Similitudes and differences between two closely related Ficus species in the synthesis by the ostiole of odors attracting their host-specific pollinators: A transcriptomic based investigation

Flower odor is often essential for attracting pollinators, and this is especially true for species-specific mutualism such as the one between Ficus species and their pollinating wasps. Receptive figs emit a stage and species specific odor composed of a mix of volatile organic compounds (VOC) that is attractive to their pollinators. Histological studies suggest that the ostiolar bracts are the main locus of fig floral VOC synthesis. To confirm a major role of the ostiole in VOC synthesis, detect the genes involved in VOC synthesis and analyze differences between closely related species, we compare the transcriptomes of pre-receptive and receptive figs of two closely related species, F. hirta and F. triloba. The two species presented similar numbers of expressed genes and similar annotation, classification results, and their up-regulated unigenes belonged to similar biosynthetic pathways. However, phenylpropanoid and terpene synthesis were the main enriched VOC pathways in receptive figs of F. hirta while terpene and jasmonate synthesis were the main enriched VOC pathways in F. triloba. The shift in gene expression between pre-receptive and receptive figs was much more marked in the ostiolar bracts than in the flowers of F. hirta, and in the fig wall, confirming an important role of the ostiole in receptive fig odor emission. Only one unigene directly involved in VOC synthesis presented signatures of positive selection, further supporting that the source of interspecific receptive fig odor differentiation is based on gene regulation rather than on gene differentiation. Regulation based receptive fig odor differentiation between species suggests a potential for rapid evolution of this interspecific barrier.