Anholocycly

The alternational discontinuity can be easily disrupted in a crop pathosystem when, for man-made reasons, the summer subsystem is able to persist indefinitely. Aphids which exhibit this phenomenon are described as anholocyclic. This term means that there is an unbroken progression of asexual reproduction, on the summer host, throughout the entire year, and that the parasite reproduction continues in this fashion indefinitely.

Anholocycly (and its equivalent in the alternating rusts, for which there is apparently no technical term) can change a summer subsystem from being discontinuous to being continuous. This is important in the present discussion because anholocycly means that alternating pathosystems are not necessarily discontinuous. The summer hosts may persist, and they may continue to produce summer subsystem colonisers and migrants, in what has now become a continuous pathosystem.

If the sexual phase has been entirely lost, this form of parasitism is clearly an evolutionary dead-end, and it is unlikely to occur in wild pathosystems. In reality, it is most unlikely that the sexual phase would ever be lost completely, and even a relatively rare sexual recombination will suffice for most evolutionary purposes. Even so, anholocycly is apparently a phenomenon of either the man-made global redistribution of species, or of agriculture. It may prove to be relatively rare, or even totally absent, in pathosystems that are both wild, and that involve old-encounter parasites.

Anholocycly usually occurs when either the parasite, or its host, or both, have been moved to a new area by agriculturists. Two modifications to the life cycle are then possible. First, a milder climate, or artificial practices, such as irrigation, or greenhouse cultivation, permit continuity of summer host tissue. Second, the hypothetical 'triggers' that stimulate the production of the alternator form, or that prevent further reproduction of the summer colonisers and migrants, may be either inadequate or lacking.

There is apparently no term corresponding to 'anholocyclic' for the rusts, but the phenomenon is known in some crop pathosystems. For example, wheat stem rust (Puccinia graminis) survives entirely as summer colonisers (uredospores) in the high altitude regions of equatorial Kenya. Winter hosts do not occur, and commercial wheat crops are cultivated for ten months of each year. Rogue wheat plants provide summer host tissue to bridge the remaining two months.

Coffee leaf rust (Hemileia vastatrix) also survives asexually on arabica coffee crops. Although autumn migrants (teliospores) have been observed, their basidiospores apparently cannot infect coffee, and no sexual stage has ever been observed on coffee. The rust is believed to be an alternating one, even though an alternate host has yet to be identified. Nutman & Roberts (1963) showed that dry season survival of the rust can be greatly enhanced by inappropriate fungicidal spraying which encourages the retention of rusted leaves during the dry season. Normally, these leaves would be lost and, as infection cannot occur without free water, the pathosystem is discontinuous.

Two points concerning anholocycly are important and should be noted. First, if the phenomenon of anholocycly is an artificial one, recognition of its artificiality is important, because it could otherwise lead to misconceptions concerning the functioning of wild alternating pathosystems.

Second, loss of the autumn alternation leads to anholocycly, while loss of the spring alternation leads to autoecy. A non-alternating parasite (i.e., an autoecious rust or a monoecious aphid) is one that does not alternate, having a single host species on which both sexual and asexual reproduction occurs. Non-alternation is more likely than anholocycly in the wild pathosystem because of the necessity to retain sexual reproduction.

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