Biology subjects

McNickle, G. G.

Publications and source records attributed to McNickle, G. G..

6 recordsLinked to original sources

bioRxiv2021

Allelopathy as an evolutionarily stable strategy

In plants, most competition is resource competition, where one plant simply pre-empts the resources away from its neighbours. Interference competition, as the name implies, is a form of direct interference to prevent resource access. Interference competition is common among animals who can physically fight, but in plants, one of the main mechanisms of interference competition is Allelopathy. allelopathic plants release of cytotoxic chemicals into the environment which can increase their ability to compete with surrounding organisms for limited resources. The circumstances and conditions favoring the development and maintenance of allelochemicals, however, is not well understood. Particularly, it seems strange that, despite the obvious benefits of allelopathy, it seems to have only rarely evolved. To gain insight into the cost and benefit of allelopathy, we have developed a 2 x 2 matrix game to model the interaction between plants that produce allelochemicals and plants that do not. Production of an allelochemical introduces novel cost associated with synthesis and detoxifying a toxic chemical but may also convey a competitive advantage. A plant that does not produce an allelochemical will suffer the cost of encountering one. Our model predicts three cases in which the evolutionarily stable strategies are different. In the first, the non-allelopathic plant is a stronger competitor, and not producing allelochemicals is the evolutionarily stable strategy. In the second, the allelopathic plant is the better competitor and production of allelochemicals is the more beneficial strategy. In the last case, neither is the evolutionarily stable strategy. Instead, there are alternating stable states, depending on whether the allelopathic or non-allelopathic plant arrived first. The generated model reveals circumstances leading to the evolution of allelochemicals and sheds light on utilizing allelochemicals as part of weed management strategies. In particular, the wide region of alternative stable states in most parameterizations, combined with the fact that the absence of allelopathy is likely the ancestral state, provides an elegant answer to the question of why allelopathy rarely evolves despite its obvious benefits. Allelopathic plants can indeed outcompete non-allelopathic plants, but this benefit is simply not great enough to allow them to go to fixation and spread through the population. Thus, most populations would remain purely non-allelopathic.

plant biology↗

bioRxiv2021

Pulling on multiple threads of evidence to reveal tree species coexistence

Understanding drivers of species coexistence is a central challenge in ecology (1, 2). Coexistence cannot be observed directly, and while species co-occurrence in time and space is necessary for coexistence, it is not sufficient to prove coexistence (3, 4). Species exclusion from a region is potentially observable, but can take decades to occur, and still might just occur stochastically (5, 6). Thus, ecologists generally use theory to identify indirect observations that are indicative of mechanisms driving coexistence or exclusion (7-9). Various methods have been developed to indirectly infer coexistence, each of which requires different data, none of which are usually conclusive on their own, and are not typically combined (e.g. (9-12)). Here, we combine three different approaches which are normally used in isolation to build a case about a lack of coexistence of multiple hardwood species. Importantly, though each method has its flaws, we find that all three point to the same conclusion which strengthens the case for a lack of coexistence. First, in an experimental planting of three mature tree species we found no relationship between productivity and species diversity, which has been hypothesized to occur due to a lack of niche differences among species. Second, we used modern coexistence theory to calculate niche and fitness differences for each pair of species, which confirmed the high niche overlap among species and competitive dominance of one species. Third, we used the United States Department of Agriculture Forest Inventory and Analysis data to examine co-occurrence patterns of our species across thousands of natural forest stands and found that, these three species were distributed randomly throughout the USA, and do not significantly co-occur as would be expected if coexistence occurred naturally. Given that these independent methods agree; we take this as strong evidence about a lack of coexistence.

ecology↗

bioRxiv2021

When Does Mutualism Offer a Competitive Advantage? A Game-Theoretic Analysis of Host-Host Competition in Mutualism

Plants due to their non-motile nature rely heavily on mutualistic interactions to obtain resources and carry out services. One key mutualism is the plant-microbial mutualism in which a plant trades away carbon to a microbial partner for nutrients like nitrogen and phosphorous. Plants show much variation in the use of this partnership from the individual level to entire lineages depending upon ecological, evolutionary, and environmental context. We sought to determine how this context dependency could result in the promotion, exclusion, or coexistence of the microbial mutualism by seeing if and when the partnership provided a competitive advantage to the plant. To that end, we created a simple 2 x 2 evolutionary game in which plants could either be a mutualist and pair with a microbe or a non-mutualist and forgo the partnership. This model included nutrients freely available to the plant, nutrients obtained only through mutualism with microbes, the cost of producing roots, the cost of trade with microbes, and the size of the local competitive neighborhood. Not surprisingly, we found that mutualism could offer a competitive advantage if its net benefit was positive. Coexistence between strategies is possible though due to competition between mutualists over the microbially obtained nutrient. In addition, the greater the size of the local competitive neighborhood, the greater the region of coexistence but only at the expense of mutualist fixation (non-mutualist fixation was unaffected). Our model, though simple, shows that plants can gain a competitive advantage from using a mutualism depending upon the context and points to basic experiments that can be done to verify the results.

