Heterosis

The term 'heterosis' was coined by Shull (1914) and in ordinary usage hybrid vigour and heterosjs are practically synonymous. However, sometimes a distinction is made between hybrid vigour and heterosis. For instance, according to Whaley (1944), 'hybrid vigour denotes the manifest effects of heterosis'. Hybrid vigour or heterosis can also be regarded as the converse of the deterioration accompanying inbreeding. In fact, Shull (1909) had concluded that the inbreeding depression and heterosis were different aspects of the same phenomenon. Powers, on the other hand, considers it as a phenomenon encountered specially in quantitative inheritance. The relationship between F₁ hybrids and their parents as shown by him is summarized in Table 47.1.


Manifestations of heterosis
Hybrid vigour or heterosis can be manifested in many ways in addition to increase in size and productiveness. It may be either morphological or physiological in nature. For instance, in certain F₁ hybrids of beans the number of nodes, leaves and pods is greater than in the parents but the gross size of plant remains unaffected. In some hybrids, the growth rate is increased with no increase in size at maturity. Earlier maturity in F₁ than in either parents, is another manifestation of heterosis and 4s sometimes accompanied with actual decrease in total plant weight. In hybrids, greater resistance to diseases and insects, and increased tolerance to vagaries of climate are some other examples of heterotic effect. However, distinction between luxuriance and true heterosis must be made clear. According to Th. Dobzhansky (1952), luxuriance-the excessive size and vigour, found in some species hybrids and in certain crosses between strains of self pollinating species, is not necessarily an expression of superior adaptation, but is regarded as an accidental condition with quite different genetic antecedents and implications of true heterosis.

Some examples of heterosis
In corn it has been shown by G.H. Shull (1909) that hybrids between inbreeds of diverse parentage generally give greater hybrid vigour than that shown by hybrids between inbreds derived from same or similar open pollinated varieties. These findings have been confirmed by other workers and wer6 used in the selection of double crosses for commercial production of hybrid corn. In a number of cases including onion, alfalfa and cabbage, it was shown that the intervarietal hybrids gave higher yields than the better of the two parental varieties.

There are also manifestations other than yield or vigour in hybrids, which are put forward as causes for heterosis. For instance in tomato, Ashby (1930) concluded that the heterosis was due to initial advantage in embryo size. However, other workers do not concur with this view. For instance, East (1936) observed that hybrid seeds were smaller than those of parents and Hatcher (1940) using the same tomato stocks which were used by Ashby concluded that embryo size was of little importance in the expression of heterosis. Sprague similarly concluded as follows : "it appears that hybrid superiority may result from a more efficient utilization of nutrient, increased rate of cell-division, greater ability to synthesize required growth substances, and possibly from other as yet unrecognized causes."