Heterosis
- The superiority of hybrid is known as heterosis and may show improved general fitness characteristics such as longevity and resistance to disease.
- Such improvement are also called ‘hybrid vigor’.
- But sometimes, such improvement may be accompanied by a deterioration in others.
- For example, hybrid of female donkey and male horse results mule is superior to both parents in fitness and in many physical aspects yet it is almost always sterile.
- For example, heterosis in corn has received marked effects on yield improvement.
- Together with improved farming methods, hybrid corn has helped to triple the average corn yield.
Important assumptions for inbreeding and heterosis
- The usual method of raising hybrid corn is to establish many inbred lines, make intercrosses and determine which hybrids are most productive at a given locality.
- More hybrid seed can be obtained from vigorous plants than from weak inbred lines, ‘double cross’ is made in which two pairs of selected hybrids are crossed to furnish ultimately harvested corn.
- To prevent self-fertilization in hybrid crosses, cytoplasmic factors which produce male sterility have been used with restorer genes.
- A cytoplasmic male sterility factor (s) prevents one of the strains from acting as male parent. A cross between this female and pollinating plant bearing restorer genes (R) can produce offspring, all or half of which are capable of producing pollen.
- The commercial hybrid seed that is planted by farmers using these strains contain an appreciable number of such restored plants, which are induced to ensure full pollination of all corn ears.
- Cytoplasmic male sterility factors prevents inbreeding have been used to produce a large number of hybrid varieties of onions, carrots and beets, etc.
Types of heterosis
- Average heterosis or Relative heterosis:
- If the hybrid is superior to the mid parents, it is called as average heterosis.
- Economic heterosis:
- It is the only estimate of heterosis, which is of commercial or practical value.
- Heterobeltiosis:
- When the heterosis is estimated over the superior parent, this is known as heterobeltiosis.