Biology subjects

Eckhart, V. M.

Publications and source records attributed to Eckhart, V. M..

4 recordsLinked to original sources

bioRxiv2022

Bet hedging is not sufficient to explain intraspecific variation in germination patterns of a winter annual plant

Bet hedging consists of life history strategies that buffer against environmental variability by trading off immediate and long-term fitness. Delayed germination in annual plants is a classic example of bet hedging, and is often invoked to explain low germination fractions. We examined whether bet hedging explains low and variable germination fractions among 20 populations of the winter annual plant Clarkia xantiana ssp. xantiana that experience substantial variation in reproductive success among years. Leveraging 15 years of demographic monitoring and 3 years of field germination experiments, we assessed the fitness consequences of seed banks and compared optimal germination fractions from a density-independent bet-hedging model to observed germination fractions. We did not find consistent evidence of bet hedging or the expected trade-off between arithmetic and geometric mean fitness, though delayed germination increased long-term fitness in 7 of 20 populations. Optimal germination fractions were 2 to 5 times higher than observed germination fractions, and among-population variation in germination fractions was not correlated with risks across the life cycle. Our comprehensive test suggests that bet hedging is insufficient to explain the observed germination patterns. Understanding variation in germination strategies will likely require integrating bet hedging with complementary forces shaping the evolution of delayed germination.

ecology↗

bioRxiv2020

Evolutionary divergence of potential drought adaptations between two subspecies of an annual plant: Do some constraints need to be broken?

PremiseWhether mechanisms of drought adaptation tend to evolve together, evolve independently, and/or evolve constrained by genetic architecture is incompletely resolved, particularly for water-relations traits besides gas exchange. We addressed this question in two subspecies of Clarkia xantiana (Onagraceae), California winter annuals that diverged approximately 65,000 years ago and that are adapted, partly by differences in flowering time, to native ranges that differ in precipitation. MethodsIn these subspecies and in F5 recombinant inbred lines (RILs) from a cross between them we scored drought-adaptation traits related to phenology (times to seed germination and to flowering) and tissue water relations (measures of succulence; pressure-volume curve parameters), in common environments. ResultsSubspecies differed distinctly. The one native to more arid environments had some trait values associated with drought adaptation (e.g., early flowering and high succulence) but had higher osmotic potential at full turgor and lost turgor at higher water potential, indicating poorer tolerance of dehydration. Traits that differed between subspecies exhibited substantial genetic variation, with broad-sense heritability from 0.09 (stem succulence) to 0.43 (time to flowering). The genetic correlation structure suggests facilitated evolution of some trait combinations that might enhance drought adaptation (e.g., high succulence plus low turgor loss point), but the subspecies exhibit some trait combinations that do not follow genetic correlations. ConclusionsAs lineages diverged in their potential to escape drought by early flowering, other traits diverged as well. Genetic architecture might facilitate some correlated evolutionary responses to drought, but particular trait combinations also can evolve despite apparent genetic constraints.

plant biology↗

bioRxiv2019

Additive genetic variance for lifetime fitness and the capacity for adaptation in the wild

The immediate capacity for adaptation under current environmental conditions is directly proportional to the additive genetic variance for fitness, VA(W). Mean absolute fitness, [Formula], is predicted to change at the rate [Formula], according to Fishers Fundamental Theorem of Natural Selection. Despite ample research evaluating degree of local adaptation, direct assessment of VA(W) and the capacity for ongoing adaptation is exceedingly rare. We estimated VA(W) and [Formula] in three pedigreed populations of annual Chamaecrista fasciculata, over three years in the wild. Contrasting with common expectations, we found significant VA(W) in all populations and years, predicting increased mean fitness in subsequent generations (0.83 to 6.12 seeds per individual). Further, we detected two cases predicting \"evolutionary rescue\", where selection on standing VA(W) was expected to increase fitness of declining populations ([Formula] < 1.0) to levels consistent with population sustainability and growth. Within populations, interannual differences in genetic expression of fitness were striking. Significant genotype-by-year interactions reflected modest correlations between breeding values across years (all r < 0.490), indicating temporally variable selection at the genotypic level; that could contribute to maintaining VA(W). By directly estimating VA(W) and total lifetime [Formula], our study presents an experimental approach for studies of adaptive capacity in the wild.

evolutionary biology↗

bioRxiv2018

Biotic interactions limit the geographic range of an annual plant: herbivory and phenology mediate fitness beyond a range margin

Species range limits offer powerful opportunities to study environmental factors regulating distributions and probe the limits of adaptation. However, we rarely know what aspects of the environment are actually constraining range expansion, much less which traits are mediating the organisms response to these environmental gradients. Though most studies focus on climatic limits to species distributions, biotic interactions may be just as important. We used field experiments and simulations to estimate contributions of mammal herbivory to a range boundary in the annual plant Clarkia xantiana ssp. xantiana. A steep gradient of increasing probability of herbivory occurs across the boundary, and herbivory drives several-fold declines in lifetime fitness at and beyond the boundary. By including in our analyses data from a sister taxon with more rapid phenology, we show that delayed phenology drives C. xantiana ssp. xantianas susceptibility to herbivory and low fitness beyond its border.

ecology↗