Comparative transcriptome analysis of galls induced by four closely related aphid species using the draft genome of the host plant (Hamamelis japonica) reveals gene expression patterns related to gall diversification

Gall-forming insects induce various types of galls on their host plants by altering gene expression in host plant organs, and recent studies have investigated gene expression in galls. However, studies comparing gene expression among galls induced by closely related insect species are very limited, even though such comparisons are important for revealing the diversification process of galls. To elucidate the genetic mechanisms involved in the diversification of gall morphology and ecology, we constructed the draft genome of a host plant species ( Hamamelis japonica ) and then compared gene expression among galls induced by four closely related aphid species ( Hamamelistes miyabei , Ha. betulinus , Ha. kagamii and Hormaphis betulae ) using the draft genome as a reference. Different expression patterns of genes associated with cell division, vascular development and phenylpropanoid biosynthesis were observed among galls induced by the four aphid species. Our results suggest that differences in the expression patterns of genes related to cell division and phloem formation contribute to morphological differences between galls induced by Hormaphis and Hamamelistes species. Additionally, differences in the expression patterns of genes related to phenylpropanoid biosynthesis may be associated with the more complex internal structures observed in galls induced by Ha. betulinus and Ha. kagamii than in those induced by Ha. miyabei . We also found the high expression of some genes belonging to gene families previously reported to be associated with gall formation in other insect species. Furthermore, the expression levels of these genes tended to differ among galls induced by four aphid species. The results of our study suggest that gene expression patterns related to tissue formation and secondary metabolism differ among galls induced by four aphid species and contribute to the diversification of gall morphology and ecology. Our results further suggest that some gene families may be commonly involved in gall formation across various insect species and that differences in the expression levels of genes belonging to these families may partly contribute to the diversity of gall morphologies.