The Wild Alternating Pathosystem

The alternating pathosystems of plants have a remarkable number of properties in common, in spite of the fact that the parasite might be either a fungus or an insect. The following description can be applied equally to the alternating pathosystem of an aphid or a rust, and it is not necessary to specify which category of parasite is being discussed (Fig. 8.1). However, it should be emphasised that a wild pathosystem is being discussed, because some crop pathosystems of alternating parasites have been misleadingly distorted by agricultural practices.

Sexual recombination in the parasite occurs on the winter host. There is some variation among species of alternating parasites in the method of over-wintering. Dormancy often occurs during parasitism of the winter host, either before or after sexual recombination. Alternatively, dormancy may occur in dead tissues of the summer host, and the autumn alternators then allo-infect their winter host only in the spring. Reduction division and the production of gametes of the parasite may thus occur either among the autumn alternators, or in the parasites of the winter host. However, this variation does not detract from the remarkable similarity of this parallel evolution in plant parasitic insects and fungi. The key feature is that, in the spring, the parasite produces segregating individuals which are the product of sexual recombination, and which are obliged to parasitise the alternate, summer host.

0 0

Post a comment