Data from: Cytology and sexuality of 11 species of Elatostema (Urticaceae) in limestone karsts suggests that apomixis is a recurring phenomenon

Limestone karsts across southern China to southeastern Asia are renowned biodiversity hotspots. The karst are characterized by exposed calcareous rocks, seasonal droughts and thin soils that are deficient in N and P but with high Ca and Mg content. The stressful habitat may result in high biodiversity through mechanisms such as niche differentiation, hybridization, polyploidy and apomixis. The genus Elatostema (Urticaceae) has particularily high species diversity in this area and can be used a model genus to explore the mechanisms of speciation. We conducted cytological studies on 11 species of Elatostema from 12 populations in Guangxi, China. We found five populations to be diploid (2n = 26), and seven triploid populations (2n = 39). We infer x = 13 as the basic chromosome number of Elatostema. The chromosome numbers 2n = 26 and 2n = 39 were both found in populations of E. longistipulum, indicating that this species comprise both diploids and triploids. Both male and female plants of Elatostema were found to be diploid. In contrast, the triploids found were all female plants, and these produce seeds, presumably by apomixis. We found no clear relationship between ploidy level or reproductive pathway and endemism in Elatostema which might be because Elatostema species are wind-pollinated and independent of biotic pollinators. However, a random sample of 11 karst species revealed that ca 2/3 appeared to be apomictic, suggesting that it is a widespread reproductive strategy of Elatostema in the limestone karsts of Guangxi. Apomixis enables plants to reproduce and disperse from a single individual, allowing ‘hopeful monsters’ adapted to a new habitat to form stable populations.中国南部至东南亚一带的石灰岩喀斯特地貌，是享誉全球的生物多样性热点区域。该类喀斯特生境以裸露碳酸盐岩、季节性干旱，以及氮（N）、磷（P）匮乏但钙（Ca）、镁（Mg）含量较高的薄层贫瘠土壤为典型特征。这类逆境生境可通过生态位分化（niche differentiation）、杂交（hybridization）、多倍化（polyploidy）与无融合生殖（apomixis）等机制，孕育出极高的生物多样性。荨麻科（Urticaceae）楼梯草属（Elatostema）在该区域物种多样性尤为突出，可作为探究物种形成机制的模式属。本研究对中国广西12个种群的11种楼梯草属植物开展了细胞学研究，发现其中5个种群为二倍体（diploid，2n=26），7个种群为三倍体（triploid，2n=39）。据此推断楼梯草属的基础染色体数为x=13。在长托楼梯草（E. longistipulum）的种群中同时检出2n=26与2n=39两种染色体倍性，表明该物种同时存在二倍体和三倍体个体。楼梯草属的雄性与雌性植株均存在二倍体类型；与之相对，本次发现的三倍体个体均为雌性，且可产生种子，推测其繁殖方式为无融合生殖（apomixis）。本研究未发现楼梯草属的染色体倍性（ploidy level）、繁殖路径与特有性（endemism）之间存在明确关联，这可能是因为楼梯草属植物为风媒传粉，无需依赖生物传粉者。不过，对11种喀斯特生境楼梯草属植物的随机抽样结果显示，约2/3的物种似乎为无融合生殖类型，这表明无融合生殖是广西石灰岩喀斯特区域楼梯草属植物广泛采用的繁殖策略。无融合生殖可使植物通过单个个体完成繁殖与扩散，使得适应新生境的“希望怪物（hopeful monsters）”能够形成稳定种群。