Estimation of the genetic composition of a near-threatened tidal marsh plant, carex rugulosa, in Japan

As human alterations such as riverbed excavation and harbor improvements have diminished natural littoral regions and estuarine basins, the numbers of organisms living in tidal marsh have declined. The tidal marsh halophyte Carex rugulosa has been commonly observed along the Japanese coastline over the past centuries; however, this species has recently been designated as near threatened. To determine the genetic structure of C. rugulosa, we examined the genetic composition within and among populations and attempted to clarify habitat salinity. Large populations along the Pacific Ocean exhibit relatively high clonal diversity and repeated seedling recruitment, which maintain the genotypic diversity in those populations. Conversely, lower clonal diversity is observed at the distributional margins, suggesting that marginal populations have low seedling recruitment. In terms of habitat salinity, high genetic diversities were observed between 5 and 15 psu in the Hii River watershed. The extant populations in Japan comprise three genetically distinct clusters whose population divisions correspond to the geological distribution and direction of ocean currents around Japan. Our results suggest that most C. rugulosa populations have relatively high inbreeding coefficients or low clonal diversities due to low seed recruitment or fragmentation and that reduction of genetic connectivity among populations is one causality for this low genetic diversity.