t2 rgl1-1 rgl2-1) were monitored under MgT and normal magnesium conditions. Two weeks of MgT treatment strongly influenced the growth of young plants, but growth inhibition of the DELLA-Q and abi1-1 mutants was less than that of the wild-type plants. Exogenous ABA further inhibited the growth of the DELLA-Q mutants, similar to that of the wild-type. Both ABA and MgT also promoted DELLA protein RGA accumulation in the nuclei. Transcriptional analysis supported these results and revealed that a complex signaling network has responded to MgT in Arabidopsis. DELLA enhancement, which depends on ABI1, contributed to the remodeling growth and development of young seedlings.

Evidence for cross-tolerance to nutrient deficiency in three disjunct populations of Arabidopsis lyrata ssp. lyrata in response to substrate calcium to magnesium ratio

Species with widespread distributions that grow in varied habitats may consist of ecotypes adapted to a particular habitat, or may exhibit cross-tolerance that enables them to exploit a variety of habitats. Populations of Arabidopsis lyrata ssp. lyrata (L.) O'Kane & Al-Shehbaz grow in a wide variety of edaphic settings including serpentine soil, limestone sand, and alluvial flood plains. While all three of these environments share some stressors, a crucial difference among these environments is soil calcium to magnesium ratio, which ranges from 25:1 in the limestone sand to 0.2:1 in serpentine soil. The three populations found on these substrates were subjected to three different Ca to Mg ratios under controlled environmental conditions during germination and rosette growth. Response to Ca to Mg ratio was evaluated through germination success and radicle growth rate, rosette growth rate, and the content of Ca and Mg in the rosette. All three populations were particularly efficient in fueling growth under nutrient deficiency, with the highest nutrient efficiency ratio for Ca under Ca deficiency and for Mg under Mg deficiency. Although the serpentine population had significantly higher leaf Ca to Mg ratio than the limestone or flood plain populations under all three Ca to Mg ratios, this increase did not result in any advantage in growth or appearance of the serpentine plants, during early life stages before the onset of flowering, even in the high Mg substrate. The three populations showed no population by substrate interaction for any of the parameters measured indicating that these populations may have cross-tolerance to substrate Ca to Mg ratio.

A common garden test of host-symbiont specificity supports a dominant role for soil type in determining AMF assemblage structure in Collinsia sparsiflora

Specialization in plant host-symbiont-soil interactions may help mediate plant adaptation to edaphic stress. Our previous field study showed ecological evidence for host-symbiont specificity between serpentine and non-serpentine adapted ecotypes of Collinsia sparsiflora and arbuscular mycorrrhizal fungi (AMF). To test for adapted plant ecotype-AMF specificity between C. sparsiflora ecotypes and field AMF taxa, we conducted an AMF common garden greenhouse experiment. We grew C. sparsiflora ecotypes individually in a common pool of serpentine and non-serpentine AMF then identified the root AMF by amplifying rDNA, cloning, and sequencing and compared common garden AMF associates to serpentine and non-serpentine AMF controls. Mixing of serpentine and non-serpentine AMF soil inoculum resulted in an intermediate soil classified as non-serpentine soil type. Within this common garden both host ecotypes associated with AMF assemblages that resembled those seen in a non-serpentine soil. ANOSIM analysis and MDS ordination showed that common garden AMF assemblages differed significantly from those in the serpentine-only controls (R = 0.643, P<0.001), but were similar the non-serpentine-only control AMF assemblages (R = 0.081, P<0.31). There was no evidence of adapted host ecotype-AMF specificity. Instead soil type accounted for most of the variation AM fungi association patterns, and some differences between field and greenhouse behavior of individual AM fungi were found.

Pollinator-mediated selection on floral morphology: evidence for transgressive evolution in a derived hybrid lineage

Hybridization between closely related lineages is a mechanism that might promote substantive changes in phenotypic traits of descendants, resulting in transgressive evolution. Interbreeding between divergent but morphologically similar lineages can produce exceptional phenotypes, but the potential for transgressive variation to facilitate long-term trait changes in derived hybrid lineages has received little attention. We compare pollinator-mediated selection on transgressive floral traits in both early-generation and derived hybrid lineages of the Piriqueta cistoides ssp. caroliniana complex. The bowl-shaped flowers of morphotypes in this complex have similar gross morphologies and attract a common suite of small insect pollinators. However, they are defined by significant differences in characters that generate pollinator interest and visitation, including floral area and petal separation. In common garden experiments, patterns of pollen deposition in early-generation recombinant hybrids indicate that Piriqueta's pollinators favour flowers with greater area and reduced petal separation. Changes in floral morphology in derived hybrid lineages are consistent with predictions from selection gradients, but the magnitude of change is limited relative to the range of transgressive variation. These results suggest that hybridization provides variation for evolution of divergent floral traits. However, the potential for extreme transgressive variants to contribute to phenotypic shifts may be limited due to reduced heritability, evolutionary constraints or fitness trade-offs.

