Sequential Discontinuity

Sequential discontinuity refers to host tissue, which may be continuously available, or only intermittently available. Host tissue is continuously available to the parasites of evergreen trees and other perennials, particularly in tropical rain forests that lack a winter and a dry season. Host tissue is only intermittently available to the parasites of annual hosts, to the aerial parasites of perennial herbs, which lose their above-ground parts during the adverse season, and to the leaf and fruit parasites of deciduous trees and shrubs.

It has been argued (see 4.11) that a gene-for-gene relationship and, hence, a vertical subsystem, can evolve only in a pathosystem that has both sequential discontinuity, and genetic diversity (spatial discontinuity). These four possibilities of sequential continuity or discontinuity, and genetic uniformity or diversity, are highly relevant to the study of alternating pathosystems. And they are doubly relevant because each alternating pathosystem has two subsystems, the summer subsystem and the winter subsystem. There are thus eight possible configurations for each alternating pathosystem.

One of the more arresting conclusions to emerge from the present study is that all alternating pathosystems appear to conform to only one of these eight possible configurations, and that this involves sequential discontinuity, and genetic diversity, in both subsystems. In other words, every alternating pathosystem apparently meets the preconditions of a gene-for-gene relationship in both its winter and summer subsystems.

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