Discontinuous pathosystems

It is now clear that the gene-for-gene relationship has evolved in many different kinds of parasites of plants, including Angiosperms (e.g., Orobanche), insects, nematodes, fungi, bacteria, and viruses. That it should have evolved analogously in so many, and in such diverse groups of parasites is a powerful argument for evolution operating by group selection on emergents at the higher systems levels. The system of locking of the gene-forgene relationship is clearly an emergent, and it is a stabilising mechanism of immense value in plant pathosystems. It is clear also that the gene-for-gene relationship evolved to stabilise mainly r-strategist parasites that are capable of major population explosions because of their asexual reproduction.

K-strategist plant hosts, such as large trees, have massive nutrient reserves and they easily survive an occasional lean season when parasites destroy seasonal tissue, such as leaves, fruit, and seeds. Many mature trees are also fire-resistant.

Many plants have recovery mechanisms that enable them to restore their population after a parasite devastation, or a major fire. These mechanisms include the nutrient reserves in the buried, dormant seeds of annuals, and the subterranean storage organs of perennial herbs, such as tubers, rhizomes, or bulbs.

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