But the conflict was very one-sided. There was vociferous and almost universal opposition to the very idea of horizontal resistance. I myself have witnessed respectable scientists so angry at the mere mention of horizontal resistance that they showed all the symptoms of incipient apoplexy. The Mendelian techniques of pedigree breeding, back-crossing, pure lines, and vertical resistance dominated the whole of crop science.
To even question this "received wisdom" was to invite trouble. The dominance of the Mendelian school is vividly illustrated by the point that, until Vanderplank published his book, very few crop scientists had even realised that there were, in fact, two kinds of resistance to the parasites of crops.
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Indeed, many crop scientists vigorously denied the very existence of horizontal resistance. A few of them still deny it, and most of them are still quite unwilling to employ it, or even to investigate it. It is now clear that the conflict over vertical and horizontal resistance was actually a revival of the original genetic conflict between the members of the Mendelian school and the biometricians.
What is depressing about this story is that the original genetic conflict started in It was resolved scientifically about thirty years later. The two kinds of resistance were recognised by Vanderplank about thirty years later still. And, thirty years after that, in the s, the whole of crop science is still dominated by the Mendelian school of genetics, the Mendelian methods of plant breeding, and the Mendelian resistances to crop parasites. We must now enquire why the two kinds of resistance to plant parasites should have evolved in plants in the first place.
The word infection has many shades of meaning in the English language. In medicine, it is sometimes taken to mean the disease itself, and we speak of a patient having a "nasty infection". In its adjectival form of "infectious", it usually means a contagious disease that is caused by a biological agent, such as a virus or bacterium.
However, we frequently speak of a laugh, or a yawn, being infectious. Throughout this book, the term infection is defined quite strictly. It means the contact made by one parasite individual, with one host individual, for the purposes of parasitism. And there are two kinds of infection, just as there are two kinds of pollination. It will be remembered that cross-pollination means that a plant is pollinated by pollen from another plant, while self-pollination means that a plant is pollinated by its own pollen.
The technical term for cross-pollination is allogamy, while self-pollination is autogamy.
These terms are derived from ancient Greek. Allo means other, or different; auto means self; and gamy means marriage or reproduction. The two kinds of infection are called allo-infection and auto-infection. Allo-infection is equivalent to cross-pollination, and it means that a host plant is infected by a parasite individual that has arrived from another, different host, or from an independent, dormant state.
The parasite had to travel to its new host. Conversely, auto-infection is equivalent to self-pollination, and it means that a host is infected by a parasite individual that was born on, or in, that same host. The parasite had no need to travel. There is a close analogy with travelling people. Think of the individual host plant as an island, surrounded by sea. Allo-infection is then equivalent to an immigrant arriving on that island, by boat or plane, from somewhere else. Auto-infection is equivalent to the colonisation of the island by the descendants of that immigrant.
This people analogy can also embrace the two kinds of resistance. Think of Ellis Island, in New York, in the bad old days. The parasite genes of a gene-for-gene relationship correspond to the immigration papers of an immigrant, and the host genes correspond to the immigration laws of the U. These papers and laws either match, or they do not match. The immigrant is accordingly allowed in, or is denied entry, as the case may be.
Horizontal resistance, on the other hand, is represented by the living conditions in the immigrant's new land, which make it either easy or difficult for that immigrant to prosper. Three further points are worth making. If the island is deserted, the first person to inhabit it must come from outside.
The first infection of any plant host must be an allo-infection. Second, colonisation can proceed only after a successful immigration. Auto-infection of a plant host can occur only after there has been a matching allo-infection. Third, whenauto-infection, or colonisation, has continued for some considerable time, possibly for many generations of colonisers, the island becomes crowded.
Some individuals may then leave the island in search for another, less crowded island, somewhere else. These explorers will be migrants, and they will allo-infect their new host, their new island. Two real-life examples will further illustrate this difference between the two kinds of infection, which is critically important. Most people are familiar with the small insects known as aphids, green flies, or green bugs. Anyone who has grown roses will know what a pest they can be. Aphids have several, morphologically different forms, and each form has a special function.
Among others, there is both a winged form, and a wingless form.
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The function of the winged individuals is clearly that of allo-infection, which is possible only by flying. The function of the wingless individuals is obviously that of auto-infection, which is possible by walking. If a rose bush is completely free of aphids, it is the equivalent of a deserted island. The only possible infection is allo-infection, and this requires a winged aphid. Once it arrives, this allo-infecting aphid, which is invariably a female, will feed on its host and begin to reproduce.
Unlike most other insects, it will reproduce without sex, and with live births rather than the laying of eggs. The sexless reproduction is the equivalent of vegetative propagation in plants, and all the progeny are genetically identical to their mother.
They constitute a clone. The loss of the egg stage saves time, because the young are born alive. They are also born without wings, because flying is not necessary for auto-infection.
The young are all female, and they grow very rapidly as a result of sucking the rich juices of their host. Soon, they too start their own sexless and eggless reproduction. There is then a population explosion of aphids, all auto-infecting the same host plant. All rose growers know how quickly a rose bush can become crowded with aphids.
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Eventually, over-crowding stimulates the birth of winged individuals, which then fly away to allo-infect a rose bush somewhere else. Ecologists have a special term for this kind of reproduction. They call it r -strategy. An r -strategist species is one that reproduces very rapidly and cheaply, with large numbers of very small offspring. It is a quantity breeder. It can exploit an ephemeral food supply very effectively by producing a population explosion. This explosion is followed by a population extinction when the food supply disappears, usually with the onset of an adverse season.
Only a very few individuals survive the winter, or the tropical dry season, but there are enough of them to produce another population explosion in the following favourable season. Most of the serious pests and diseases of our crops are r -strategists, and it is their population explosions that can be so alarming, so damaging, and so very difficult to control. The second real-life example concerns a disease of coffee trees called rust. This fungus parasite, like its coffee host, is a native of Africa.
In , coffee leaf rust appeared for the first time in Brazil, which is the world's largest coffee producer, and a chill of fear spread among everyone in the coffee trade. Fortunately, the disease was riot nearly as serious in the New World as people had feared, and all of us can still have our morning cup of coffee. Coffee rust is caused by a microscopic fungus which reproduces by means of spores so small that they are invisible.
These spores are similar in size and shape to the pollen cells of flowering plants.
When pollen cells are seen en masse, they are yellow, and when rust spores are seen en masse, they are the colour of rusty iron. Just as iron rust will leave an orange smudge on your finger or clothing, so will coffee rust. Hence its name. Scientists in East Africa discovered that the spores of coffee rust are sticky, and that they are highly resistant to becoming air-borne, and to being dispersed by wind. But they are freely dis-persed in water, and every coffee tree gets wet when it rains. Shortly after this discovery was made, it became obvious that the newly introduced disease in Brazil was spreading at a rate of hun-dreds of miles each year.
Brazilian scientists showed that the rust spores were wind-borne. One of those silly scientific disputes arose, with everyone assuming that, if one side were right, the other must be wrong. The spores had to be either water-borne, or wind-borne, and that was that.