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Electronic supplementary material is available online at https://dx.Life is built on cooperation between genes, which makes it vulnerable to parasitism. Selfish genetic elements that exploit this cooperation can achieve large fitness gains by increasing their transmission relative to the rest of the genome. This leads to counter-adaptations that generate unique selection pressures on the selfish genetic element. This arms race is similar to hostparasite coevolution, as some multi-host parasites alter the host's behaviour to increase the chance of transmission to the next host. Here, we ask if, similarly to these parasites, a selfish genetic element in house mice, the t haplotype, also manipulates host behaviour, specifically the host's migration propensity. Variants of the t that manipulate migration propensity could increase in fitness in a meta-population. We show that juvenile mice carrying the t haplotype were more likely to emigrate from and were more often found as migrants within a long-term free-living house mouse population. This result may have applied relevance as the t has been proposed as a basis for artificial gene drive systems for use in population control.

Endocrine data from wild populations provide important insight into social systems. However, obtaining samples for traditional methods involves capture and restraint of animals, and/or pain, which can influence the animal’s stress level, and thereby undesirable release of hormones. Here, we measured corticosterone, testosterone and progesterone in the hair of 482 wild-derived house mice that experienced sexual competition while living under semi-natural conditions. We tested whether sex, age, weight and indicators of sexual maturity, reproduction and social conflicts predict hormone concentrations measured in hair (sampling at endpoint). We show that body weight, sex and age significantly predict cumulative testosterone and progesterone levels, allowing the differentiation between subadults and adults in both sexes. Corticosterone was only slightly elevated in older males compared to older females and increased with the level of visible injuries or scars. Testosterone in males positively correlated with body weight, age, testes size, and sperm number. Progesterone in females significantly increased with age, body weight, and the number of embryos implanted throughout life, but not with the number of litters when controlled for age and weight. Our results highlight the biological validity of hair steroid measurements and provide important insight into reproductive competition in wild house mice.

Meiotic drivers are genetic entities that increase their own probability of being transmitted to offspring, usually to the detriment of the rest of the organism, thus ‘selfishly’ increasing their fitness. In many meiotic drive systems, driver-carrying males are less successful in sperm competition, which occurs when females mate with multiple males in one oestrus cycle (polyandry). How do drivers respond to this selection? An observational study found that house mice carrying the t haplotype, a meiotic driver, are more likely to disperse from dense populations. This could help the t avoid detrimental sperm competition, because density is associated with the frequency of polyandry. However, no controlled experiments have been conducted to test these findings. Here, we confirm that carriers of the t haplotype are more dispersive, but we do not find this to depend on the local density. t -carriers with above-average body weight were particularly more likely to disperse than wild-type mice. t -carrying mice were also more explorative but not more active than wild-type mice. These results add experimental support to the previous observational finding that the t haplotype affects the dispersal phenotype in house mice, which supports the hypothesis that dispersal reduces the fitness costs of the t .

8Life is built on cooperation between genes, which makes it vulnerable to par-9 asitism. However, selfish genetic elements that exploit this cooperation can 10 achieve large fitness gains by increasing their transmission unfairly relative to 11 the rest of the genome. This leads to counter-adaptations that generate unique 12 selection pressures on the selfish genetic element. This arms race is similar to 13 host-parasite co-evolution. Some multi-host parasites alter the host's behaviour 14 to increase the chance of transmission to the next host. Here we ask if, similarly 15 to these parasites, a selfish genetic element in house mice, the t haplotype, also 16 manipulates host behaviour, specifically the host's migration propensity. Vari-17 ants of the t that manipulate migration propensity could increase in fitness in 18 a meta-population. We show that juvenile mice carrying the t haplotype were 19 more likely to emigrate from and were more often found as migrants within a 20 long-term free-living house mouse population. This result may have applied rel-21 evance as the t has been proposed as a basis for artificial gene drive systems for 22 use in population control. 23 25 interests are not identical [1]. This causes conflict, because not all genes in an organism 26 will be transmitted in equal numbers to the next generation. Consequently, a fair chance of 27 2 transmission is necessary for cooperation within the genome over evolutionary time. Genes 28 that violate this rule by unfairly increasing their chance of transmission can gain large fitness 29 advantages at the detriment of those that act fairly [2]. This leads to selection for selfish 30 adaptations and, as a result, counter-adaptations to this selfishness, initiating an arms race 31 between these selfish genetic elements and the rest of the genome. This arms race is similar to 32 the one between hosts and parasites, where some parasites even manipulate their hosts. For 33 example, rats infected with the multi-host parasite Toxoplasma gondii show decreased fear of 34 cat odour [3]. This is expected to increase the risk of predation by cats, the final host of T. 35 gondii, thereby increasing the transmission of the parasite. The parasite furthermore increases 36 the attractiveness of its host, circumventing the avoidance of the infected individual by other 37 rats [3]. Similar manipulations have been observed, for example, in fungi-infected ants that 38 climb vegetation and remain latched onto it post-mortem, which leaves their infested bodies 39 conspicuous to predators [4]. 40Host defences against parasites and "parasitic" [5,6] selfish genetic elements range from 41 behavioural changes to increased resistance in infected populations. For example, populations 42 of the amphipod Gammarus pulex that are not naturally infected with the parasite Pom-43 phorhynchus laevis are more sensitive to the parasite's manipulation than naturally infected 44 populations [7]. This is evidence of an arms race. A similar counter-adaptation to selfish ge-45...