Adaptive genetic differentiation in a predominantly self-pollinating species analyzed by transplanting into natural environment, crossbreeding and Q(ST)-F(ST) test

• Both genetic drift and natural selection result in genetic/phenotypic differentiation over space. I analyzed the role of local adaptation in the genetic differentiation of populations of the annual grass Hordeum spontaneum sampled along an aridity gradient. • The study included the introduction of plants having desert vs nondesert origin into natural (desert) environment, analysis of population differentiation in allozymes and random amplified polymorphic DNA (RAPD) markers vs phenotypic traits (Q(ST) -F(ST) comparison), and planting interpopulation hybrids under simulated desert conditions in a glasshouse. • The results of the home advantage test, Q(ST) -F(ST) comparison and crossbreeding were consistent with local adaptation; that is, that differentiation of the desert plants from plants of nondesert origin in phenotypic traits was adaptive, giving them home advantage. Each method used provided additional, otherwise unavailable, information, meaning that they should be viewed as complementary rather than alternative approaches. • Gene flow from adjacent populations (i.e. populations experiencing the desert environment) via seeds (but not pollen) had a positive effect on fitness by enhancing natural selection and counteracting drift. At the same time, the effect of genes from the species distributional core (nondesert plants) by either seed or pollen had a negative fitness effect despite its enriching effect on neutral diversity. The pattern of outbreeding depression observed in interpopulation hybrids (F(1) ) and their segregating progeny (F(2) ) was inconsistent with underdominance, but indicated the presence of additive, dominance and epistatic effects.

Phylogenetic perspectives on diversification, biogeography, and floral evolution of Collinsia and Tonella (Plantaginaceae)

PREMISE OF THE STUDY

Collinsia was the subject of classic biosystematic studies by Garber and colleagues and is increasingly investigated to address major evolutionary questions. Lack of phylogenetic data from more than one gene region and one taxonomic exemplar has left relationships, diversity, and phytogeography of Collinsia in question and has limited understanding of its diversification.

METHODS

Phylogenetic analyses representing 179 populations of Collinsia and closely related Tonella were conducted based on DNA sequences of nuclear ribosomal transcribed spacers, the single-copy nuclear gene CYCLOIDEA-1, and part of the chloroplast matK/trnK intron region to reexamine systematic hypotheses and extend understanding of the importance of floral characters, chromosome evolution, interfertility, crossability, hybridization, edaphic factors, and ecogeographic barriers to diversification in the group.

KEY RESULTS

Informal "sections" of Collinsia are artificial, although pedicel length and other traditional deep-level taxonomic characters are more conservative evolutionarily than flower size. Evolutionary loss of crossability and interfertility in Collinsia appears to be largely a byproduct of divergence. Although most taxa appear to have arisen by divergent evolution, multiple lines of evidence indicate a homoploid hybrid constitution of C. tinctoria, possibly explaining an occurrence of convergent chromosome evolution. Phylogeographic and cryptic diversity is extensive.

CONCLUSIONS

Diversity in Collinsia is greater than previously documented. Recently divergent lineages are often associated with distinct habitat (including soil) and geographic factors, different flower sizes, and contrasting chromosomal arrangements. Evidence for a hybrid constitution of diploid C. tinctoria is consistent with lack of strong intersterility barriers between closely related taxa.

Viability selection prior to trait expression is an essential component of natural selection

Natural selection operates throughout the life cycle of an organism. Correlative studies typically fail to consider the effects of viability selection prior to trait expression. A 3-year field experiment on the wildflower Mimulus guttatus demonstrates that this unmeasured component of selection can be very strong. As in previous studies, we find that fecundity is positively related to flower size. However, survival to flowering is much lower in large-flowered genotypes than in small-flowered genotypes. Aggregating viability and fecundity, lifetime fitness through female function generally favoured smaller flowered genotypes. This result differs from the great majority of field studies, which suggest strong positive selection on flower size. It has important cautionary implications for studies of natural and sexual selection on adult characters generally, in both plants and animals.

