Transcriptome profile of Dictyostelium discoideum slugs in clonal and chimeric development

Social interactions are typically characterised by conflict and competition. This antagonism can play a critical role in evolutionary processes, such as promoting diversity through maintenance of alternative strategies or driving accelerated evolution through arms-race like escalation. However, despite our sophisticated understanding of how conflict shapes social traits, we still have limited knowledge of how it impacts molecular evolution across the underlying ‘social genes’. To address this problem, we analysed the genome wide impact of social interactions in a microbe. To understand broad-scale processes shaping molecular evolution at social genes we have used four different, but complementary, approaches to identify sets of social genes. For ease, we have named these sets ‘sociality’, ‘chimerism’, ‘antagonism’ and ‘cheater’ genes. Chimerism genes are defined as those up-regulated in chimeric aggregations in comparison to clonal aggregations. This is based on the logic that chimeric development will be characterised by conflict, and hence these genes will show the signatures of conflict driven evolution. Using genome sequences from 67 Dictyostelium discoideum strains, we find that social genes often exhibit enhanced polymorphism and accelerated evolution. However, these patterns are not consistent with the expectation of conflict driven processes, but instead reflect relatively relaxed purifying selection. This pattern reflects the fact that social interactions are conditional, and therefore selection on genes expressed in social interactions is diluted by generations of inactivity. This results in the ‘Red King’ process, wherein dilution of selection by inactivity enhances the role of drift, resulting in increased polymorphism and accelerated evolution. Two conditions (clonal development of three strains: NC34.2, NC58.1 and NC87.1 vs chimeric development of 50%:50% pairwise mixtures : NC34.2+NC58.1, NC34.2+NC87.1 and NC87.1+NC58.1). Aggregates isolated at 14hrs, when slugs had formed. Two biological replicates for sample in each condition.