Abstract Pandanus boninensis , endemic to the Ogasawara Islands, Japan, is distributed on both the older Bonin and younger Volcano Islands. In this study, we conducted population genetic analyses of P. boninensis on these islands to examine the population diversity and structure across old and young islands, to assess potential differences in population demography with island age, and to collect any evidence of migration between old and young islands. We found that the genetic diversity of expressed sequence tag (EST)–based microsatellite (SSR) markers, the nucleotide diversity of nuclear DNA sequences, and the haplotype diversity of chloroplast DNA on young islands were lower than those on old islands. Clustering analyses of EST‐SSR indicated that populations on old islands were strongly diverged from those on young islands. Approximate Bayesian computation analysis of EST‐SSR suggested that population expansion occurred on old islands while population reduction occurred on young islands. We also found evidence of migration among old islands (mostly from south to north), while it appears that there have been very few migration events between old and young islands. These differences could be due to the fact that young islands tend to be geographically isolated and support smaller populations that began a shorter time ago from limited founders. The P. boninensis populations on the Volcano Islands are interesting from an evolutionary perspective as they constitute a classic example of the early stages of progressive colonization on oceanic islands with small effective population sizes and low genetic diversity.
Hybrid zones are areas where gene flow between related species is currently occurring, so information on the compatibility between related species and their hybrids is essential for predicting the dynamics of such zones generated by introgressive hybridization. In this study, we quantified the compatibility among Magnolia stellata, M. salicifolia, and their hybrids in a hybrid zone using gene dispersal modeling.After determining the genealogical classes of adult trees in the hybrid zone, the paternity of 574 open-pollinated seeds from 37 known maternal trees was analyzed with microsatellite markers. A neighborhood-based Bayesian gene dispersal model developed by us for estimating compatibility was then applied to the paternity data.When M. stellata or M. salicifolia were mothers, interspecific mating to produce F1 hybrids yielded significant incompatibility, but backcrossing with F1 hybrids did not. Furthermore, when F1 hybrids became mothers, no significant incompatibility resulted from backcrossing to parental species or intra-F1 mating to produce F2 hybrids. The estimated proportion of F1 hybrids in the outcrossed seeds (1.7%) in the hybrid zone was much lower than that in the adult trees (14.0%).While it is difficult to obtain F1 hybrids, their low incompatibility makes it easy to produce advanced generation hybrids, once they have been successfully obtained. Although the production of F1 seeds is rare, heterosis and/or weak selection pressure in an empty niche between the parental species' niches may have contributed to the increased proportion of adult F1 hybrids in the hybrid zone.
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.
Abstract Vicia sepium (bush vetch) is a perennial legume widely distributed throughout the Eurasian continent. However, its distribution in Japan is limited to Mt. Ibuki and small parts of central and southern Hokkaido. Therefore, each Japanese V. sepium lineage has been considered to have been introduced separately from Europe. Here, we examined whether the species was introduced or not on the basis of cpDNA sequences and genome-wide SNPs from Japanese and overseas samples. Both the cpDNA haplotype network and the nuclear DNA phylogenetic tree showed that Japanese V. sepium is monophyletic. Furthermore, although the nuclear DNA phylogenetic tree also showed that each lineage is clearly monophyletic, genetic admixture of the genetic cluster dominated in the Hokkaido lineage was also detected in the Mt. Ibuki lineage. Population divergence analysis showed that the two lineages diverged during the last glacial period. The Mt. Ibuki lineage showed a sudden population decline 300–400 years ago, indicating that some anthropogenic activity might be involved, while the Hokkaido lineage showed a gradual population decline from 5000 years ago. Consequently, these two lineages show low current genetic diversity compared with overseas lineages. These results show that the Japanese V. sepium is not introduced but is native.
Abstract Aim Islands are ‘nature's laboratories of evolution’. Most island biogeographical studies have focussed on remote volcanic island chains. Here, we challenge island biogeographical patterns using a slowly evolving recent colonist present on five islands in a nonlinear chronosequence island chain. Location The Lesser Antilles (Caribbean). Taxon Magnolia dodecapetala (Magnoliaceae). Methods Genetic diversity was characterised using Sanger sequencing of 21 individuals amplified for 11 DNA markers, plus microsatellite data of 195 individuals genotyped with 19 simple sequence repeat (SSR) markers. Sanger sequencing data were used to construct a Bayesian phylogenetic hypothesis, while SSR markers were used to run approximate Bayesian computation (ABC) demographic analyses and calculate population statistics. Results Both types of molecular data support stepwise colonisation, decoupled from known island ages. The ABC analyses support a north to south migration while the Sanger sequencing data indicate a mixture of island progression rule and stepping stone dispersal. The SSR data show strong genetic structuring per island and significant inbreeding in all populations except in Saint Lucia. The lowest genetic diversity is found in the population from Saint Vincent. A high amount of genetic linkage occurs in a subpopulation from Dominica. Main Conclusions Biogeographical patterns in the complex geological setting of the Lesser Antilles are uncovered using a slowly evolving study species. All genetic data support treating each island as distinct Management Units for conservation and call for a re‐evaluation of the species limits. Inbreeding threatens the survival of island populations and the populations of Saint Vincent and Dominica represent conservation priorities.
The Bonin Islands, comprising of the Mukojima, Chichijima, and Hahajima Islands, are known for their isolated and distinctive habitats, hosting a diverse array of endemic flora and fauna. In these islands, adaptive radiation has played a remarkable role in speciation, particularly evident in the Callicarpa genus that is represented by three species: Callicarpa parvifolia and Callicarpa glabra exclusive to the Chichijima Islands, and Callicarpa subpubescens , distributed across the entire Bonin Islands. Notably, C. subpubescens exhibits multiple ecotypes, differing in leaf hair density, flowering time, and tree size. In this study, we used double-digest restriction site–associated DNA sequencing to analyze species, ecotypes and geographical variations within Callicarpa in the Bonin Islands. We aimed to determine detailed phylogenetic relationships, investigate species and ecotype diversification patterns, estimate divergence times, and explore cryptic species using genetic and phenotypic data. Genetic analysis revealed that C. parvifolia and C. glabra formed a single, distinct genetic groups. Conversely, C. subpubescens showed seven genetic groups corresponding to different ecotypes and regions, with one ecotype derived from the hybridization of two others. Phylogenetic and population demography analyses, focusing on six Chichijima and Hahajima Islands–based species/ecotypes, indicated the divergence of an ecotype adapted to tall mesic forests approximately 170 kya, whereas the other five species/ecotypes diverged nearly simultaneously around 73–77 kya. Environmental changes during the glacial period likely contributed to this process of adaptive radiation. Moreover, leaf morphology, flowering time, and genetic analyses suggested the presence of two cryptic species within C. subpubescens .
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