Seed size selection in the wild in Dithyrea californica

Seed size is regarded as a functional trait with very important consequences for the fitness of plant species. Seedlings emerging from larger seeds are more competitive but are more costly to produce than seedlings from smaller seeds. Seed size is also a trait with transgenerational effects, affecting both the fitness of the parent as well as that of the offspring. Theory on the evolution of offspring size predicts an optimum balance between size and number, seen from the parent's perspective; while empirical studies often show selection for larger seeds, seen from the offspring's perspective. Seed size selection arising from post germination traits is, however, often not unidirectional, nor operating with the same strength in all life history stages of the plant. Seed size selection is also environmentally dependent. Even environmental influence might not operate with the same consistency and strength uniformly through the plant's life cycle. This dissertation is intended to study these questions concerning the dynamics of seed size selection in the wild. This work is to my knowledge, the first to document how seed size selection operates through the whole life cycle, with naturally germinated annual plants from the Sonoran Desert. In my first chapter I explored the offspring fitness consequences of seed size in a multiyear observational study using plant demography and relating vital rates (germination, survival, and fecundity) to the size of the seeds that originate individual plants and the environmental variables of precipitation and competition. I detected positive directional selection operating both through survival and fecundity. Water availability increased both survival and fecundity but also strengthened survival selection and had no effect on fecundity selection. Competition detrimental effects were only observed in fecundity but not in plant survival. In my second chapter I ask whether seed size-specific germination could influence seed size selection later in the life cycle. We found that because germination is differential in relation to seed size, the time of optimal conditions for germination in the field would determine the variance of seed size in the germinated fraction and thus influencing the strength of seed size selection operating through survival. In my third chapter I explored the dispersal consequences of phenotypic plasticity in seed provisioning. We found that mother plants that experienced more competition made smaller seeds and affected the seed dispersal process. Smaller seeds were better able to disperse farther away from their mothers and therefore increased their probability of escaping competition in the next growing season. These studies demonstrated that seed size selection varies through the life cycle and in intensity depending on interactions with the environment

Selection on seed size experiment in the wild

Data from a natural selection experiment where we manipulated water avaliability and conspecific denisty to investigate the effects of the environment on selection on seed size. We used naturally germinated plants and we related relative fitness to standardized seed diameter to test slope differences between treatments

Data from: Selection for seed size: the unexpected effects of water availability and density

1. Seed size is a functional trait with important fitness consequences that potentially extend throughout the life cycle of plants. Dithyrea californica experiences selection for larger seeds in post‐germination stages but it is still uncertain how environmental factors mediate the strength and the direction of natural selection on seed size. 2. D. californica represents a unique opportunity to investigate selection on seed size in natural conditions due to a persistent seed ring that stays attached to the root throughout the plant's life. This makes it possible to unearth plants at any stage and measure the size of the seed from which they originated. 3. We conducted a factorial experiment manipulating water availability and intraspecific competition using plants that naturally germinated in the wild. 4. Selection on seed size via survivorship was nil because all individuals survived to reproduce. The strength and the direction of selection on seed size via fecundity depended on water availability and conspecific density. 5. Contrary to our predictions, increasing conspecific density relaxed directional selection favoring larger seeds, but only in the wettest conditions and an increase in water availability strengthened it, but only at low density. A possible explanation of these counter‐intuitive results relies on the observed absence of survival selection and increased plant growth rates under high water and low density. 6. Larger seeds require more resources to construct and when this cost is taken into account; there is no overall fitness increase with seed size. This nicely follows the life history theory predictions for optimal seed size. At the evolutionary equilibrium, if seeds could be larger, per seed fitness would still increase, which is what we observed. But cost corrected fitness should be flat. Maternal fitness equals per seed fitness times seed number, so any increase to per seed fitness of making a bigger seed is balanced by the resulting cost to seed number. Our results indicate flat cost corrected fitness of seed size as theory predicts.Selection on seed size experiment in the wildData from a natural selection experiment where we manipulated water avaliability and conspecific denisty to investigate the effects of the environment on selection on seed size. We used naturally germinated plants and we related relative fitness to standardized seed diameter to test slope differences between treatments.wd.csvFunding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: 0817121, 125679

Data from: Selection for seed size: the unexpected effects of water availability and density

