Effects of inbreeding on the magnitude of inbreeding depression in a highly self-fertilizing tree, Magnolia obovata

Somatic Mutation and Evolution in Plants

Somatic mutations are common in plants, and they may accumulate and be passed on to gametes. The determinants of somatic mutation accumulation include the intraorganismal selective effect of mutations, the number of cell divisions that separate the zygote from the formation of gametes, and shoot apical meristem structure and branching. Somatic mutations can promote the evolution of diploidy, polyploidy, sexual recombination, outcrossing, clonality, and separate sexes, and they may contribute genetic variability in many other traits. The amplification of beneficial mutations via intraorganismal selection may relax selection to reduce the genomic mutation rate or to protect the germline in plants. The total rate of somatic mutation, the distribution of selective effects and fates in the plant body, and the degree to which the germline is sheltered from somatic mutations are still poorly understood. Our knowledge can be improved through empirical estimates of mutation rates and effects on cell lineages and whole organisms, such as estimates of the reduction in fitness of progeny produced by within- versus between-flower crosses on the same plant, mutation coalescent studies within the canopy, and incorporation of somatic mutation into theoretical models of plant evolutionary genetics.

Sex expression and inbreeding depression in progeny derived from an extraordinary hermaphrodite of Salix subfragilis

BACKGROUND

An extraordinary hermaphrodite of dioecious willows provides us an opportunity to examine the inheritance of sex expression and the magnitude of inbreeding depression using a progeny assay of the hermaphrodite.

RESULTS

We indentified 165 progeny of an open-pollinated hermaphrodite of Salix subfragilis as siblings selfed (Self) or crossed with another hermaphrodite (Cross_H) or a male (Cross_M) using microsatellite genotypes. There were more selfed progeny (110 in Self) than outcrossed progeny (31 in Cross_H and 24 in Cross_M), suggesting the absence of barriers to selfing in the maternal hermaphrodite. The sex ratio (female:male:hermaphrodite) of the progeny differed among the sibling groups (27:17:66 in Self, 3:16:12 in Cross_H and 9:8:7 in Cross_M). Nearly half of the selfed progeny were hermaphrodites, suggesting that an identical combination of parental alleles in progeny reproduced the hermaphroditism of the parent. We measured fitness components of growth (stem height and basal area), survival and fertility (pollen germination proportion, number of ovules and seed set). The magnitudes of inbreeding depression in growth and survival (0.29-0.70) were higher than those in fertility (0.00-0.16).

CONCLUSIONS

The findings suggest a genetic basis of extraordinary hermaphroditism and substantial inbreeding depression in survival and growth in the dieocious S. subfragilis.

Interpopulation variation in mating system and late-stage inbreeding depression in Magnolia stellata

Inbreeding has the potential to cause evolutionary changes in populations, although these changes are likely to drive populations to extinction through inbreeding depression and reductions in genetic diversity. We investigated the mating system and late-stage inbreeding depression (delta) in 10 populations of Magnolia stellata using nine microsatellite markers and evaluated the effects of population size and the degree of population isolation through inbreeding and inbreeding depression on the persistence of populations. The outcrossing rates were very similar (approximately 0.7) among populations, but the correlations of paternity, fractions of biparental inbreeding and inbreeding coefficients at the seed stage (F(S)) varied among populations, suggesting that the level of outcrossing was similar among populations, while the quality of it was not. A significant negative correlation was detected between F(S) and population size. The average value of delta was 0.709, and the values in six of the 10 populations were significant. The values of delta differed among populations, although clear relationships with population size and the degree of population isolation were not detected. However, in one population, which was very small and located in the edge of the species' range, we obtained a very low value of delta (-0.096), which may be indicative of purging or the fixation of deleterious alleles. Existing M. stellata populations that are small (and thus might be expected to have higher frequencies of inbreeding) and have large values of delta may be in danger of declining, even if the populations are located within the central region of the species' range.

Estimation of outcrossing rates at hierarchical levels of fruits, individuals, populations and species in Magnolia stellata

In plant species with mixed mating systems, differences in diverse factors-including their pollination system, flowering phenology, life form and susceptibility to inbreeding depression-cause variation in outcrossing rates among fruits within individuals, among individuals within populations and among populations within species. To quantify this hierarchical variation, we examined outcrossing rates at the seed stage in five populations of Magnolia stellata, a self-compatible, insect-pollinated and protogynous tree species. For this purpose, we sampled 1498 seeds within 204 fruits obtained from 56 individuals of the five populations, determined genotypes of the sampled seeds and maternal individuals at six polymorphic microsatellite loci, then estimated outcrossing rates and their variance components at four hierarchical levels (fruits, individuals, populations and species) using a nested analysis of variance-type linear model with a Bayesian approach. The species-level outcrossing rate was 0.730 (95% credible interval, 0.595-0.842), indicating that this species has a mixed mating system. Outcrossing rates were not significantly different among populations, but were significantly different among individuals within populations. Variance components at the levels of individual and fruit were statistically supported and were highest for the former. Thus, factors influencing outcrossing rates at the individual level, such as differences in flowering phenology and early-stage inbreeding depression, appear to have important effects within these M. stellata populations, but not among them. The method of hierarchically estimating outcrossing rates using a Bayesian approach, as applied in this study, is compared with conventional methods for estimating outcrossing rates, and the statistical properties of the Bayesian approach are discussed.