Diversifying selection of the anthocyanin biosynthetic downstream gene UFGT accelerates floral diversity of island Scutellaria species.

Bing-Hong Huang,Chia-Lung Huang,Jian Gao,Pei-Chun Liao,Yi-Wen Chen

doi:10.1186/s12862-016-0759-0

Abstract

BackgroundAdaptive divergence, which usually explains rapid diversification within island species, might involve the positive selection of genes. Anthocyanin biosynthetic pathway (ABP) genes are important for floral diversity, and are related to stress resistance and pollination, which could be responsible for species diversification. Previous studies have shown that upstream genes of ABP are subject to selective constraints and have a slow evolutionary rate, while the constraints on downstream genes are lower.ResultsIn this study, we confirmed these earlier observations of heterogeneous evolutionary rate in upstream gene CHS and the downstream gene UFGT, both of which were expressed in Scutellaria sp. inflorescence buds. We found a higher evolutionary rate and positive selection for UFGT. The codons under positive selection corresponded to the diversified regions, and the presence or absence of an α-helix might produce conformation changes in the Rossmann-like fold structure. The significantly high evolutionary rates for UFGT genes in Taiwanese lineages suggested rapid accumulation of amino acid mutations in island species. The results showed positive selection in closely related species and explained the high diversification of floral patterns in these recently diverged species. In contrast, non-synonymous mutation rate decreases in long-term divergent species could reduce mutational load and prevent the accumulation of deleterious mutations.ConclusionsTogether with the positive selection of transcription factors for ABP genes described in a previous study, these results confirmed that positive selection takes place at a translational level and suggested that the high divergence of ABP genes is related to the floral diversity in island Scutellaria species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0759-0) contains supplementary material, which is available to authorized users.

Highlights

Adaptive divergence, which usually explains rapid diversification within island species, might involve the positive selection of genes
The UDP-glucose:flavonol 3-O-D-glucosyltransferase (UFGT) sequence of the outgroup Tinnea rhodesiana was identical to a Turkish endemic species, S. salviifolia, and was grouped with S. indica, which is widespread in East, Southeast, and South Asia
Several studies have indicated genetic conservation in genes that are upstream of Anthocyanin biosynthetic pathway (ABP) genes, whereas there has been a relaxation of selective constraints in downstream genes [2,3,4]

Summary

Introduction

Adaptive divergence, which usually explains rapid diversification within island species, might involve the positive selection of genes. Several studies have shown that upstream CHS has a low non-synonymous substitution rate (dN) and this suggests that it evolves with stronger selective constraints than downstream genes, which have an elevated dN [3, 4]. It has been suggested that floral diversification is mostly caused by downstream gene diversity [8, 9] and/or diversity in the corresponding TFs [10,11,12], and that regulatory gene variation is more important than structural mutation for floral anthocyanin diversity in rapidly evolving species [13]. Mutations of upstream genes (e.g., CHS) typically cause large physio-ecological changes, e.g., the elimination of most flavonoids by the suppression of CHS [14, 15] They have been shown to affect pollination [15, 16]

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