Abstract
How endemic species originated in eastern Asia has interested botanists for a long time. In this study, we combined experimental and computational modeling approaches to examine the morphological and genetic divergence and reproductive isolation of two tentative species of Sinalliaria (Brassicaceae) endemic to eastern China, S. limprichtiana and S. grandifolia. Most of the examined morphological characters (including hairs of leaf blades and stems, corolla length and width, and flower stalk length) were well-delineated between two species at the same ploidy level, and there was clear evidence of reproductive isolation between them (mainly due to post-pollination barriers) in the common garden environment. There were also strong and consistent divergences in the population genetic data. Coalescent simulations based on sequence variation of the nuclear genes suggest that interspecific divergence began during the Pleistocene when the climate oscillated in eastern Asia. Gene flow between two species appears to have been very limited and asymmetrical. Our results suggested that both species are well-differentiated and that the fast divergence between them might have been together shaped by both stochastic processes and habitat selection pressures.
Highlights
For several decades, biogeographic researchers have considered the reasons for the high species diversity of plants and numerous endemics in eastern Asia, especially compared with eastern North America, which has a similar size, climate, and floristic components (e.g., Tiffney, 1985; Axelrod et al, 1998; Qian and Ricklefs, 2000)
We examined the species’ speciation history using the following demographic parameters derived from analyses of the low-copy nuclear genes: population-split time (T), bidirectional migration rate m, and effective sizes of the ancestral (θ A) and descendant populations (θ L and θ G)
Hairs were present on calyxes, leaf blades and stems of S. limprichtiana, but not S. grandifolia (Figure 2A)
Summary
Biogeographic researchers have considered the reasons for the high species diversity of plants and numerous endemics in eastern Asia, especially compared with eastern North America, which has a similar size, climate, and floristic components (e.g., Tiffney, 1985; Axelrod et al, 1998; Qian and Ricklefs, 2000). Many young species (for example, four Dysosma species, D. pleiantha, D. versipellis, D. difformis, and D. majoensis) have originated through fast divergence probably driven by Pleistocene climate oscillations in conjunction with physiographic heterogeneity (Ikeda et al, 2012; Han et al, 2016; Wang et al, 2017). Both hypotheses have been partly tested based on biogeographic analyses of a few genera and phylogeographic examinations of some widely. Ikeda et al (2012), Han et al (2016), and Wang et al (2017) have revealed the Pleistocene origins of a pair of endemic alpine species, two coastal wild radish lineages and four understory herbs based on population genetic data, it remains unknown whether these species or lineages were morphologically differentiated with stable distinct characters and reproductively isolated as “good species” based on the integrate species concept (Liu, 2016)
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