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

Abstract

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.

Discovery of a diverse cave flora in China

“…Of the flora we document, whilst ferns and spikemosses have previously been associated with low-light environments [ 7 – 10 , 56 ], Urticaceae have not. That the Urticaceae genus Elatostema were the most species-rich group in both the entrance and twilight zone may reflect their understory habitat and high diversity on karst [ 57 ] and so further supports the notion of cave floras as extensions of the terrestrial forest understory.…”

A Preliminary Evaluation of The Karst Flora of Brazil Using Collections Data

“…Features which are exclusive to karst include the export of weathered material below ground in solution, as opposed to as solids or in suspension, and above ground, high levels of Ca, Mg and K and very slow rates of soil formation 6 . High concentrations of Ca, Mg and K, the absence of surface water and very slow rates of soil formation pose several challenges for colonising plants 7 and have led in some places to the development of a specialised flora, often derived from rapid diversifications 8,9 . Combined with the high heterogeneity of microhabitats 6,10 these features could be expected to result in a relatively high frequency of endemic species as has been documented in Southeast Asia, Mexico and the Greater Antilles 6,10–14 .…”

Checklist to the Elatostema (Urticaceae) of Vietnam including 19 new records, ten new combinations, two new names and four new synonyms

“…& G Forst. (Urticaceae) comprises several hundred species of succulent herbs and subshrubs that grow in shade in forests, gorges, stream sides and caves (Fu et al, 2017; Monro et al, 2018). There are currently 626 accepted names within the genus Elatostema (The Plant List, 2018).…”

“…Elatostema is distributed throughout tropical and subtropical Africa, Madagascar, Asia, Australia and Oceania, with highest diversity on limestone karst in China and Southeast Asia (Yahara, 1984; Lahav-Ginott & Cronk, 1993; Wang & Chen, 1995; Lin, Friis & Wilmot-Dear, 2003; Wang, 2014). The previous studies suggest that this diversity has been driven by the challenges of colonising karst substrates, which once overcome leads to species radiations (Chung et al, 2014; Fu et al, 2017), and that past temperature fluctuations and East Asian monsoons have driven the rates of plant diversification in karst ecosystems by accelerating the rate of karstification (Kong et al, 2017).…”