Linking divergent selection on vegetative traits to environmental variation and phenotypic diversification in the Iberian columbines (Aquilegia)

Divergent selection is a key in the ecological theory of adaptive radiation. Most evidence on its causes and consequences relies on studies of pairs of populations or closely related taxa. However, adaptive radiation involves multiple taxa adapted to different environmental factors. We propose an operational definition of divergent selection to explore the continuum between divergent and convergent selection in multiple populations and taxa, and its links with environmental variation and phenotypic and taxonomic differentiation. We apply this approach to explore phenotypic differentiation of vegetative traits between 15 populations of four taxa of Iberian columbines (Gen. Aquilegia). Differences in soil rockiness impose divergent selection on inflorescence height and the number of flowers per inflorescence, likely affecting the processes of phenotypic and, in the case of inflorescence height, taxonomic diversification between taxa. Elevational variation imposes divergent selection on the number of leaves; however, the current pattern of divergent selection on this trait seems related to ecotypic differentiation within taxa but not to their taxonomic diversification.

Fluctuating patch boundaries in a native annual forb: the roles of niche and dispersal limitation

Abiotic, biotic, and dispersal constraints jointly control the spatial distributions of species, but few studies have directly evaluated how these forces interact and vary over time to create dynamic spatial distributions. Through three years of observation and two years of field manipulation, I investigated simultaneous constraints on the spatial distribution of Lupinus nanus, a native annual legume that grows in dense patches in California grasslands. I transplanted L. nanus across its own patch boundaries, yet within apparently suitable habitat, and assessed the demographic success of naturally occurring and seeded plants in plots with and without competitor removal in years that varied in temperature and rainfall. Core sites were defined as those consistently densely occupied, whereas peripheral sites were densely occupied only during some years, and exterior sites were consistently unoccupied or very sparsely occupied. Site types (core, periphery, and exterior) differed in soil moisture, P, and NO3. Competition limited emergence in all site types in the dry/warm year and in patch peripheries in the wet/cool year. Population fitness (seeds produced per seed added) was > 1.0 in cores during all years. Peripheral sites had fitness near replacement in the wet/cool year, which was greatly increased by competition removal. Exterior fitness was < 1.0 in both experimental years, regardless of seed addition and competitor removal. Seed addition did not increase site-specific fitness, and a seed bank was found to be present in all site types. Herbivory was greater in patch cores and peripheries than in exteriors. Soil variation exerted the most consistent control over patch limits, while competition played an intermittent role in excluding Lupinus from patch peripheries. The dynamic distribution of L. nanus is the product of temporal variation in specific abiotic and biotic niche axes, primarily soil characteristics and competition, rather than dispersal limitation.

Serpentine and non-serpentine ecotypes of Collinsia sparsiflora associate with distinct arbuscular mycorrhizal fungal assemblages

Although plant adaptation to serpentine soils has been studied for several decades, the mechanisms of plant adaptation to edaphic extremes are still poorly understood. Arbuscular mycorrhizal fungi (AMF) are common root symbionts that can increase the plant hosts' establishment and growth in stressful environments. However, little is known about the role plant-AMF interactions play in plant adaptation to serpentine. As a first step towards understanding this role, we examined the AMF assemblages associated with field populations of serpentine and non-serpentine ecotypes of California native plant Collinsia sparsiflora. We sampled roots of C. sparsiflora from three serpentine and three non-serpentine sites in close proximity (110 m to 1.94 km between sites) and analysed the small subunit ribosomal DNA gene amplified from root DNA extracts using AMF-specific primers. A total of 1952 clones from 24 root samples (four from each site) were sequenced. We used sequence similarity and phylogenetic analysis to determine operational taxonomic units (OTU) resulting in 19 OTUs representing taxa from six AMF genera, including one serpentine-specific OTU. We used Bray-Curtis similarity, multidimensional scaling and analysis of similarity to compare root sample AMF assemblages. These analyses clearly showed that plant ecotypes associated with distinct AMF assemblages; an Acaulospora OTU-dominated serpentine, and a Glomus OTU-dominated non-serpentine assemblages. Species diversity and evenness were significantly higher in serpentine assemblages. Finally, relate analysis showed a relationship between ecotype AMF assemblages and soil nutrients. This study reveals a strong relationship between AMF associates and plant adaptation to edaphic extremes.