Data from: Inbreeding depression at the sapling stage and its genetic consequences in a population of the outcrossing dominant tree species, Castanopsis sieboldii

Highly frequent pollination at short distances combined with spatial genetic structure (SGS) among reproductive individuals, as observed in populations of many forest tree species, may lead to highly frequent pollinations between genetically related individuals located close to each other. This could increase frequencies of biparental inbreeding and strengthen the degree of SGS across generations. However, the degree of SGS observed in forest tree populations is not usually strong, suggesting that such processes are restricted by other factors, which may include inbreeding depression. To test this hypothesis, we examined inbreeding depression in Castanopsis sieboldii saplings in a population known to have significant SGS and high frequencies of short-distance pollination. We determined genotypes at eight microsatellite loci of 125 adult trees and 899 saplings in a 4-ha plot. The neighborhood model approach, assuming an exponential power function, showed highly frequent pollination at short distance among parents of saplings. SGS was significantly stronger among saplings than among adults. The average value of F is (individual inbreeding coefficient based on kinship coefficients between genes within individuals) was also significantly higher among saplings than among adults. Furthermore, average values for saplings were relatively high (0.036–0.052) in 30–50 to 90–110 cm height classes and rapidly decreased to 0.006 for the ≥110-cm class. Analysis of covariance, taking into account the canopy state, showed that inbreeding depression had a significant negative effect on the saplings’ height. Thus, the level of inbreeding in saplings decreased with size classes increasing, probably due to pronounced inbreeding depression resulting in mortality or reduced growth of inbred saplings, and hence, the inbreeding depression may prevent increasing of the level of inbreeding and the degree of SGS in subsequent generations.许多森林树种种群中已观测到，短距离高频授粉与繁殖个体间的空间遗传结构（SGS）相结合，可能导致近距离的遗传相关个体间发生高频授粉事件。这会提升双亲近交的发生频率，并跨世代强化空间遗传结构的程度。然而，林木种群中观测到的空间遗传结构通常并不强烈，这表明这类过程受到其他因素的限制，其中可能包括近交衰退。为验证这一假说，我们对存在显著空间遗传结构且短距离授粉频率较高的日本锥栗（Castanopsis sieboldii）种群的幼树近交衰退情况展开了研究。我们对一块4公顷样地内的125株成年植株与899株幼树的8个微卫星位点进行了基因分型。采用基于指数幂函数的邻域模型方法，结果显示幼树亲本间存在高频的短距离授粉现象。幼树群体的空间遗传结构显著强于成年植株群体。幼树的平均Fis值（基于个体内基因间亲缘系数计算得到的个体近交系数）同样显著高于成年植株。此外，在树高30~50 cm至90~110 cm的龄级中，幼树的平均Fis值相对较高（0.036~0.052），而在≥110 cm的龄级中则快速降至0.006。考虑冠层状态的协方差分析结果显示，近交衰退对幼树树高具有显著的负向影响。综上，幼树的近交水平随龄级增大而降低，这可能是因为显著的近交衰退导致近交幼树死亡或生长受阻，因此近交衰退或可阻止后续世代近交水平与空间遗传结构程度的上升。