Abstract
Coastal plants with simple linear distribution ranges along coastlines provide a suitable system for improving our understanding of patterns of intra-specific distributional history and genetic variation. Due to the combination of high seed longevity and high dispersibility of seeds via seawater, we hypothesized that wild radish would poorly represent phylogeographic structure at the local scale. On the other hand, we also hypothesized that wild radish populations might be geographically differentiated, as has been exhibited by their considerable phenotypic variations along the islands of Japan. We conducted nuclear DNA microsatellite loci and chloroplast DNA haplotype analyses for 486 samples and 144 samples, respectively, from 18 populations to investigate the phylogeographic structure of wild radish in Japan. Cluster analysis supported the existence of differential genetic structures between the Ryukyu Islands and mainland Japan populations. A significant strong pattern of isolation by distance and significant evidence of a recent bottleneck were detected. The chloroplast marker analysis resulted in the generation of eight haplotypes, of which two haplotypes (A and B) were broadly distributed in most wild radish populations. High levels of variation in microsatellite loci were identified, whereas cpDNA displayed low levels of genetic diversity within populations. Our results indicate that the Kuroshio Current would have contributed to the sculpting of the phylogeographic structure by shaping genetic gaps between isolated populations. In addition, the Tokara Strait would have created a geographic barrier between the Ryukyu Islands and mainland Japan. Finally, extant habitat disturbances (coastal erosion), migration patterns (linear expansion), and geographic characteristics (small islands and sea currents) have influenced the expansion and historical population dynamics of wild radish. Our study is the first to record the robust phylogeographic structure in wild radish between the Ryukyu Islands and mainland Japan, and might provide new insight into the genetic differentiation of coastal plants across islands.
Highlights
Coastal plants offer several advantages as a suitable system for improving our understanding of patterns of intra-specific distributional history and genetic variation [1]
Investigations of the geographic distribution of intra-specific genetic variation in coastal plants have often resulted in the detection of clear phylogeographic structures; e.g., Hordeum marinum [3], Triglochin maritima [4], Eryngium maritimum [5], and Carex extensa [6] in Mediterranean and European coasts, Zostera marina in Northern Hemisphere coasts [7], and Hibiscus tiliaceus in Pacific and Indian Ocean regions [8]
A highly positive relationship was observed between pairwise genetic distance and geographic distance over total populations (R2 = 0.2953, p
Summary
Coastal plants offer several advantages as a suitable system for improving our understanding of patterns of intra-specific distributional history and genetic variation [1]. Investigations of the geographic distribution of intra-specific genetic variation in coastal plants have often resulted in the detection of clear phylogeographic structures; e.g., Hordeum marinum [3], Triglochin maritima [4], Eryngium maritimum [5], and Carex extensa [6] in Mediterranean and European coasts, Zostera marina in Northern Hemisphere coasts [7], and Hibiscus tiliaceus in Pacific and Indian Ocean regions [8]. These patterns were primarily explained by historical processes (e.g., Pleistocene glaciations). Factors related to specific properties of species, such as clonal growth and/or long-distance dispersal, have been proposed to account for colonization success
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