One aphid species induces three gall types on a single plant: Comparative histology of one genotype and multiple extended phenotypes

In aphids, usually only the fundatrix (F1) induces galls. In Smynthrodes betae (Fordini) however, three gall types may coexist on the leaflets of a single host species, Pistacia atlantica (Anacardiaceae). The fundatrix induces pea-shaped galls on the leaflet midvein early in spring and its genetically identical daughters (F2) disperse from these galls and induce spindle-shaped galls on leaflet margins. In some of the fundatrix' galls, the daughters (F2) continue to reproduce and complete their normal life cycle within the same gall (CLC galls). Using a comparative anatomy approach, the aim of this study is to evaluate the role of different aphid generations and gall location in controlling gall structure. Histological sections of the three gall types as well as galled and ungalled leaflets were examined. The leaflet's mesophyll is highly modified in all gall types expressing hyperplasia and hypertrophy and collateral veins occupy the inner part of the gall's wall. The epidermis lining the chamber of only the F1 and the CLC galls is covered with trichomes. The F1 galls have a rather simple structure, composed mostly of large parenchyma cells. The F2 and CLC galls have two parenchyma layers separated by a sclereid layer that creates a protective hardened structure. The structural similarity of the F2 and the CLC galls indicates that most histological modifications are controlled by the aphids and depend on either the continuous or cumulative activity of the following aphid generations in the gall.