Kinship between Mates Reduces Offspring Fitness in a Natural Population of 1

1 Background and Aims Reduction in offspring fitness resulting from mating between 2 neighbors is interpreted as biparental inbreeding depression. However, little is known 3 about the relationship between the genetic relatedness of parents and the biparental 4 inbreeding depression in their progeny in natural populations. Here, the effect of kinship 5 between mates on the fitness of their progeny and the extent of spatial genetic structure 6 in a natural population of Rhododendron brachycarpum were assessed. 7 Methods The kinship coefficients between 11782 pairs among 154 plants were 8 estimated a priori using six microsatellite markers in a natural population of R. 9 brachycarpum. Pairs of individuals were selected from the genotyped plants to vary the 10 level of the kinship coefficient from self-pollination to related plants and then to 11 unrelated plants. After a hand-pollination experiment among the 60 selected plants, the 12 offspring’s fitness was measured at the following stages: seed maturation under natural 13 condition, and seed germination and seedling survival under greenhouse. In addition, 14 spatial autocorrelation analysis was performed to assess the genetic structure of the 15 population. 16 Key Results The offspring’s fitness decreased significantly with increasing kinship 17 between mates. However, the magnitude and timing of this effect was not uniform 18 among the life-cycle stages. The standard measurements of inbreeding depression were 19 0.891 for seed maturation, 0.122 but not significant for seed germination, and 0.506 for 20 seedling survival. Local population structure was significant, and the physical distance 21 between parents mediated the level of inbreeding between mates. 22 Conclusions The level of inbreeding between mating individuals is a key determinant in 23 offspring fitness in R. brachycarpum, especially during seed maturation. Genetic 24