Two Kinds of Population

A plant pathosystem involves the two populations of host and parasite, and either of these populations may be genetically uniform, or genetically diverse. Generally, the host population controls the parasite population in this respect, and a parasite population will be heterogeneous only under the compulsion of host population heterogeneity. If the host population is genetically uniform, the parasite population will usually acquire a comparable uniformity.

A wild host population is genetically diverse. It exhibits variation for every genetically controlled, quantitative variable and, usually, this variation has a normal distribution. It may also possess a mixture of Mendelian characters such as vertical resistances.

A cultivated plant population is usually genetically uniform. A clone is genetically uniform because all the individuals within it are descended by vegetative propagation from a single individual. A pure line is uniform because the population is both homogeneous and homozygous. And a hybrid variety is uniform because it is the result of crossing inbred lines.

Some crops are genetically diverse. Alfalfa (Medicago sativa), for example, is cultivated as an 'improved' but genetically diverse population because of the exigencies of seed production. Most subsistence crops in the tropics are also genetically diverse.

It should be noted that genetic uniformity and diversity can also occur over periods of time. For example, it is possible to achieve diversity over time by cultivating a genetically uniform crop, but by using a different cultivar, with a different vertical resistance, each season (see also: Person model, 4.18, and Fig. 4.7).

Heterogeneous and homogeneous populations should not be confused with heterozygous and homozygous individuals. This is a difference of systems level. Both involve the difference between genetic mixture and genetic uniformity, but the one involves populations, while the other involves the genetic constitution of the individual.

Note the difference between homogeneous (uniform descent, as with a clone or pure line) and homogenous (uniform composition, as with homogenised milk). The prefix 'homo-' is Greek and should be pronounced with a short 'o'. It should not be confused with the Latin Homo (= man), which is pronounced with a long 'o'.

In the context of plant populations, the term 'multiline' should be noted. This is a mixture of several closely similar, homozygous lines which differ in their vertical resistances. It is thus a heterogeneous population of several different homozygous vertical pathodemes. A multiline is an attempt to produce a system of locking (see 4.15) with vertical resistances in agriculture. However, severe practical difficulties limit this approach.

Plants may be open-pollinated (allogamous) or self-pollinated (autogamous). In wild ecosystems, open-pollination is the norm and it ensures genetic recombination and variation. Self-pollination is apparently a survival mechanism for annual species that are totally dependent on seeds for their survival from one season to the next, and which lack an effective mechanism of seed dormancy. Should there be adverse seasons, in which natural cross-pollination was severely restricted, autogamous species survive because they are able to produce seed by self-pollination. However, even self-pollinating species invariably have a small percentage of cross-pollination to provide the heterozygosity and heterogeneity that are essential for a response to ecological and evolutionary selection pressures. (An alternative survival mechanism for such species involves dormant seeds, which remain in the soil but do not germinate for several seasons).

It transpired that the Mendelian, single-gene, pedigree breeding was ideally suited to self-pollinated species, in which relatively few crosses are necessary. The biometricians' many-gene, recurrent mass selection was more suited to breeding open-pollinated species, in which many crosses are necessary. The early success of the Mendelians persuaded plant breeders to use this pedigree breeding in most of the important crops, even in open-pollinated species (e.g., sugarcane) and vegetatively propagated species (e.g., potatoes), in which recurrent mass selection would have been more suitable.

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