Permissive aggregative group formation favors coexistence between cooperators and defectors in yeast

Abstract

In Saccharomyces cerevisiae, the FLO1 gene encodes flocculins that lead to formation of multicellular flocs, that offer protection to the constituent cells. Flo1p was found to preferentially bind to fellow cooperators compared to defectors lacking FLO1 expression, resulting in enrichment of cooperators within the flocs. Given this dual function in cooperation and kin recognition, FLO1 has been termed a 'green beard gene'. Because of the heterophilic nature of Flo1p binding however, we hypothesize that kin recognition is permissive and depends on the relative stability of FLO1 + /flo1 − versus FLO1 + /FLO1 + bonds, which itself can be dependent on environmental conditions and intrinsic cell properties. We combine single cell measurements of adhesion strengths, individual cell-based simulations of cluster formation and evolution, and in vitro flocculation experiments to study the impact of relative bond stability on defector exclusion as well as benefit and stability of cooperation. We hereto vary the relative bond stability by changing the shear flow rate and the inherent bond strength. We identify a marked trade-off between both aspects of the green beard mechanism, with reduced relative bond stability leading to increased kin recognition, but at the expense of decreased cluster sizes and benefit of cooperation. Most notably, we show that the selection of FLO1 cooperators is negative-frequency dependent, which we directly attribute to the permissive character of the Flo1p bond. Taking into account the costs associated to FLO1 expression, this asymmetric selection results in a broad range of ecological conditions where coexistence between cooperators and defectors is stable. Although the kin recognition aspect of the FLO1 'green beard gene' is thus limited and condition dependent, the negative-frequency dependency of selection can conserve the diversity of flocculent and non-flocculent phenotypes ensuring flexibility towards variable selective pressures.studying ST 9-11 and CT 12-14 group formation, although S. cerevisiae does not have any known obligate multicellular descendants 4 . A key gene family involved in group formation in yeast comprises the FLO genes, which encode for flocculins, proteins involved in cell adhesion 4,12,[15][16][17][18][19] . These flocculins possess an N-terminal domain protruding from the cell surface, a central domain of tandem repeated sequences, and a C-terminal glycosylphosphatidylinositol (GPI) domain anchored in the cell wall 15,20 . Based on the N-terminal domain, two types of flocculins can be distinguished. Flo11p harbors a fibronectin type III-like domain that confers homophilic protein-protein interaction with neighbouring cells 11,21 . Flo11p-mediated adhesion partakes in multiple ST group phenotypes such as biofilm 11,22 and pseudohyphae formation 23 . In contrast, the FLO1 gene encodes for a PA14-like N-terminal domain that binds to mannose residues on the cell wall of neighbouring cells, a mechanism that is heterophilic in nature 24,25 . Flo1p controls the CT flocculatio...

The influence of flocculation upon global gene transcription in a yeast CYC8 mutant

“…This feedback could therefore act to ensure the optimal level of flocculation is achieved once the cells are triggered to embark on what should be a survival strategy and not a cause of further stress. Indeed, this regulation could act to balance the cost-benefit ratio of the flocculation survival strategy [72,73].…”

The role of recognition error in the stability of green-beard genes

“…It is not mechanistically impossible for yeast cells to adopt more reliable means of aggregation. For example, FLO11 , one of the flocculin-encoding genes along with FLO1 , has the trait of homophilic adhesion ( Belpaire et al, 2022 ; Brückner et al, 2020 ). Hence, the adoption of FLO11 rather than FLO1 as the primary adhesion molecule would enhance the accuracy of the green-beard recognition.…”

The effect of recognition on competition among clones in spatially structured microbial communities

“…It is within this context that it is possible to ask whether a recognition mechanism in which cooperators only provide growth benefits to other cooperators might provide a fitness advantage, which was tested in our greenbeard and non-cooperator simulations. For biofilms, these simulations capture the idea of only adhering to cells also expressing adhesins (Smukalla et al, 2008; Belpaire et al, 2022). And finally, we ask if restrictive cooperation in which cooperators provide growth benefits exclusively to specific cooperator types might provide a further fitness advantage, tested in our polychromatic greenbeard simulations.…”

“…In this first set of simulations, we also investigated greenbeard cooperation in a community with non-cooperators. In microbes, this kind of cooperation is usually in the form of cell-cell adherence, and has been investigated experimentally and computationally (Xavier and Foster, 2007; Smukalla et al, 2008; Nadell et al, 2010; Schluter et al, 2015; van Gestel and Wagner, 2021; Belpaire et al, 2022). Once again, our simulations qualitatively confirm findings from these studies: the ability to adhere selectively promotes spatial segregation and can even be used as a weapon against non-adhering strains.…”

“…For a greenbeard system to function, there must be relatedness at the locus, but not necessarily across the entire genome. Microbial examples include cell adhesion proteins that allow cells to attach to one another, as has been reported in yeast and social amoebae (Queller et al, 2003;Smukalla et al, 2008;Strassmann et al, 2011;Belpaire et al, 2022).…”