Interspecific Divergence of Two Sinalliaria (Brassicaceae) Species in Eastern China

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.

Analyses of genome‐scale variation reveal divergence of two Sinalliaria species (Brassicaceae) with continuous but limited gene flow

“…The genome‐scale variation observed in this work agrees very well with that reported previously based on analyses of SSRs, low‐copy genes, and chloroplast DNA (Hu et al, , ; Zhang et al, ). The two studied species are well differentiated morphologically; notable distinguishing traits include the presence/absence of hairs on leaf blades and stems, the number of flowers produced by each individual, and corolla and flower sizes (Zhang et al, ). In addition, crossing experiments between the two species revealed strong post‐pollination isolation: interspecific crossing consistently produced plants with reduced fruit set.…”

“…Among the proposed four models, our comparisons supported speciation involving continuous but very modest and asymmetric gene flow, with migration rates of 1.91 Â 10 À6 from S. grandifolia to S. limprichtiana and 7.04 Â 10 À7 in the opposite direction (Table 3). This weak and asymmetric gene flow is consistent with previous conclusions based on population genetic data for nine low-copy-number genes (Zhang et al, 2018). However, the results presented here reveal for the first time that there has been continuous gene flow since the two species' initial divergence.…”

“…In addition, our samples did not cover all distributional ranges of two species and the sampled individuals for each species are limited. However, distributions of both species are extremely narrow and the size of each population is extremely small according to the specimen records and field explorations (Zhou et al, ; Hu et al, ; Zhang et al, ). Our samples cover most distributional ranges of two species except for one distantly disjunct population of S. limprichtiana because of the failure to collect viable seeds.…”

“…In such scenarios, the outcome is primarily governed by natural selection. However, the current distributions of S. limprichtiana and S. grandifolia are completely allopatric with no overlapping occurrence at any site or mountain (Zhang et al, ). Although we cannot exclude the possibility that these species might be sympatrically or parapatrically distributed in the past, their current distributions are strongly indicative of allopatric distribution.…”

“…A). Morphological distinctions between these species include differences in flower stalk length, corolla size, and the presence of hairs on leaf blades and stems (Zhang et al, ) (Fig. B).…”