Department of Integrative Biology, University of California Berkeley, Berkeley, CA, USA.
Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA, USA.
Mol Biol Evol. 2022 Feb 3;39(2). doi: 10.1093/molbev/msab362.
The diversity of herbivorous insects is attributed to their propensity to specialize on toxic plants. In an evolutionary twist, toxins betray the identity of their bearers when herbivores coopt them as cues for host-plant finding, but the evolutionary mechanisms underlying this phenomenon are poorly understood. We focused on Scaptomyza flava, an herbivorous drosophilid specialized on isothiocyanate (ITC)-producing (Brassicales) plants, and identified Or67b paralogs that were triplicated as mustard-specific herbivory evolved. Using in vivo heterologous systems for the expression of olfactory receptors, we found that S. flava Or67bs, but not the homologs from microbe-feeding relatives, responded selectively to ITCs, each paralog detecting different ITC subsets. Consistent with this, S. flava was attracted to ITCs, as was Drosophila melanogaster expressing S. flava Or67b3 in the homologous Or67b olfactory circuit. ITCs were likely coopted as olfactory attractants through gene duplication and functional specialization (neofunctionalization and subfunctionalization) in S. flava, a recently derived herbivore.
食草昆虫的多样性归因于它们倾向于专门食用有毒植物。在进化的转折中，当食草动物将毒素作为寻找宿主植物的线索而采用时，毒素会暴露出其携带者的身份，但这一现象背后的进化机制还知之甚少。我们专注于 Scaptomyza flava，一种专门以产生异硫氰酸酯（ITC）的植物（芸薹属）为食的食草果蝇，并鉴定出随着芥菜特异性食草性进化而三倍化的 Or67b 基因的同源物。通过表达嗅觉受体的体内异源系统，我们发现 S. flava 的 Or67bs 对 ITC 有选择性反应，但微生物取食相关的同源物没有，每个同源物检测不同的 ITC 亚群。与此一致的是，S. flava 被 ITC 吸引，表达 S. flava Or67b3 的果蝇在同源的 Or67b 嗅觉回路中也被 ITC 吸引。在最近衍生的食草动物 S. flava 中，通过基因复制和功能特化（新功能化和亚功能化），ITC 可能被选为嗅觉引诱剂。
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