1. Seed size is a functional trait with important fitness consequences that potentially extend throughout the life cycle of plants. Dithyrea californica experiences selection for larger seeds in post‐germination stages but it is still uncertain how environmental factors mediate the strength and the direction of natural selection on seed size. 2. D. californica represents a unique opportunity to investigate selection on seed size in natural conditions due to a persistent seed ring that stays attached to the root throughout the plant's life. This makes it possible to unearth plants at any stage and measure the size of the seed from which they originated. 3. We conducted a factorial experiment manipulating water availability and intraspecific competition using plants that naturally germinated in the wild. 4. Selection on seed size via survivorship was nil because all individuals survived to reproduce. The strength and the direction of selection on seed size via fecundity depended on water availability and conspecific density. 5. Contrary to our predictions, increasing conspecific density relaxed directional selection favoring larger seeds, but only in the wettest conditions and an increase in water availability strengthened it, but only at low density. A possible explanation of these counter‐intuitive results relies on the observed absence of survival selection and increased plant growth rates under high water and low density. 6. Larger seeds require more resources to construct and when this cost is taken into account; there is no overall fitness increase with seed size. This nicely follows the life history theory predictions for optimal seed size. At the evolutionary equilibrium, if seeds could be larger, per seed fitness would still increase, which is what we observed. But cost corrected fitness should be flat. Maternal fitness equals per seed fitness times seed number, so any increase to per seed fitness of making a bigger seed is balanced by the resulting cost to seed number. Our results indicate flat cost corrected fitness of seed size as theory predicts

Heritability and variance components of seed size in wild species: influences of breeding design and the number of genotypes tested.

Seed size affects individual fitness in wild plant populations, but its ability to evolve may be limited by low narrow-sense heritability (h2). h2 is estimated as the proportion of total phenotypic variance (σ2P) attributable to additive genetic variance (σ2A), so low values of h2 may be due to low σ2A (potentially eroded by natural selection) or to high values of the other factors that contribute to σ2P, such as extranuclear maternal effects (m2) and environmental variance effects (e2). Here, we reviewed the published literature and performed a meta-analysis to determine whether h2 of seed size is routinely low in wild populations and, if so, which components of σ2P contribute most strongly to total phenotypic variance. We analyzed available estimates of narrow-sense heritability (h2) of seed size, as well as the variance components contributing to these parameters. Maternal and environmental components of σ2P were significantly greater than σ2A, dominance, paternal, and epistatic components. These results suggest that low h2 of seed size in wild populations (the mean value observed in this study was 0.13) is due to both high values of maternally derived and environmental (residual) σ2, and low values of σ2A in seed size. The type of breeding design used to estimate h2 and m2 also influenced their values, with studies using diallel designs generating lower variance ratios than nested and other designs. e2 was not influenced by breeding design. For some breeding designs, the number of genotypes included in a study also influenced the resulting h2 and e2 estimates, but not m2. Our data support the view that a diallel design is better suited than the alternatives for the accurate estimation of σ2A in seed size due to its factorial design and the inclusion of reciprocal crosses, which allows the independent estimation of both additive and non-additive components of variance

Appendix C. Dispersal distance beanplot.

Dispersal distance beanplot

Ten years (2006-2016) of annual growth rates for 13 of saguaro populations in the Sonoran Desert

Premise of the study: The saguaro cactus is an iconic species of the Sonoran Desert. Its individual growth rates have been investigated for over one hundred years. Its growth dynamics have been studied using phenomenological models intended to estimate growth, but not to understand the underlying biological processes. Most studies have suggested summer rainfall as the sole factor determining saguaro growth overlooking the influence of other factors related to the process of growth. Methods: We analyzed the annual growth rates for 13 saguaro populations in the Sonoran Desert using non-linear models which are better suited to analyze growth since they consider the fact that maximum growth rates diminish just before the onset of reproduction and related model parameters to the local climate. Key results: We found that the most parsimonious model was the Ricker function that described growth considering its decline with age. We also found that the variance in temperature, rather than precipitation, was more closely related to growth. Higher variance in temperature at the beginning of the warm season was detrimental to saguaro growth. Conclusions: Simple non-linear equations model growth rate with biologically interpretable parameters related to climate factors. As the temperature is projected to increase in both mean and variance by climate change, the population dynamics of this iconic cactus are likely to be affected

Appendix A. Seed mass as a function of mericarp diameter.

Seed mass as a function of mericarp diameter