Biology subjects

Broersma, C. M. E.

Publications and source records attributed to Broersma, C. M. E..

4 recordsLinked to original sources

bioRxiv2025

Evolutionary dynamics of the tgr gene family in Dictyostelium allows escape from Crozier's Paradox

Croziers Paradox states that genetic kin recognition will select for its own demise, and theoretical analyses over the past decades have supported this conclusion. Here we examine the molecular evolution of two kin recognition genes in the social amoeba Dictyostelium discoideum, tgrB1 and tgrC1, which enable cells to recognize and reject unrelated cells during cooperative multicellular development. Our results reveal extraordinary polymorphism in these genes, placing them amongst the most rapidly evolving genes in the genome. Co-occurring amoebae, isolated from just a few grams of soil, show highly diverged recognition alleles, indicating plentiful sequence variation that can impact social decision-making on a micro-scale. Analyses of closely related gene duplicates show dynamic evolution of the gene family as a whole, suggesting a mechanism for replenishing genetic variation lost through Croziers Paradox. Our results provide evidence that kin recognition loci can retain sufficient genetic variation in real-world settings and suggest that large gene families may be crucial to retaining genetic variation necessary to evade Croziers paradox.

evolutionary biology↗

bioRxiv2025

Being bigger is better: Size-dependent advantages of plasmodial fusion in a multinucleated slime mould

Larger body size often enhances survival and reproduction. While most organisms grow to achieve larger sizes, some can rapidly increase size through fusion. Fusion decisions are influenced by genetic relatedness and environmental factors, balancing potential benefits and costs. In Physarum polycephalum, a multinucleated slime mould, fusion is thought to occur mainly between genetically identical or highly related individuals. However, the frequency of fusion among close relatives, and the drivers and benefits of fusion, remain unclear. This study explored fusion frequency, morphology, and benefits among clonal and closely related individuals of Physarum. We also assessed the impact of abiotic stress on fusion and whether genetic relatedness between fusion partners influences survival during exposure to stress. Our results revealed variation in fusion frequency and morphology among different plasmodial lines and pairs of close relatives. Abiotic stress increased fusion rates between both clonal and compatible non-clonal pairs. However, the benefits of fusion did not depend on the whether fusion occurred between clones or non-clones. These findings demonstrate plasmodial fusion provides size-related survival benefits. Furthermore, the promotion of fusion in stressful conditions highlights its adaptive role in responding to environmental challenges. Collectively, these results underscore the evolutionary importance of fusion in Physarum, shaping its ecological success and resilience. Competing Interests StatementThe authors declare no competing financial interests.

evolutionary biology↗

bioRxiv2024

Natural variation in fruiting body morphology in the amoeba Dictyostelium discoideum

Reproductive altruism, where some individuals reproduce and others do not, is considered one of the pinnacles of cooperative societies. However, the optimal level of reproductive altruism is likely to depend on inclusive fitness considerations, including the relatedness of reproducing to non-reproducing individuals, as well as the benefits and costs accruing to each, respectively. In the social amoeba Dictyostelium discoideum, thousands of cells aggregate to form a multicellular fruiting body. During this process, some cells die, forming a rigid stalk that supports the rest of the cells, which become viable spores. The level of stalk investment by the social group can therefore be considered a metric of altruism investment. Importantly, genetically unrelated cells can co-aggregate to produce chimeric fruiting bodies, and selection can favour genotypes that behave selfishly by preferentially forming spores and avoiding forming the stalk. Owing to the extreme differences in fitness consequences of stalk cells versus spores, the level of altruism investment is likely to be under strong selection. Here we examined clonal fruiting body morphology in four natural populations to assess the extent to which stalk investment varies within populations and is maintained to different extents among populations. We found variation in fruiting body size and stalk investment, at both a cm-scale and between geographically isolated populations. These findings indicate the divergent evolution of altruism investment with and among populations and demonstrate widespread potential for cheating.

evolutionary biology↗

bioRxiv2024

Dominance hierarchies are linear but shallow in the social amoeba Dictyostelium discoideum.

Social groups often form dominance hierarchies, and these hierarchies are almost always linear. However, why linear dominance hierarchies emerge is not well understood. In the social amoeba Dictyostelium discoideum, cells form a multicellular fruiting body when starved, which consists of a ball of viable spores held aloft by a stalk of dead cells. In genetically mixed ( chimeric) fruiting bodies, conflicts can arise over the equitable sacrifice of cells to the dead stalk, and some strains predictably dominate others in the spores. Using pairwise mixes of strains that co-occurred in small soil samples, we determined the dominance hierarchies in four natural populations of Dictyostelium. These hierarchies were significantly linear in two of four populations, but also extremely shallow, indicating that co-occurring strains are competitively similar. We used quantitative genetic analyses to assess the causes of social dominance. Each strains solo spore production was a significant predictor of its performance in pairs. However, we detected additional genetic contributions of both the focal and partner strain, indicating additional cryptic traits that mediate social competitiveness. In contrast to earlier studies showing strong fitness differences among strains collected over a larger spatial scale, we show that co-occurring strains are remarkably competitively equivalent, resulting in linear yet shallow hierarchies. Our results underscore the importance of biologically relevant spatial scales in assessing fitness interactions among microbes. They also explain why social trait diversity might be observed despite dominance hierarchies that should eliminate this variation.

evolutionary biology↗