Kinship between parents reduces offspring fitness in a natural population of Rhododendron brachycarpum

Abstract

Background and aims: A reduction in offspring fitness resulting from mating between neighbours is interpreted as biparental inbreeding depression. However, little is known about the relationship between the parents' genetic relatedness and biparental inbreeding depression in their progeny in natural populations. This study assesses the effect of kinship between parents on the fitness of their progeny and the extent of spatial genetic structure in a natural population of Rhododendron brachycarpum.

Methods: Kinship coefficients between 11,858 pairs of plants among a natural population of 154 R. brachycarpum plants were estimated a priori using six microsatellite markers. Plants were genotyped, and pairs were selected from among 60 plants to vary the kinship from full-sib to unrelated. After a hand-pollination experiment among the 60 plants, offspring fitness was measured at the stages of seed maturation (i.e. ripening) under natural conditions, and seed germination and seedling survival under greenhouse conditions. In addition, spatial autocorrelation was used to assess the population's genetic structure.

Key results: Offspring fitness decreased significantly with increasing kinship between parents. However, the magnitude and timing of this effect differed among the life-cycle stages. Measures of inbreeding depression were 0.891 at seed maturation, 0.122 (but not significant) at seed germination and 0.506 at seedling survival. The local population spatial structure was significant, and the physical distance between parents mediated the level of inbreeding between them.

Conclusions: The level of inbreeding between individuals determines offspring fitness in R. brachycarpum, especially during seed maturation. Genetic relatedness between parents caused inbreeding depression in their progeny. Therefore, biparental inbreeding contributes little to reproduction and instead acts as a selection force that promotes outcrossing, as offspring of more distant (less related) parents survive better.

Fig. 1.

Distribution of sampled individuals of Rhododendron brachycarpum in the study plot. The location of the plants sampled for hand-pollination treatments are shown as closed circles. The area surrounded by the thick grey line represents the ‘core area’ in which all flowering adults of the species were mapped. The open squares represent stations for researchers.

Fig. 2.

Fully mature seeds from outcross-pollinated fruits (A) and presumed aborted seeds from self-pollinated fruits (B) of Rhododendron brachycarpum. Scale bars = 5 mm.

Fig. 3.

Relationships between kinship coefficient and offspring fitness at three life-cycle stages: (A) seed maturation, (B) germination, and (C) seedling survival. Fitted curves predicted by the generalized linear mixed model are presented when the effect of the kinship coefficient was significant (see Table 2), with hand-self and hand-outcross treatments as indicated. Solid and dashed curves represent the results for all the analysed samples and for the samples after selfing was excluded, respectively. Dashed horizontal lines indicate the level of offspring fitness under open-pollinated conditions.

Fig. 4.

Spatial genetic structure and predicted inbreeding depression. (A) Correlogram for the kinship coefficient among pairs of Rhododendron brachycarpum individuals, expressed for nine distance classes (values on the x-axis represent the upper limit of each distance class). Dashed lines represent the 95 % confidence interval for the null hypothesis that assumes no genetic structure based on 9999 randomizations. Asterisks indicate significance (P < 0·05) for each distance class. (B) Predicted inbreeding depression as a function of the spatial genetic structure for stages.