There are two further kinds of infection, just as there are two kinds of pollination. Cross pollination (allogamy) means that the pollen came from a different parent. Self-pollination (autogamy) means that the pollen came from the same parent, and usually the same flower. By the same token, allo-infection is infection in which the parasite originates away from the host that it is infecting The parasite had to travel to its host. Auto-infection is infection in which the parasite originated on the same host that it infects. The parasite had no need to travel.
Allo-infection means that the two individuals of host and parasite have to come together. Usually, it is the parasite that must find the host, but other arrangements occur. Vertical resistance operates against various categories of allo-infection as follows. • Airborne _parasites: Typical air-borne parasites are the aphids and rusts. In a discontinuous pathosystem, these parasites have to allo-infect newly emerged seedlings, or the newly emerged leaves of a deciduous tree. In a wild pathosystem, the host population is heterogeneous. A wide variety of different vertical resistances will provide a system of locking (see 4.14) that ensures that most of these allo-infections are non-matching.
Contaminated seed occurs typically with the covered smuts of the small grain cereals, in which the smut spores are carried on the outside of the seed, or in the soil, and the actual allo-infection occurs at the time of seed germination. The frequency of matching can be greatly reduced by the system of locking produced by vertical resistances.
Infected seed occurs typically with the loose smuts of small grain cereals, in which the smut spores allo-infect the stigma in order to penetrate the seed. Once again, the system of locking produced by vertical resistances can greatly reduce the frequency of matching. It is in these loose smut diseases that the analogy between vertical resistance and pollen self-incompatibility is at its closest.
Auto-infection is infection in which the parasite originates on, or in, the host that it infects. The parasite has no need to travel.
With asexual reproduction, the auto-infecting parasite is a member of a clone, and it is descended from an allo-infecting parasite that matched the vertical resistance of the host. It follows that all auto-infection is matching infection. Even when the allo-infecting parasite has sexual reproduction only, the non-matching segregants will be lost, and the auto-infecting parasite population will quickly achieve homogeneity of the matching vertical pathotype.
The individual host has been usefully compared to an island, with the parasite being compared to immigrants and colonisers. Allo-infection is the equivalent of an immigrant arriving, by boat or plane, on that island. Auto-infection is the equivalent of the descendants of that immigrant colonising the island.
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