Limestone karsts across southern China to southeastern Asia are renowned biodiversity hotspots. Karst is characterized by exposed rocks, seasonal droughts and thin soils that are deficient in N and P but with super abundance of Ca and Mg. The habitats stress may result in high biodiversity through niche differentiation and genetic mechanisms such as hybridization, polyploidy and apomixis. The genus Elatostema (Urticaceae) exhibits high species diversity in this area and can be used a model genus to explore the genetic mechanisms of speciation. We conducted cytological studies of 11 species of Elatostema from 12 populations in Guangxi, China. We found five populations to be diploid (chromosome number 2n = 26) and seven populations triploid (chromosome number 2n = 39). We infer x = 13 as the basic chromosome number of Elatostema. Chromosome numbers 2n = 26 and 2n = 39 are reported in two populations of E. longistipulum indicating this species to be diploid and triploid. Our cytological studies and recent field observations show that the sampled populations 2n = 26 (diploid) occur in both male and female plants of Elatostema. However, 2n = 39 (triploid) occurs only in female plants which 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 found that ca 2/3 were apomictic suggesting that it is a widespread reproductive strategy. We suspect that apomixis might be a recurring phenomenon in Elatostema of limestone karsts of Guangxi, China. Apomixis enables plants to reproduce and disperse from a single individual, allowing ‘hopeful monsters’ adapted to a new habitat to form stable populations.
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