Studies in gall induction with special reference to the pontania-salix system
An investigation was conducted into gall induction in the leaves of Salix fragilis L. var. russelliana (Sm.) Koch by Pontania proxima (Lepeletier) (Hymenoptera: Tenthredinidae). The work was divided into four parts:- a) Reviews of the available literature were undertaken on: hypotheses concerning cecidogenesis of galls initiated by insects, the history of cecidology as it related to galls formed by insects, the ontogeny and morphology of galls produced by sawflies, ecological aspects of gall biology and the effects of galling on the host plant. b) Methods of rearing both insect and host were established; these resulted in an extension of the insect's flight period from five to eight months with three broods per year instead of the usual two. Further work established the topological specificity of oviposition. Preliminary studies were completed on the tissue culture of Salix leaf-disc explants and the fusion of protoplasts released from normal and galled tissue. c) Using light and electron microscopy, a study was made of the reproductive system and associated glands of Pontania proxima females, together with the first stages of the procecidium they initiated. Using these techniques, no microorganisms, viral or otherwise, were observed in the gall, in the lumina of the reproductive system and associated glands or in surrounding tissues of the insect. Evidence for a secretory role for the lateral oviducts was found. A structure termed the vaginal valve was described and it was hypothesized that this functioned to separate the fluids produced by the accessory glands from the contents of the oviduct, until oviposition. In the plant, it was observed that the gall effect was limited and that the presence of an egg or larva was not required for the formation of a procecidium. Gall growth was mainly due to periclinal divisions of the provascular tissues of the leaf. d) A bioassay, based on microinjection techniques, was developed .This demonstrated that the cecidogen was contained in the colleterial fluid produced by the accessory glands. Further analysis showed that the cecidogen had a molecular weight of less that 3 kDa.