evolutionary biology↗

bioRxiv2020

A global database of photosynthesis model parameters, and phylogenetically controlled analysis of photosynthetic responses from every major terrestrial plant clade

Plant photosynthesis is a major part of the global carbon cycle and climate system. Carbon capture by C3 plants is most often modelled using the Farquhar-von-Caemmerer-Berry (FvCB) equations. We undertook a global synthesis of all parameters required to solve the FvCB model. The publicly available dataset we assembled includes 3663 observations from 336 different C3 plant species among 96 taxonomic families coming from every major vascular plant clade (lycophytes, ferns, gymnosperms, magnoliids, eudicots and monocots). Geographically, the species in the database have distributions that span the majority of the globe. We used the model to predict photosynthetic rates for a hypothetical average plant in each major terrestrial plant clade and find that generally plants have dramatically increased their photosynthetic abilities through evolutionary time, with the average monocot (the youngest clade) achieving maximum rates of photosynthesis almost double that of the average lycophyte (the oldest clade). We also solved the model for different hypothetical average plant functional types (PFTs) and find that herbaceous species generally have much higher rates of photosynthesis compared to woody plants. Indeed, the maximum photosynthetic rate of graminoids is almost three times the rate of the average tree. The resulting functional responses to increasing CO2 in average hypothetical PFTs would suggest that most groups are already at or near their maximum rate of photosynthesis. However, phylogenetic analysis showed that there was no evidence of niche conservatism with most variance occurring within, rather than among clades (K=0.357, p=0.001). This high within-group variability suggests that average PFTs may obscure important plant responses to increasing CO2. Indeed, when we solved the model for each of the 3663 individual observations, we found that, contrary to the predictions of hypothetical average PFTs, that most plants are predicted to be able to increase their photosynthetic rates. These results suggest that global models should seek to incorporate high within-group variability to accurately predict plant photosynthesis in response to a changing climate.

physiology↗

bioRxiv2020

Pisum sativum has no competitive responses to neighbours: a case study in reproducible ecology

Plant-plant competition is ubiquitous in nature. However, studying below ground behaviour of roots has always posed certain difficulties. Pea (Pisum sativum L.) has become a sort of model species for ecological studies about how plant roots respond to neighbouring plant roots and barriers in soil. However, published results point in several different directions. This has sometimes been interpreted as pea having sophisticated context dependent responses that can change in complex ways depending on its surroundings. To explore this further, here, we combine the result of five new experiments with published results to examine 18 unique experiments from 7 different studies for a total of 254 replicates. We used a Bayesian hierarchical meta-analysis approach to estimating the likely effect size from the available data, as well as quantify heterogeneity among different experiments, studies and cultivars. The posterior distributions show that, at the coarsest possible scale of total root production, it is unlikely that P. sativum root growth is influenced by either neighbours or barriers to root growth imposed by the walls of pots that vary in volume. We suggest that further work should consider repeating experiments that reported finer scale root plasticity in pea at the rhizosphere scale, and also consider alternative model species for the study of plant root responses to external cues.

ecology↗

bioRxiv2020

THE BENEFITS OF MYCORRHIZAE ARE FREQUENCY-DEPENDENT: A CASE STUDY WITH A NON-MYCORRHIZAL MUTANT OF PISUM SATIVUM

O_LIMutualisms are remarkably common in the plant kingdom. The mycorrhizal association which involves plant roots and soil fungi is particularly common, and found among members of the majority of plant families. This association is a resource-resource mutualism, where plants trade carbon-based compounds for nutrients, such as phosphorus and nitrogen, mined by the fungi. C_LIO_LIEvolutionary models usually assume that a mutation grants a small number of individual plants the ability to associate with mycorrhizal fungi, and that this subsequently spreads through the population resulting in the evolution of mutualism. This frequency-dependent hypothesis has been difficult to test, because it is rare to have members of the same species that are capable and incapable of forming the mutualism. C_LIO_LIHere we describe the results of an experiment that took advantage of a mutant pea (Pisum sativum L. R25) that is incapable of forming mycorrhizal (or rhizobial) associations, and differs from the wildtype (P. sativum cv. Sparkle) by a single recessive Mendelian allele (Pssym8). We grew each genotype either alone or in every combination of pairwise mixed- or same-genotype. We also present an evolutionary matrix game, which we parameterize from the experimental 15N results, that allows us to estimate the costs and benefits of the mutualism. C_LIO_LIWe find that there was no difference between R25 and WT when grown with a competitor of the same genotype, but when R25 and WT compete, WT has a significant fitness advantage. From the model, we estimate that the benefit in units of fitness (g pod mass) obtained from direct plant nitrogen uptake is 22.2 g, and mycorrhizae increase this by only 0.6 g. The costs of plant nitrogen uptake are 9.4 g, while the cost of trade with mycorrhizae is 0.1g. C_LIO_LIFrom the model and experiment, we conclude that this relatively small cost-benefit ratio of the mycorrhizal association is enough to drive the evolution of mutualism in frequency-dependent selection. However, without the mutant R25 genotype we would not have been able to draw this conclusion. This validation of frequency-dependent evolutionary models is important for continued theoretical development. C_LI

ecology↗