The spring alternation

The 'spring alternators' have to travel to the summer host. Note that the first infection of each summer host individual must be an allo-infection. If there is a vertical subsystem, there must also be a matching allo-infection on each summer host individual before the parasitism of that host can commence. The sexual recombination of the parasite on its winter host ensures a wide diversity of vertical pathotypes among the spring alternators but, nevertheless, the parasite faces formidable obstacles.

Sexual recombination during the winter ensures a great diversity of vertical pathotypes among the spring alternators at a time when heterogeneity is needed most. This is because a wide diversity of summer hosts, with different vertical resistances, must be matched at the onset of the summer epidemic. It is reasonable to speculate that only vertical pathotypes with n/2 vertical parasitism genes will be produced on the winter host, and that they will be produced with an equal frequency, and a random distribution, in accordance with the n/2 model (see 4.15). It is also likely that only summer hosts with n/2 vertical resistance genes will occur, and that they too will occur with equal frequency and a random distribution. On average, all vertical pathotypes should find matching hosts with an equal frequency.

The majority of the initial summer host allo-infections will be non-matching infections. This proportion will depend on n, the number of pairs of matching genes in the gene-for-gene relationship (Fig. 4.6). This means that the majority of spring alternators will fail to find a matching host, and will be lost.

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