Origin of the Eleusine indica population on the active volcanic Nishinoshima Island, Japan

Abstract

Nishinoshima Island, located approximately 1,000 km south of mainland Japan and 130 km from the Ogasawara Islands, emerged as a new island due to volcanic activity in 1973. On this isolated island, grasses such as Eleusine indica and Portulaca oleracea have been observed growing, though their routes of invasion remain unknown. Numerous seabirds have been spotted inhabiting and breeding around Nishinoshima Island, potentially contributing to the spread of these plants without any possibility of anthropogenic influence. The aim of this study was to trace the origin of the E. indica population on Nishinoshima Island using genetic methods. We collected samples of E. indica from Nishinoshima Island, mainland Japan, the Ryukyu Islands, the Ogasawara Islands, and other locations. In this study, we utilized two types of DNA with different inheritance modes: chloroplast DNA (cpDNA) and nuclear DNA. For cpDNA, five polymorphic regions were analyzed, while genetic diversity and structure were also assessed for nuclear DNA using MIG-seq (Multiplexed inter-simple sequence repeat (ISSR) genotyping by sequencing), a method for detecting nucleotide polymorphisms with next-generation sequencing technology. Genetic diversity analysis revealed high genetic diversity in the mainland Japan population. Additionally, cpDNA analysis confirmed the presence of the Nishinoshima haplotype on several islands, including Ogasawara Island. Nuclear DNA analysis showed a close relationship between the Nishinoshima population and those on Ogasawara, Hachijojima, Okinawa and Indonesia. Given instances of seabirds carrying tiny seeds over long distances, it is plausible that E. indica seeds were transported from distant locations by migrating birds. Based on the genetic diversity of E. indica on Nishinoshima, it was considered that this population originated from a single or a few seed as a result of an extremely rare seed dispersal event.