Rice paddies are wetland ecosystems recognized as important habitats for many organisms; however, the hybridization-related extinction risk of native plant species has not been investigated in this system so far. Eclipta L. (Compositae) is a common paddy weed in Japan; however, its genetic composition might be altered due to the hybridization between the native E. thermalis and the closely related exotic E. alba. We examined Eclipta's genetic composition using 12 microsatellite markers (612 samples collected from 109 populations) and found (i) widespread geographical distribution of E. alba in Japan, (ii) hybridization with E. thermalis, and a large number of later-generation hybrids, and (iii) widely varying situations among regions and populations. Eclipta alba appears to have invaded an open niche in northern Japan but has not yet reached southern Japan. Both E. alba and E. thermalis were found in central Japan; however, the latter had become rare due to hybridization-mediated processes such as competition, and demographic and genetic swamping. Notably, endogenous and exogenous selection plays an important role in the invasion of E. alba, but to varying degrees among different areas. In summary, considering the genetic variability in E. thermalis, the genetic cluster of mainland Japan is in a highly critical situation due to the invasion of E. alba.

中文翻译：

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旱莲草，一种以前常见的杂草，受到外来同类植物白花旱莲草在日本稻田中扩张的威胁

稻田是湿地生态系统，被认为是许多生物的重要栖息地；然而，到目前为止，该系统尚未研究与杂交相关的本地植物物种灭绝风险。Eclipta L.（菊科）是日本常见的水稻杂草；然而，它的遗传组成可能会由于本地热桉和密切相关的外来白桉之间的杂交而改变。我们使用 12 个微卫星标记（从 109 个种群收集的 612 个样本）检查了旱莲草的遗传组成，发现 (i)白旱莲草在日本有广泛的地理分布，(ii) 与热旱莲草杂交，以及大量后来的旱莲草。 (iii) 不同地区和人群的情况差异很大。白旱莲草似乎已经侵入日本北部的开放生态位，但尚未到达日本南部。E. alba和E. Thermalis均发现于日本中部；然而，由于竞争、人口和基因泛滥等杂交介导的过程，后者已变得罕见。值得注意的是，内源和外源选择在白桉的入侵中发挥着重要作用，但不同地区作用程度不同。综上所述，考虑到热桉的遗传变异性，日本本土的遗传簇因白桉的入侵而处于高度危急的境地。