Alternational Discontinuity

There are two occasions in each life cycle of an alternating parasite when a change of host species is obligatory. This is the definitive characteristic of the alternating aphids and rusts. Real and functional discontinuity are possible in non-alternating pathosystems, but alternational discontinuity is possible only in alternating pathosystems. The critically important fact is that no alternator can auto-infect or parasitise the species of host on which it originated, even if host tissue is available. The alternator is compelled to infect a different species of host. This means that, effectively, every alternating pathosystem has a sequential discontinuity in both its summer and winter subsystems. (Note: some species of alternating aphid have one or two asexual reproduction cycles on the winter host, before alternation. To this extent, limited auto-infection of the winter host can occur. But, strictly speaking, it is not the spring alternators that auto-infect. Indeed, it is possible that a physiological inability to auto-infect is the mechanism that compels alternation.)

0 0

Post a comment