It is unclear how mutualistic relationships can be stable when partners disperse freely and have the possibility of forming associations with many alternative genotypes. Theory predicts that high symbiont relatedness should resolve this problem, but the mechanisms to enforce this have rarely been studied. We show that African fungus-growing termites propagate single variants of their Termitomyces symbiont, despite initiating cultures from genetically variable spores from the habitat. High inoculation density in the substrate followed by fusion among clonally related mycelia enhances the efficiency of spore production in proportion to strain frequency. This positive reinforcement results in an exclusive lifetime association of each host colony with a single fungal symbiont and hinders the evolution of cheating. Our findings explain why vertical symbiont transmission in fungus-growing termites is rare and evolutionarily derived.

BackgroundOne of the big remaining challenges in evolutionary biology is to understand the evolution and maintenance of meiotic recombination. As recombination breaks down successful genotypes, it should be selected for only under very limited conditions. Yet, recombination is very common and phylogenetically widespread. The Red Queen Hypothesis is one of the most prominent hypotheses for the adaptive value of recombination and sexual reproduction. The Red Queen Hypothesis predicts an advantage of recombination for hosts that are coevolving with their parasites. We tested predictions of the hypothesis with experimental coevolution using the red flour beetle, Tribolium castaneum, and its microsporidian parasite, Nosema whitei.ResultsBy measuring recombination directly in the individuals under selection, we found that recombination in the host population was increased after 11 generations of coevolution. Detailed insights into genotypic and phenotypic changes occurring during the coevolution experiment furthermore helped us to reconstruct the coevolutionary dynamics that were associated with this increase in recombination frequency. As coevolved lines maintained higher genetic diversity than control lines, and because there was no evidence for heterozygote advantage or for a plastic response of recombination to infection, the observed increase in recombination most likely represented an adaptive host response under Red Queen dynamics.ConclusionsThis study provides direct, experimental evidence for an increase in recombination frequency under host-parasite coevolution in an obligatory outcrossing species. Combined with earlier results, the Red Queen process is the most likely explanation for this observation.

The extreme environmental conditions that prevail in cities are known to cause selection pressures leading to adaptive changes in wild, city-dwelling, organisms (urban evolution). The urban heat island, elevated temperatures in the city centre due to a combination of generation, reflection, and trapping of heat, is one of the best recognised and most widespread urban environmental factors. Here, we use a citizen-science approach to study the effects of urban heat on genetically-determined shell colour in the land snail Cepaea nemoralis in the Netherlands. We use smartphone applications to obtain colour data on almost 8000 snails throughout the country. Our analysis shows that snails in urban centres are more likely to be yellow than pink, an effect predicted on the basis of thermal selection. Urban yellow snails are also more likely to carry dark bands at the underside of the shell; these bands might affect thermoregulation in yet underexplored ways.

12The extreme environmental conditions that prevail in cities are known to cause selection pressures leading to 13 adaptive changes in wild, city--dwelling, organisms ("urban evolution"). The urban heat island, elevated 14 temperatures in the city centre due to a combination of generation, reflection, and trapping of heat, is one of the 15 best recognised and most widespread urban environmental factors. Here, we used a citizen--science approach to 16 study the effects of urban heat on genetically--determined shell colour in the land snail Cepaea nemoralis in the 17Netherlands. We used smartphone applications to obtain colour data on almost 8,000 snails throughout the 18 country. Our analysis shows that snails in urban centres are more likely to be yellow than pink, an effect predicted 19 on the basis of thermal selection. Urban yellow snails are also more likely to carry dark bands at the underside of 20 the shell; these bands might affect thermoregulation in yet underexplored ways. 21 22

BackgroundCoevolution with parasites and population size are both expected to influence the evolution of mating rates. To gain insights into the interaction between these dual selective factors, we used populations from a coevolution experiment with the red flour beetle, Tribolium castaneum, and its microsporidian parasite, Nosema whitei. We maintained each experimental population at two different population sizes. We assayed the mating behaviour of both males and females from coevolved and paired non-coevolved control populations after 24 generations of coevolution with parasites.ResultsMales from large, coevolved populations (i.e. ancestors were exposed to parasites) showed a reduced eagerness to mate compared to males from large, non-coevolved populations. But in small populations, coevolution did not lead to decreased male mating rates. Coevolved females from both large and small populations appeared to be more willing to accept mating than non-coevolved females.ConclusionsThis study provides unique, experimental insights into the combined roles of coevolving parasites and population size on the evolution of mating rate. Furthermore, we find that males and females respond differently to the same environmental conditions. Our results show that parasites can be key determinants of the sexual behaviour of their hosts.