Internal structure of developing aucuba fruit as a defence increasing oviposition costs of its gall midges Asphondylia aucubae

Abstract. 1. Damage to juvenile plant tissues can cause reductions in fitness. Therefore, plants are expected to have evolved various defences for juvenile organs; however, so far, little attention has been paid to mechanical defence, as they have been considered to constrain the growth of juvenile organs. This study revealed that the dioecious tree Aucuba japonica uses mechanical defences to protect young developing fruit from the gall midge Asphondylia aucubae. 2. Young fruit of A. japonica have a hard layer of endocarp covering the integument. Midges oviposit on the surface of the integument, where larval chambers are later formed. The endocarp gradually becomes cracked as the embryo sac develops. 3. Oviposition by the midges is successful only when the ovipositors happen to pass through cracks in the endocarp. Thus, to successfully lay eggs, midges must insert the ovipositor repeatedly. This should decrease the fecundity of the midges, and subsequently their infectiousness, because their adult lifespan is short and they do not consume food during this time. 4. Expansion of the cracks in the endocarp simplifies oviposition over time; however, the embryo sac continues to grow, increasing its volume relative to that of the ovule. This appears to deplete available space and tissue used in the construction of larval chambers, gradually making the fruit less susceptible to midge attacks. 5. The temporary nature of this defence should prevent it from constricting the growth of young fruit. This exemplifies a novel strategy for a mechanical defence of young developing plant tissues.