Phylogeographic analysis of Saxifraga fortunei complex (Saxifragaceae) reveals multiple origins of morphological and ecological variations in the Japanese Archipelago.

Phenotypic polymorphism within a species is a notable phenomenon in evolutionary biology to understand the process of adaptive speciation and other historical events. The Saxifraga fortunei complex is a widespread herb found in East Asia. It includes several ecotypic taxa corresponding to their habitat environments. The distribution of the various ecotypes in a limited area of the Japanese Archipelago makes the species a suitable model to investigate the impact of population demographic history and natural selection on lineage diversification. Here, Sanger-based sequencing was used to estimate the divergence timeframe between populations of the Eurasian continent and Japan. Genome-wide SNPs obtained by ddRAD sequencing were used to investigate the phylogeographic origins of ecotypic taxa. The phylogenetic analyses revealed the divergence of the Japanese population from the continental population in the late Miocene. Two distinct regional clades of North and South Japan were identified; phenotypic diversification was evident only in the southern clade. The South Japan clades displayed a historical distribution expansion from north to south. The phenotypic variations appeared to have generated during the expansion. The ecotypic boundaries were incongruent with the genetic grouping. We propose that morphological and ecological specialization in Japanese populations was repeatedly generated by local natural selection.

Comparative analysis of spatial genetic structures in sympatric populations of two riparian plants, Saxifraga acerifolia and Saxifraga fortunei.

PREMISE: The genetic structure between plant populations is facilitated by the spatial population arrangement and limited dispersal of seed and pollen. Saxifraga acerifolia, a local endemic species in Japan, is a habitat specialist that is confined to waterfalls in riparian environments. Its sister species, Saxifraga fortunei, is a generalist that is widely distributed along riverbanks. Here, we examined sympatric populations of the two Saxifraga species to test whether the differences in habitat preference and colonization process influenced regional and local genetic structures. METHODS: To reveal genetic structures, we examined chloroplast microsatellite variations and genome-wide nucleotide polymorphisms obtained by genotyping by sequencing. We also estimated the gene flow among and within populations and performed landscape genetic analyses to evaluate seed and pollen movement and the extent of genetic isolation related to geographic distance and/or habitat differences. RESULTS: We found strong genetic structure in the specialist S. acerifolia, even on a small spatial scale (<1 km part); each population on a different waterfall in one river system had a completely different predominant haplotype. By contrast, the generalist S. fortunei showed no clear genetic differentiation. CONCLUSIONS: Our findings suggest that the level of genetic isolation was increased in S. acerifolia by the spatially fragmented habitat and limited seed and pollen dispersal over waterfalls. Habitat differentiation between the sister taxa could have contributed to the different patterns of gene flow and then shaped the contrasting genetic structures.

Development and characterization of EST-SSR markers for Saxifraga fortunei var. incisolobata (Saxifragaceae).

PREMISE: Saxifraga fortunei (Saxifragaceae) includes several infraspecific taxa that are ecologically and morphologically distinct. To investigate the evolutionary history of phenotypic polymorphisms in this species, we developed expressed sequence tag-simple sequence repeat (EST-SSR) markers for S. fortunei. METHODS AND RESULTS: We developed 26 polymorphic markers based on transcriptome data obtained from Illumina HiSeq 2000. Within three populations of S. fortunei var. incisolobata, the number of alleles ranged from four to 25, and the levels of observed and expected heterozygosity ranged from 0.200 to 0.847 and from 0.209 to 0.930, respectively. Furthermore, all 26 loci showed transferability for S. fortunei var. obtusocuneata and S. fortunei var. suwoensis, and 18 loci were also successfully amplified in S. acerifolia. CONCLUSIONS: These newly developed EST-SSR markers will prove useful to infer the evolutionary history of S. fortunei var. incisolobata and its relatives in population genetic studies.