Ichiro Tamaki

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 (δ) 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 (~0.7) among populations, but the correlations of paternity, fractions of biparental inbreeding and inbreeding coefficients at the seed stage (FS) 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 FS and population size. The average value of δ was 0.709, and the values in six of the 10 populations were significant. The values of δ 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 δ (–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 δ 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.

Asymmetric introgression between Magnolia stellata and M. salicifolia at a site where the two species grow sympatrically

In order to understand the ongoing evolutionary relationships between species, it is important to elucidate patterns of natural hybridization. In the zone where two species are sympatrically distributed, we examined 274 individuals of Magnolia stellata, Magnolia salicifolia, and their putative hybrids by means of 16 nuclear and three chloroplast microsatellite markers. Hybrid classes of individuals were estimated by admixture analyses. Morphological traits were also investigated for 64 of the 274 individuals. Admixture analyses revealed that 66 of the 274 individuals were classified as hybrids, comprising 17 F1 and 19 F2 individuals, 27 backcrosses to M. salicifolia, and 3 individuals of unknown origin. Morphological data from the 64 individuals agreed well with their genetic admixture rates. Spatial locations of F1 and F2 hybrids at the study site were intermediate between the two purebred species, indicating that the site preferences of hybrids are intermediate. The occurrences of F2 and backcross hybrids indicate that F1 hybrids are fertile. The chloroplast DNA haplotypes of all F1 hybrids corresponded to those detected in M. salicifolia, so that maternal parents of the F1 hybrids were all M. salicifolia. Furthermore, no hybrid individuals derived from a backcross to M. stellata were detected. These results suggest that the direction of hybridization and the subsequent introgression have been quite asymmetric and that the introgression occurred from M. stellata into M. salicifolia.

Genetic diversity and structure of remnant Magnolia stellata populations affected by anthropogenic pressures and a conservation strategy for maintaining their current genetic diversity

Understanding the factors shaping rare species’ current genetic diversity and structure, particularly the impact of recent anthropogenic pressures, is important in order to develop appropriate conservation strategies based on robust predictions. Thus, we have genotyped all 585 surviving individuals of Magnolia stellata from six remnant populations and seven isolated tree sites in northern Mie Prefecture, Japan, using nuclear and chloroplast microsatellites. Three genetic clusters were detected by STRUCTURE analysis, with an oldest divergence time between pairs within 25 generations according to coalescent analysis. We attribute this recent divergence to recent anthropogenic environmental changes. Evidence of only one significant recent migration event between pairs of the six populations was detected, indicating that most of the remnant populations are isolated now. Their future genetic status was predicted using Monte Carlo simulations, under four scenarios. It declined more than twice as rapidly in a scenario assuming variations in fecundity among both female and male parents than in a scenario assuming no fecundity variations, but strongly improved in a scenario including promotion of migrations between adjacent pairs of populations. These predictions indicate that sexual reproduction of the species should be promoted by providing more suitable habitats and migration between populations should be enhanced by restoring remnant isolated tree sites and extinct populations. In addition, all the remnant populations should be conserved because they host current genetic variation that may be important for coping with future climate change, and they could provide important stepping-stones for gene flow.

Population demographic history of a temperate shrub, Rhododendron weyrichii (Ericaceae), on continental islands of Japan and South Korea

Abstract Continental islands provide opportunities for testing the effects of isolation and migration on genetic variation in plant populations. In characteristic of continental islands is that the geographic connections between these islands, which are currently distinguished by seaways, have experienced fluctuations caused by sea‐level changes due to climate oscillations during the Quaternary. Plant populations on the islands have migrated between these islands via the exposed seafloors or been isolated. Here, we examined the demographic history of a temperate shrub, Rhododendron weyrichii, which is distributed in the southwestern parts of the Japanese archipelago and on an island of South Korea, using statistical phylogeographic approaches based on the DNA sequences of two chloroplast and eight nuclear loci in samples analyzed from 18 populations on eight continental islands, and palaeodistribution modeling. Time estimates for four island populations indicate that the durations of vicariance history are different between these populations, and these events have continued since the last glacial or may have predated the last glacial. The constancy or expansion of population sizes on the Japanese islands, and in contrast a bottleneck in population size on the Korean island Jeju, suggests that these islands may have provided different conditions for sustaining populations. The result of palaeodistribution modeling indicates that the longitudinal range of the species as a whole has not changed greatly since the last glacial maximum. These results indicate that exposed seafloors during the glacial period formed both effective and ineffective migration corridors. These findings may shed light on the effects of seafloor exposure on the migration of plants distributed across continental islands.