With the recognition of Mendel's work on inheritance, in 1900, a foolish scientific dispute arose between the older biometricians, and the new Mendelians. The biometricians dealt with the inheritance of quantitative characters, while the Mendelians dealt with qualitative characters. The biometricians had practical agriculture on their side. All agriculturally valuable traits, in both plants and animals, appeared to be inherited quantitatively. There seemed to be no valuable characters that were inherited qualitatively.

The Mendelians had science on their side. Mendel's experiments were repeatable, and the discovery of chromosomes gave them powerful supporting evidence. The biometricians were not able to explain their quantitative inheritance in such scientific terms.

The dispute became acrimonious, with both sides assuming that, if one side was right, the other must be wrong. However, the dispute was resolved a couple of decades later, when it was shown that the inheritance of some characters could be controlled by more than one gene. In other words, quantitative inheritance was controlled by many genes, called polygenes, which individually obeyed Mendel's laws.

However, in the meanwhile, Biffin (1905) had shown that the inheritance of resistance to wheat rust (Puccinia striiformis) was qualitative and Mendelian. The Mendelian school of genetics now had a qualitative character of economic importance. It was soon shown the inheritance of resistance to various other plant diseases was also controlled qualitatively. The Mendelians pursued this advantage with such vigour that they soon dominated plant breeding. And they have dominated it ever since.

For most of the past century, plant breeders have chosen to work with single-gene resistances, and to use gene-transfer breeding techniques in order to incorporate resistance genes into crops, usually from wild host plants. This method would normally result in a new crop variety that consisted of a homogeneous population that was a pure line, a clone, or a hybrid variety. Every plant in such a population would carry the same single gene for resistance. However, several resistance genes would sometimes be used in combination, in a so-called pyramid, to provide a more complex resistance that was common to all the individuals in the homogenous population.

This qualitative resistance of the Mendelians was controlled by single genes derived from a gene-for-gene relationship, and it was the kind of resistance that is now called 'vertical resistance'. This resistance provided either a complete protection or no protection at all. The so-called 'gene-transfer' breeding techniques were developed to enable plant breeders to transfer a resistance gene from a wild plant to a cultivar by repeated back-crossing. This became the classic breeding method of the twentieth century, and it was known as 'pedigree breeding'. Unfortunately, vertical resistance operates against some strains of the parasite but not others. These strains were known as a physiologic races, pathologic races, biotypes, or vertical pathotypes. When a matching vertical pathotype appeared in the agro-ecosystem in question, the vertical resistance stopped functioning and, in common usage, it was said to have 'broken down'. In fact, the resistance remained unaltered, and it was the parasite population that had changed.

The quantitative resistance of the biometricians was inherited polygenically. It was the resistance that is now called 'horizontal resistance'. The level of this kind of resistance normally exhibits every degree of difference between a minimum and a maximum, usually with a normal distribution. In crops, the minimum level of horizontal resistance is characterised by a complete loss of crop in the absence of crop protection chemicals. The maximum level is characterised by a negligible loss of crop in the absence of crop protection chemicals. Horizontal resistance has the advantage that it is durable. Indeed, for all practical purposes, it is permanent resistance.

Although the scientific dispute between the biometricians and the Mendelians was resolved within a couple of decades, the dispute between the two schools of practical plant breeders continued and, indeed, it continues still. When Vanderplank (1963) published his classic work on plant pathology, defining and clarifying this situation, most plant breeders vehemently denied the very existence of horizontal resistance. They had strong, if false, grounds for their conclusion (see 7.16) and, being Mendelians, they had no wish to give way to quantitative breeding methods.

Sadly, a comparable dispute is now reigning between the molecular biologists and the traditional plant breeders. It really is essential that the genetic engineers undertake some fundamental studies concerning the durability of their transgenic resistances, before claiming that they can solve crop parasite problems.

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