Concentrations of total phenolics and condensed tannins and the number of galls combined in each sample

Gall-forming insects induce various types of galls on their host plants by
altering gene expression in host plant organs, and recent studies have
been conducted for gene expression in galls. However, the evolutionary
trajectories of gene expression patterns and the resulting phenotypes have
not yet been studied using multiple related species. We investigated the
speciation and the diversification process of galls induced by four
closely-related aphid species (Hormaphidini) on a host plant species
(Hamamelis japonica) by examining the phylogenetic congruence between the
geographic divergences of aphids and the host plant, and by comparing
their gene expression patterns and resulting phenotypes. Phylogenetic
analysis of aphids and the host plant showed that geographic isolation
among host plant populations has interrupted gene flow in aphids and
accelerated the speciation process. The concentration of phenolics and the
complexity of the internal structure of galls were correlated with the
expression levels of genes for the biosynthesis of phenolics and
morphogenesis, respectively. These results suggest that the expression
levels of genes for the biosynthesis of phenolics and morphogenesis have
evolutionarily increased in galls accelerated by the speciation process of
aphids due to the distribution change of the host plant, leading to the
related phenotypic evolution. Our study showed the evolutionary process of
phenotypic traits in galls in the wild from both gene expression and
actual phenotype levels.