Recent Progress Regarding the Evolution and Molecular Aspect of Insect Gall Formation

Galls are characteristic plant structures formed by hypertrophy (excessive increase in cell size) and/or hyperplasia (cell proliferation) induced by parasitic or pathogenic organisms. Insects are a major inducer of galls, and insect galls can occur on plant leaves, stems, floral buds, flowers, fruits, or roots. Many of these exhibit unique shapes, providing shelter and nutrients to the insects. To form unique gall structures, all-inducing insects are believed to secrete certain effector molecules and hijack host developmental programs. However, the molecular mechanisms of insect gall induction and development is still largely unknown because of the difficulty of studying non-model plants in the wild. Recent progress in next-generation sequencing has allowed us to determine the structure of biological processes in non-model organisms, including gall-inducing insects and their host plants. In this review, we first summarize the evolutionary aspects of gall-inducing life histories and their adaptive significance for insects and plants. Then, we briefly summarize recent progress regarding the molecular aspects of insect gall formation.