“Heterosis”is also known as hybrid vigor, this refers to the improved morphological trait of an individual in terms of growth, vigor etc, produced as a result of cross between two individuals not closely related.
It is out breeding that generally produces heterosis, because, the cross involves two different strains or stocks. Heterosis has been noticed both in plants and animals bringing about around improvement in terms of production, germination, resistance and quality (of meat, egg, milk, taste etc).
Heterosis or hybrid vigour has been observed since a long time. Kolereuter, a German plant breeder of the pre Mendelian period, – had observed, that hybrids in tobacco plants were more vigorous, than their parents. Beal (1880) demonstrated a similar behavior in corn.
Image Source: genome.cshlp.org
ADVERTISEMENTS:
Shull (1908), East (1908), Jones, Hayes and others, who have made detailed studies in maize, have also come to the conclusion of hybrid vitality, after comparing progenies obtained by self and cross pollination.
Out breeding of various crop plants like, wheat, sugar cane, potato etc., have been practiced to yield a better progeny.
Extensive experiments on hybrid vigor in maize have been conducted in USA resulting in the yield per hectare of about 75 lbs.
ADVERTISEMENTS:
Besides yield there was improvement of other traits also like size of ears, resistance to drought, disease etc. Hybridization experiments involving two varieties of Nicotiana – One measuring 31″ and the other 54″, have yielded progenies with a height of 64″.
Instances of cross breeding, resulting in hybrid vigor are not as numerous in animals as in plants possibly because of the sex chromosomes. The cross between horse and ass resulting in the production of mule is a classical example of our breeding.
But the animal is sterile even though it is stronger and swifter than both the parents. Vicario of USA working on white rats has reported that the offspring of a cross between two different races, had a better mental ability, and learned more quickly as to how to run through a maze than their parents. This type of heterosis is often described as Psychological heterosis.
According to Buchholz, physiological heterosis has been noticed in Pines (Finns spp) due to our breeding. Similarly it has been reported in guinea pigs, that crossing the domestic Variety (Cavia cobaya) with the wild variety produces offspring of a bigger size.
ADVERTISEMENTS:
In sheep also, hybrid vigour is clearly demonstrable. The Rambovillet variety has fine short wool, while the Lincoln variety has good meat. The product of these two known as Columbia breed has intermediate wool and high quality of meat.
Likewise, a cross between black faced Hampshire and Rambovillet sheep produces a hybrid with many desirable traits.
There is also a report on heterosis noticed in Drosophila melanogaster (Clarke and Smith, 1955), wherein cross breeding produces a hybrid female with the capacity of higher production of eggs.