Abstract
Deceptive pollination is key to the species richness of Orchidaceae. However, the genetic basis of species diversification is still under study. Section Trigonopedia is a monophyletic clade of genus Cypripedium distributed in the southwest of China. The species of this section are pollinated by different flies. Pollinator differentiation makes section Trigonopedia an ideal group for studying the genetic basis underlying species diversification. Here, we sequenced the transcriptomes of eight species of the genus Cypripedium, including six co-flowering species of section Trigonopedia and two species outside this section as an outgroup. We reconstructed the phylogeny of the section with the combined 1572 single-copy genes extracted from the eight species and produced a highly resolved tree of the section. Furthermore, we combined substitution rate estimation and differential expression analysis to identify candidate genes, including genes related to floral scent synthesis and environmental adaptation, involved in species differentiation. Field investigations showed that these species have adapted to different habitats. We propose that the species diversification in this section is initiated by floral scent differentiation, followed by habitat differentiation, finally leading to speciation. This study sheds novel light on the diversification of closely related orchid species in the Qinghai-Tibetan region.
Highlights
Speciation, which is important to ecology and evolution, is a key question puzzling orchid biologists
Several diverse metabolic pathways are involved in floral scent synthesis, and the differential expression of candidate genes contributes to the odour difference[11]
Our results showed that neighbouring species are more closely related, e.g., C. bardolphianum and C. micranthum, with sister relationships grow in a sympatric distribution in Sichuan; C. lentiginosum is distributed in Yunnan at the base of the other clade, followed by C. margaritaceum; while C. fargesii and C. sichuanense, with overlapping distributions, form the sister clade, which agrees with the south to north niche shift of the genus[45]
Summary
Speciation, which is important to ecology and evolution, is a key question puzzling orchid biologists. Pollinators are attracted by floral colour, reward, morphology, size, and scent. The inner genetic basis of deceptive pollination is still not well understood for other species. Next-generation sequencing provides an opportunity to investigate this inner genetic basis and provide clues regarding the candidate genes and pathways involved in floral scent biosynthesis[11]. This speciation model should be investigated in more non-model species. Sedeek et al.[8] sequenced the transcriptomes and proteomes of three Ophrys species and identified candidate genes for pollinator attraction and reproductive isolation among sexually deceptive orchids. Section Trigonopedia of Cypripedium, pollinated by different flies, is endemic to the Qinghai-Tibetan Plateau. It is interesting that the predominant compound emitted by C. bardolphianum is ethyl acetate (70.4%) with a fruity wine flavour, whereas the compounds emitted by C. micranthum are dominated by pentyl ester-acetic acid (44.8%), 2-(2-methoxyethoxy)-ethanol (27.1%), and hydroxyacetic acid (15.3%)[37]
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