The Importance of Discontinuity

A gene-for-gene relationship can function and, consequently, can evolve, only in a discontinuous pathosystem. There must be both sequential discontinuity and spatial discontinuity.

4.12.1 Sequential discontinuity

Vertical resistance occurs only in seasonal tissues, and in discontinuous epidemics. This is because it functions as a system of locking (see 4.14). Consider a parasite of the leaves of a deciduous tree. With leaf-fall, the leaves die, and the parasite survives usually as a sexually produced dormant form. In the spring, the new leaves are parasite-free, and the only possible infection is allo-infection. The vertical resistance of the tree, which was matched (i.e., 'broken down') in the previous season, is now unmatched and functioning. This is the converse of a 'breakdown' and it represents the 'recovery' of the vertical resistance. In terms of a system of locking, a 'breakdown' is equivalent to unlocking, and 'recovery' is equivalent to re-locking. Note also that new parasite individuals in the spring are the result of sexual recombination, and they are segregating genetically with respect to their vertical parasitic abilities. Their chances of matching the host individual in whose leaves they survived the winter are no different from those of any other parasite individual in the n/2 model (see 4.15).

Occasionally, an agro-ecotype of a deciduous species, produced by artificial selection, is so susceptible that a leaf parasite can survive an adverse season in the bark. For example, the apple cultivar 'Cox's Orange Pippen' can harbour over-wintering mycelium of apple scab (Ventura inaequalis). The pathosystem is then functionally continuous.

Now consider an evergreen tree in which host tissue is continuously available to the parasite. Vertical resistance will not evolve in such a species because it can protect the host only until the first matching allo-infection occurs. From then on, the parasitism can be continuously maintained by auto-infection. And all auto-infection is matching infection. We are forced to conclude that vertical resistance can evolve only in discontinuous pathosystems. This conclusion is supported by all the established examples of vertical resistance, although there are a few misinterpretations in the scientific literature. As noted above (see 4.6), it is easy to mistake some differential interactions for a gene-for-gene relationship.

Because every epidemic, whether continuous or discontinuous, has matching infections, we are forced to conclude that every host has horizontal resistance to every one of its parasites. The available evidence supports this conclusion, even if the level of horizontal resistance in some modern cultivars is at a very low level. To postulate any other conclusion would be to postulate an absolute susceptibility, and this possibility has never been demonstrated.

There are some apparent exceptions to this conclusion that vertical resistance occurs only in discontinuous pathosystems.

These exceptions could be misleading if their real discontinuity was not understood.

Coffee leaf rust: Coffea arabica is an evergreen perennial and its leaf parasites apparently have a continuous epidemics. Coffee leaf rust (Hemileia vastatrix) requires free water on the leaf surface in order to infect. During the dry season, all rust-infected leaves are shed, and rust dies with them. Coffee is thus functionally deciduous with respect to rusted leaves only, and the epidemic is discontinuous (see 8.11). Coffea arabica is an allo-tetraploid that survives only in cultivation, and it presumably obtained this functional deciduousness from one of its wild, diploid ancestors.

Barley rust: Winter barley germinates in the fall and can be infected with rust (Puccinia hordei), in an apparently continuous epidemic. However, the barley grows slowly throughout the winter, but the rust does not grow at all, and it cannot infect the new leaves. In the spring, all rusted leaves have died, and the rust has died with them (Parlevleit & Van Ommeren, 1976).

South American Leaf Blight of Rubber (SALB): It seems that a vertical subsystem has not been conclusively demonstrated in SALB (Microcyclus ulei) of rubber (Hevea brasiliensis), but hypersensitivity mechanisms occur, and both oligogenic and polygenic resistances are known (Holliday, 1970). The Amazon

Valley is permanently warm and wet and its pathosystems are apparently continuous. However, the rubber tree is deciduous, in spite of its growing in these conditions of continuous epidemics. Its pathosystem is consequently discontinuous.

Potato blight in Mexico: The wild potatoes (Solanum demissum, and other spp.) of Mexico are perennials but they have vertical resistances to blight (Phytophthora infestans). Their aerial parts are annual, however, dying out entirely during the winter. The tubers never get diseased because the blight fungus is apparently unable to survive in the soil. The epidemic is thus discontinuous, and the fungus survives the winter as oospores which are the result of sexual recombination, and which have variable vertical pathogenicities.

White pine blister rust: Rust (Cronartium ribicola) of the five-needle pines (Pinus spp.) of North America is believed to be a new-encounter disease, apparently introduced from Eurasia. There is a gene-for-gene relationship in the white pines and it is thought that this vertical subsystem is ancient. It may have evolved to function with a local pathotype of the rust, and it is possible that the local pine ecotypes have little horizontal resistance to a new-encounter allopatric ecotype introduced from Eurasia (see 8.15).

Taro blight: This tropical crop (Colocasia esculenta) is also known as cocoyam, dasheen, or eddoe. It is an evergreen perennial and it was domesticated from a wild progenitor that had a continuous pathosystem. In many of the islands of the South Pacific, a newly introduced blight (Phytophthora colocasiae) is causing severe damage. It is thought that vertical resistance to blight occurs in a wild relative of taro in the Himalayas, where this wild relative has discontinuous epidemics. These vertical resistance genes have apparently been introduced into the tropical taro of the South Pacific in the course of plant breeding.

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