How are Galls formed?
Mechanisms of gallwasp gall induction remain little known, and understanding the molecular tools they use to manipulate plant development is the Holy Grail of current cynipid research. Cynipid gall development can be divided into three phases: initiation, growth, and maturation. Initiation begins with oviposition by the female gallwasp. She determines the host plant, gall location on the host, and (by the number of eggs she lays) the number of larvae developing in the resulting gall. Gallwasps can apparently only induce galls in certain plant tissues - specifically, those containing cells whose developmental future is not yet fixed. Such cells (sometimes called 'totipotent' because they could develop into any of a number of tissue types) are found in the growth zones of plants - the meristems in shoot and root tips and buds, and the cambium tissue associated with plant fluid transport systems (such as leaf veins). When the female lays her egg, neighbouring plant cells disintegrate to produce a small fluid-filled space. After hatching from its egg, the gallwasp larva enters this space, and controls all subsequent gall development. Each larva develops in its own chamber, and galls induced by a specific gallwasp species are usually either single-chambered (unilocular) or many-chambered (multilocular).
For a diagram of cynipid gall structure, go here!
The next phase - gall growth - takes place while the gallwasp larva is very small. The larval chamber enlarges, a network of fluid-transporting channels develops that joins those supplying water and nutrients to the surrounding oak tissues, and layers of outer tissues develop around the larval chamber. In all cynipid galls, the larval chamber is lined with nutritive cells and in most cases these tissues are contained in a thin shell of sclerenchyma tissue that will ultimately become brittle and hard. Work by Karsten Schšnrogge and co-workers has shown thatÊ the nutritive tissue consists of large thin-walled cells whose chromosome structure, protein content and physiology are very similar to seed tissue. These cells represent the gallwasp's sole source of food throughout development. The larval chambers are morphologically similar in most cynipid galls, and the differences in gall structure between gallwasp generations and species result from variation in the development of the outer tissues (particularly the parenchyma and epidermis in the diagram linked above). The complexity of these outer tissues varies enormously across gallwasp tribes, among species, and between the generations of cyclically parthenogenetic species. Some examples of oak gall diversity are shown here.
During the growth phase, the gall draws in nutrients from surrounding host plant tissues, acting as a 'sink' for mineral nutrients and the carbon fixed into sugars during photosynthesis. Some gallwasps that gall short-lived plant structures (such as catkins) are able to prolong the lifespan of these structures on the host, so extending the period available for development The outer gall tissues synthesise tannins and phenolic compounds that were originally thought to be a deterrent against insect larvae such as caterpillars that might feed on the gall. However, these compounds are now known to be feeding stimulants for some larvae, and they may function in addition (or instead) to prevent attack by fungi. Whatever their function, tannins reach far higher levels in galls than in normal plant tissues. This concentration lies behind the long history of using gallwasp (and other) galls as tannin sources in human tanning and dye-making. Through most or all of this phase the larva stays tiny. One idea, returned to below, is that this strategy allows the gallwasp's defences to be in place before it presents a bigger target to its natural enemies.
The maturation phase is characterised by decreased rates of cell division and the gall ceases to draw heavily on host plant resources. The gallwasp larva now grazes the nutritive cells lining the larval chamber and grows quickly. Feeding continues until the sclerenchymatous shell of the larval chamber is reached. The larval intestine is closed between the midgut and the hindgut for most of the larval life, and only opens for defaecation immediately prior to pupation, thus avoiding fouling of the larval chamber. In many oak galls the tissues surrounding the larval chamber become lignified (hard and woody), and, in some cases, regional tissue death results in internal air spaces. Lignification makes the tissue unusable for other herbivores and in some species the onset of lignification determines when the galled plant part (e.g. catkins, leaves or acorns) is shed from the host. The timing of lignification is under larval control, and may have important consequences, particularly in galls that fall from the tree to overwinter on the ground. Many galls fall slightly before leaf fall in the autumn, so ensuring a covering of leaves and a suitable microclimate in which pupate or enter diapause.
