Inbreeding and Selection
An issue related to selection differential is the total number of plants selected (i.e., the number of parents) each cycle. Addressing this issue involves the concept of inbreeding. Inbreeding occurs whenever related individuals mate. The most extreme form of inbreeding is self-pollination because the same plant is both the female and male parent. In any closed group of plants (“closed” means that no new plants from outside the group are allowed to enter and mate), all the plants will be related. Such a closed group of intermating individuals is referred to in genetics as a population. The degree of relationship among individuals in a population depends on the number of generations that has passed since the population became closed and the number of parents that formed each new generation. The more generations and the fewer the number of parents, the greater the average degree of relatedness among individuals; that is, the greater the level of inbreeding.
Inbreeding in corn causes loss of vigor. Therefore, inbreeding should be minimized though it cannot be totally avoided. Loss of yield due to inbreeding is not reflected in the breeders’ equation. One way to minimize inbreeding is to have many rather than few parents. This needs to be done every generation, because the inbreeding that occurs by having few parents one generation cannot be undone by having many parents during subsequent generations. The problem with many parents is that this can lower the selection differential. The actual effect on the selection differential depends on the total number of individuals whose phenotypes are measured. Selecting 100 individuals versus selecting only 10 individuals when the total number of individuals measured is 10,000 will have little negative impact on the selection differential. However, selecting 100 versus selecting 10 will reduce the selection differential substantially if the total number of individuals measured is only 200. Choosing a sufficient number of parents and maintaining a large selection differential is a balancing act. It is not a problem if large numbers of individuals can be evaluated. But time and resources for evaluation are not unlimited. A reasonable minimum number of plants to be used as parents in each generation in a mass selection program in corn is 100.
Are there steps that can be taken other than using large numbers of parents to minimize inbreeding? The answer is yes. One step is to make sure that an equal amount of seed from each parental ear is taken to plant the next generation. Being exact is not critical, but the more equal the better. Another step is occasionally introducing new material into the population. This is known as immigration. The rate of immigration need not be very high. In fact, a rate of 1% per generation can be quite effectual in minimizing inbreeding (1% = 1 parent out of 100 is from outside the population). It is important that introduced individuals not have performance that is much under the performance of the population. This will become increasingly difficult as continual selection improves the population.
A third way of minimizing inbreeding is having more than one selection program. Before starting the mass selection program, the population is split into two sub-populations, which are then selected independently in the same manner. When inbreeding increases to undesirable levels (as indicated by loss of vigor) in either or both sub-populations, then they are crossed to restore the vigor. Although inbreeding has occurred in both sub-populations, the genes at which inbreeding has occurred in one of the sub-populations likely will be different than the genes at which inbreeding has occurred in the other. This is why crossing the sub-populations restores much of the lost vigor. After crossing, the new population could again be divided into two sub-populations, and this process could be repeated. The drawback to this approach is that it requires twice the work and resources if the selection differential in each sub-population is the same as it would be in a single population.
The steps a corn breeder can take to minimize inbreeding during selection are summarized below:
i) Select many rather than few parents.
ii) Use an approximately equal number of kernels from each selected ear to plant the next generation.
iii) Incorporate outside individuals into the population (immigration).
iv) Conduct selection independently in sub-populations.
In this lesson, the discussion of mass selection and the improvements expected from this type of selection has assumed that phenotypic value (P), genotypic value (G), and environmental value (E) are related according to the equation below.
This assumption often is not valid. The real world is usually a little more complicated. This complication is the topic of the lesson on genotype-by-environment interaction in this series.