Rstrategist Parasites

The function of r-strategist reproduction is to exploit an ephemeral food supply as efficiently as possible by producing a very rapid population explosion of biologically cheap, small individuals. With plant parasites, these population explosions usually involve parasites of seasonal host tissue, and they are stabilised by the gene-for-gene relationship.

Among fungal parasites of plants, asexual reproduction is common. It occurs typically after a matching allo-infection has occurred. Asexual reproduction produces microscopic spores in really huge numbers. If the system of locking ensures that only an infrequent allo-infection is a matching infection, the positive feedback in the population explosion is reduced, and the epidemic is stabilised. Sexual reproduction occurs at the end of the epidemic, and in association with dormancy. At this point, the asexual spores are no longer propagules, and they become gametes. With the start of a new epidemic, sexual recombination will have ensured that there will be many different vertical pathotypes. This variability is necessary if each vertical pathotype is to become established.

Among insects, r-strategist reproduction is achieved by asexual reproduction, by viviparous reproduction, and by reproduction in immature, larval stages of development. Sexual reproduction, and sexual recombination in final instars, occur only at the end of the r-strategist phase of reproduction. This combination of two methods of reproduction is clearly very similar to that of the fungi. The aphids typically have asexual, viviparous reproduction, leading to rapid population explosions.

Gould (1978) has discussed the cecidomyian gall midges (Cecidomyiidae), which feed on mushroom populations that are of short duration. The r-strategist reproduction is asexual and viviparous, and it occurs in immature female larvae, which produce females only. These reproducing larvae undergo only one moult, and each produces up to 38 offspring in five days. The progeny devour the mother from inside and, within two days, their own offspring are beginning to devour them. With crowding, and a shortage of food, the reproduction returns to normal, with a sexual production of eggs and mixed broods by adults of both sexes. The normal sexual adults require two weeks to develop. However, it is not known whether a gene-for-gene relationship occurs in this host-parasite association.

Plant parasites as unrelated as fungi, aphids, gall midges, and beetles, have asexual r-strategist reproduction, and many of them have gene-for-gene relationships. Other plant parasites, such as the bacteria and viruses, have only asexual, r-strategist reproduction. Many of these also have gene-for-gene relationships. All of these r-strategist parasites, that have a gene-for-gene relationship, exploit seasonal host tissue in discontinuous pathosystems. This is a remarkable example of the parallel evolution of a complex phenomenon being repeated many times.

Chapter Five

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