Parental care maintains genetic variation by relaxing selection

21 Benevolent social behaviours, such as parental care, are predicted to relax selection against 22 deleterious mutations, enabling them to persist. We tested this prediction experimentally 23 using burying beetles Nicrophorus vespilloides , which make an edible nest for their larvae, 24 whom they nourish and defend. For 20 generations, we allowed replicate experimental 25 burying beetle populations to evolve either with post-hatching care (‘Full Care’ populations) 26 or without it (‘No Care’ populations). Lineages were seeded from these experimental 27 populations and then inbred to expose differences in their mutation load. Outbred lineages 28 served as controls. Half the lineages received post-hatching care, half did not. We found that 29 inbred lineages derived from the Full Care populations had lower breeding success and went 30 extinct more quickly than lineages derived from the No Care populations – but only when 31 offspring received no post-hatching care. We infer that Full Care lineages carried more 32 recessive deleterious mutations. When parents provided care, the developmental environment 33 was sufficiently benign that broods had higher survival, whether the population had a high 34 mutation load or not. We suggest that the increased mutation load caused by parental care 35 increases a population’s dependence upon care. This could explain why care is seldom lost 36 once it has evolved. 37


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Classical population genetics models imagine that populations attain an equilibrium level of 40 genetic variation (known as mutation-selection balance [1][2][3][4][5]). New genetic mutations arise 41 spontaneously, through diverse mechanisms, and increase genetic variation in the population 42 [e.g. 5, 6]. However, since the majority of new mutations are mildly deleterious [e.g. 5, 6], 43 they are quickly purged by natural selection. Mutation-selection balance is theoretically 44 achieved when the rate of input of new genetic variants through spontaneous mutation is 45 perfectly balanced by the rate of their elimination by selection [1][2][3][4][5].

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The concept of mutation-selection balance has long been used as a theoretical reference point 48 for understanding the effects of mutation rate on the health of human populations, partly 49 because it is recognised that humans can modify their own environment and so change the 50 forces of natural selection to which they are exposed [1][2][3][4][5]. Better quality housing, improved 51 diets, and benevolent social activities, such as a welfare state or the universal provision of 52 medical care, are suggested to have been particularly influential in preventing natural 53 selection from purging deleterious mutations in human populations [1,2,4,5]. Consistent 54 with this suggestion, recent comparative genomic analyses have revealed a greater incidence 55 of genetic pathologies in western industrialised populations than in traditional, pre-industrial 56 human societies which are more exposed to natural selection [4,5,[7][8][9]. Nevertheless, it is 57 impossible to demonstrate that a more benign physical and social environment, in which 58 selection is relaxed, has caused this difference. insect species that cooperate with each other and live socially [10]. For these species, any 63 causal effects of this social and physical environment on genetic variation can more easily be 64 investigated. A complication in other animal societies, however, is that additional factors 65 might perturb the mutation-selection balance. For example, animals that breed cooperatively 66 also tend to produce fewer, larger offspring. This life history strategy is known to reduce 67 genetic diversity [11] and could potentially oppose, or even conceal, any increases in genetic 68 variation that are due to cooperation buffering the effects of natural selection. Cooperative 69 animal societies are also commonly associated with a high incidence of reproductive skew. 70 Since only a few dominant individuals are typically able to reproduce, the effective 71 population size is greatly reduced [12]. This can lead to a reduction in the efficiency of natural 72 selection and a greater influence of genetic drift [6], potentially confounding any increases in 73 genetic variation that are due solely to relaxed selection. Similarly, animal societies typically 74 comprise related individuals that derive kin-selected benefits from their cooperative social 75 interactions. Theoretical analyses have shown that kin selection acts more weakly than direct 76 selection [13]. Consequently, loci under kin selection are predicted to harbour more sequence 77 variation than loci under direct selection [3,13].

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We tested the effect of kin-selected cooperative actions on the maintenance of genetic 80 variation by focusing on parental care, a widespread form of cooperation [14]. Since care is 81 commonly exhibited by pair-breeding individuals, this form of cooperation is unlikely to 82 change effective population size -eliminating this potentially confounding effect. By building 83 protective nests, defending their brood from attack and nourishing them, animal parents shield 84 their young from environmental stressors [15] and weaken the correlation between the 85 phenotypic variation seen by selection and the underlying genetic variation [3,16]. In these 86 ways, parents relax selection on the offspring phenotype [3,[15][16][17][18]  half of all our treatments parents were allowed to provide care after their offspring hatched, 115 while in the remainder they were prevented from supplying post-hatching care. This    hatching care as well as pre-hatching care. In the other two 'No Care' lines, parents engaged 148 in pre-hatching care but at each generation they were removed from the breeding box around 149 53 h after they were paired, so that they never interacted with their larvae. The work reported 150 here began when these lines had been exposed to 20 generations of experimental evolution 151 under these contrasting regimes of care. week with raw beef mince. Adults were bred at 2-3 weeks post-eclosion in a breeding box (17 183 x 12 x 6cm) with soil and a mouse carcass (11-13 g for all treatments except for the 184 individuals derived from the Full Care lines, that were outbred under Full Care conditions (8-185 14 g)). To ease the considerable burden of work, data for broods in this treatment were 186 collected from the ongoing experimental evolution lines in the laboratory. Carcass size was 187 included, where appropriate, as a factor in the statistical analyses (see below).

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For the inbreeding treatments, we paired full siblings (one pair per family) whereas for the 189 outbreeding treatments we paired males and females at random and did not pair siblings or  (1) and (2)   210 To test predictions (1) and (2)

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To test predictions (1) and (2) we initially focused on the data collected from the first 249 generation of breeding in the Evolutionary History Experiment. In support of prediction (1), 250 we found that exposure to a No Care environment reduced reproductive success, regardless of 251 the evolutionary history of the lineage (Figure 1, Table 1). However, in support of prediction

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(2), we found that a supply of post-hatching care enabled more broods to survive, even if they 253 12 were inbred -and regardless of the evolutionary history of their lineage (Figure 1, Table 1 To test prediction (3), we continued to examine inbred families in the first generation of 257 breeding in the Evolutionary History Experiment. In this generation, we found an interaction 258 between evolved history and the current environment in inbred but not outbred lineages 259 (Table 2). We split the dataset by the current level of care supplied, to be able to examine the  (Table 2).   By supplying care, parents shield their young from relatively harsh environmental conditions: 306 larvae receiving parental care had higher survival than those that had no care. Indeed, we 307 found that when parents provided care, the developmental environment was sufficiently 308 benign, and the strength of selection then sufficiently weak, that diverse genetic variants were 309 able to survive -even those that were inbred, just as previous work has shown [19].  This suggests that some of the additional mutations present in the Full Care populations were 317 recessive and / or only mildly deleterious [5]. Given the relatively short timeframe of this 318 experiment, we presume that these mutations were present in the founding populations of 319 wild-caught beetles but were removed from the No Care populations by selection acting more 320 strongly against them. In this sense, our findings are similar to previous work on Tribolium 321 which found that deleterious genetic variation was purged when populations were exposed 322 experimentally to more intense sexual selection [30].

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Although it is now well-understood why individuals evolve cooperative behaviour, the 325 mechanisms that cause cooperation to persist and diversify remain relatively unclear [31].