In this essay we will discuss about the outbreeding which is a negative genetic assortative mating.
When a mating involves individuals that are more distantly related than the average of the selected group, it is classified as outcrossing or outbreeding which is a negative genetic assortative mating. Outbreeding involves crossing individuals belonging to different families or crossing different inbred varieties of plants or crossing different breeds of livestock.
Outbreeding increases heterozygosity and enhances the vigour of the progeny, i.e., hybrid has superior phenotypic quality but often has poor breeding value than the parental populations. Thus, the two inbred parents homozygous for different genes, if crossed produce F1 progeny heterozygous for all the genes.
Such a F1 progeny or hybrid may have improved general fitness, resistance to diseases and it may show remarkable growth and vigour. The superiority of the hybrid over the best parent is called heterosis, a term coined by Shull (1914) for describing hybrid vigour.
In ordinary usage, the terms heterosis and hybrid vigour are used as synonyms, however, sometimes a distinction is made between these two terms. According to Shull, the developed superiority of the hybrids is the ‘hybrid vigour’ and heterosis is the mechanism by which this superiority is developed.
Also, according to Whaley (1944) ‘hybrid vigour denotes the manifest effects of heterosis’. Powers regarded the heterosis as a phenomenon encountered in quantitative inheritance and he has shown following relationship between F1 hybrids and their parents (see Table 52.1).
Cross-Breeding and Male Production:
Mating of individuals from entirely different races or even different species is called cross – breeding. This represents the most extreme form of outbreeding that is possible among animals. Cross-breeding produces sterile hybrids in comparison to normal out-breeding’s.
Example:
A mule is a hybrid of a male donkey (Equus asimus, 2n =62) and a female horse (Equus caballus, 2n = 64). The hybrid from the reciprocal cross (i.e., a female donkey or jenny and a male horse or stallion) is called henny.
Mule shows hybrid vigour and because of this it has served mankind as a patient beast of burden since time immemorial. Mules are larger than the donkey and sturdier than the horse. However, they are sexually sterile and have to be produced every time anew. Donkey stallions have been imported by the Indian army from Europe for breeding mules.
There are two kinds of mules which are used by the Indian army:
(1) General service type and
(2) Mountain artillery type.
The latter are very important as they are firm-footed animals that can carry heavy loads on steep Himalayan mountain terrain.
Manifestation of Heterosis:
In addition to increase in size and productiveness, heterosis is manifested in many ways. It may be either morphological or physiological in nature. For example, in some crosses of beans certain F1 hybrids contain greater number of nodes, leaves and pods than their parents; however, the gross size of plant remain unaffected. In some hybrids, the growth rate is increased but there occurs no increase in size of mature plant.
Further, earlier maturity of F1 hybrids than in either parent is another manifestation of heterosis and is sometimes accompanied with actual decrease in total plant weight. In hybrids, greater resistance to diseases and to insect infestation and increased tolerance to abnormal climatic condition are some other examples of heterotic effects in plants.
Sprague forwarded the following explanation for the hybrid vigour:
“It appears that hybrid superiority may result from a more efficient utilization of nutrients, increased rate of cell division, greater ability to synthesize required growth substances, and possibly from other as yet unrecognised causes.”
Some Examples of Heterosis in Plants:
G.H. Shull (1909) has shown that in com or maize, hybrids between inbreeds of diverse percentage generally give greater hybrid vigour than that shown by hybrids between inbreeds derived from same or similar open-pollinated varieties.
However, such an exploitation of heterosis in maize posed one problem; most inbred lines were so infertile that they did not produce enough seeds for commercial plantings. Shull’s method was modified by a method called double cross method.
Double cross method for producing hybrid corn:
Starting with four inbred lines (A, B, C, D), a single-cross is made between A and B by growing the two lines together and removing the tassels from line A so that A cannot self-fertilize, and, thus, received only B pollen.
In another locality, the same method is followed for lines C and D. The yield of single-cross hybrid seed is usually low because the inbred parent lacks vigour and produces small cobs. Plant that germinate from single-cross seeds are usually vigorous hybrids with large cobs and many kernels.
It is undesirable for the single-cross hybrid to self-fertilize, as this inbreeding process commonly produces less vigorous progeny. Therefore, a double cross is made by using pollen from the CD hybrid on the AB hybrid.
Heterosis in other plants:
In a number of cultivated plants such as onion, alfalfa and cabbage, it was shown that the intervarietal hybrids gave higher yields than the better of the parental varieties.
Genetical Basis of Heterosis:
The genetical basis of heterosis is still a subject of controversy and following two hypotheses have been propounded to explain it:
1. Dominance Hypothesis of Heterosis:
The dominance hypothesis of heterosis holds that increased vigour and size in a hybrid is due to combination of favourable growth genes by crossing two inbred races. In other words, the hybrid vigour is a result of action and interaction of dominant or fitness factors or cumulative (polygenic) effect of dominant genes.
Example:
If we suppose, that a quantitative trait is governed by four genes, each recessive genotype contributes one unit to the phenotype and each dominant genotype contributes two units to the phenotype. A out-cross (outbreeding) between two inbred lines can produce more heterotic F1 individuals than the parents, in following manner-
The dominance hypothesis of heterosis has been supported by certain experiments. For example, Quinby and Karper (1946) studied heterosis in Sorghum and observed that the heterozygote Mama is significantly late in maturity and produces a greater weight of grain than either of the homozygote parents, Mama or mama.
Keeble and Fellow studied two varieties of pea both semi-dwarf, one with thin stem and long internodes and the other with thick stems and short internodes. The F1 hybrid was much taller than either of the parents, combining the long internodes of one parent and many nodes of the other.
2. Overdominance Hypothesis of Heterosis:
Overdominance hypothesis was proposed by Shull and East, independently in 1908. They considered that there is a physiological stimulus to development that increases with the diversity of the uniting gametes.
In Mendelian terms, it means that there are loci at which the heterozygote is superior to either homozygote and that vigour increases in proportion to the amount of heterozygosity. The overdominance hypothesis is variously known as single gene heterosis, cumulative action of divergent alleles, or stimulation of divergent alleles. Fisher (1930) called it superdominance.
Example:
If we suppose that four gene loci are contributing to a quantitative trait, homozygous recessive genotype contribute 1 unit to the phenotype, heterozygous genotypes contribute 2 units to the phenotype and homozygous dominant genotypes contribute ½ units.
Then the results can be represented as follows:
Application of Heterosis:
Heterosis has been exploited at commercial scale both in plants and animals. Among plants, it is applied to crop plants, ornamentals and fruit crops. It is found more important in vegetatively propagated perennial plants.
Thus, in fruit plants and ornamental plants, if heterosis is once achieved, it may be maintained for long. Among cross-pollinated crops, heterosis is exploited in the form of hybrids, composites and synthetic seeds.
Sometimes, intermediate phenotypes are preferred, in such a case heterosis involves mating of parents having opposite phenotypes. For example, general purpose cattle can be produced by crossing beef type with a dairy type.
The offspring commonly produce an intermediate yield of milk and hang up a fair carcass when slaughtered. The same is true of the offspring from crossing an egg type (such as Leghorn breed of chicken) with a meat type (such as the Cornish). Crossing phenotypic opposite may also be made to correct specific defects.
For example, ‘weedy’ relatives of agriculturally important crops may carry genes for resistance to specific diseases. Hybrids from such crosses may acquire disease resistance, and successive rounds of selection combined with back crossing to the crop variety can eventually fix the gene or genes for disease resistance.