Heterosis and in breeding depression and quantitative traits of the day k

Heterosis and in breeding depression

1. Heterosis and Inbreeding Depression

2. Heterosis The term heterosis was first used by Shull in in 1914 Heterosis is defined as the superiority of F1 hybrid over both its parents in terms of yield or some other character.

3. Types of heterosis Average heterosis Superiority of F1 over the average of the two parents or the mid parent, such superiority is regarded as average heterosis or relative heterosis .

4. Heterobeltiosis When heterosis is estimated over the superior parent, such estimate is referred to as heterobeltiosis . Economic heterosis : superiority of F1 over the best commercial variety, generally known as check variety.It is also known as standard heterosis and useful heterosis . Economic heterosis is the only estimate of heterosis , which is of practical value

5. Heterosis and dominance in relation to parental values Position and mean value of the parents Mean value of the F1 hybrid Phenomenon >10 Heterosis Parent A (10) 10 Complete dominance <10 but >8 Partial Dominance Mid parent (8) 8 No Dominance <8 but >6 Partial Dominance Parent B (6) 6 Complete dominance <6 hETEROSIS In 1944, Powers suggested that the term heterosis should be used only when the hybrid is either superior or inferior to both the parents.. Other situations should be regarded as partial or complete dominance

6. Genetic basis of heterosis There are three main theories to explain heterosis Dominance theory Over dominance theory Epistasis hypothesis

7. Dominance hypothesis The dominance hypothesis was first proposed by Davenport in 1908. In simplest terms, this hypothesis suggests that at each locus the dominant allele has a favourable effect while the recessive allele has an unfavorable effects. In heterozygous state, thozygous due to inbreeding.e deleterious effects of recessive alleles are masked by their dominant alleles. Thus heterosis results from the masking of harmful effects of recessive allels by their dominant alleles which become hom

8. Overdominance Hypothesis According to overdominance hypothesis, heterozygote at atleast some of the loci are superior to both the relevant homozygote. Thus heterozygote Aa at would be superior to both the homozygotes AA and aa.

9. Epistatic Hypothesis Influence of one locus on the expression of another locus may be involved in heterosis

10. Inbreeding Depression Inbreeding is mating between individuals related by descent or ancestry. When individual are closely related, the degree of inbreeding is high. The highest degree of inbreeding is achieved by selfing . The chief effect of inbreeding is an increase in homozygosity in the progeny, which is proportionate to the degree of inbreeding. Infact , the measure of degree inbreeding is provided by the degree of homozygosity in the progeny. E.g selfing reduces the heterozygosity by a factor of ½ in each generation and degree of inbreeding increases in the same proportion.

11. Inbreeding depression It is defined as the reduction or loss in vigour and fertility as a result of inbreeding.

12. Effects of inbreeding Appearance of lethal and sublethal alleles : Inbreeding results in the appearance of recessive lethal (leading to death), sublethal and subvital (reducing survival or reproduction rate) characteristics. Such characteristics include chlorophyll deficiencies, rootless seedlings , defects in flower structure etc. Reduction in vigour : There is general reduction in vigour of the population. Plants become shorter and weaker because of general reduction in the size of various plant parts. Reduction in reproduction ability: The reproduction ability of the population decreases rapidly. Many lines (plant progenies) reproduce so poorly that they can not be maintained.

13. Increase in homozygosity: Each line become increasingly homozygous following inbreeding. Consequently, the variation within a line decreases rapidly. Ultimately, after 7 to 8 generations of selfing , the lines become almost uniform, since they approach (>99 per cent homozygosity). The lines, which are almost homozygous due to continued inbreeding and are maintained through close inbreeding, preferably by selfing known as inbred lines or inbred. Reduction in Yield: Inbreeding generally leads to reduced yields. Inbred lines that can be maintained, yield much less than the open pollinated varieties from which they were derived.

14. Degrees of inbreeding depression Inbreeding depression in different crop species may range from very high to very low or it may be absent. The degree of inbreeding depression observed in various plant species may be grouped as follos : High Inbreeding Depression:Several plant species e.g. alfalfa, carrot etc. show very high inbreeding depression A large proportion of plants produced by selfing show lethal characteristivs and do not survive. The loss in vigour and fertility is so great that few lines can be maintained after 3 or 4 generations of inbreeding. The lines that do not survive show greatly reduced yields, generally less than 25 percent of the yield of the open pollinated varieties.

15. Moderate Inbreeding Depression: Many crop species such as maize, sorghum, bajra etc show moderate inbreeding depression. Many lethal and sublethal types appear in the selfed progeny, but a substantial proportion of lines can be maintained under self pollination. There is appreciable reduction in fertility and many lines reproduce so poorly that they are lost. However, a large number of inbred lines can be obtained, which yield up to 50 per cent of the open pollinated varieties. Production and maintenance of inbred lines are relatively easier in these species than in those showing a high degree of inbreeding.

16. Low inbreeding Depression: Several crop plants e.g. onion, many cucurbits, rye, sunflower etc show only a small degree of inbreeding depression. Further, only a small proportion of the plants produced by inbreeding show lethal or subvital characteristics. The loss in vigour and fertility is small, rarely a line can not be maintained due to poor fertility The reduction in yield is small or absent. Some inbreds lines may yield as much as the open pollinated varieties which they were developed.

17. Lack of inbreeding depression: Self pollinated species do not show inbreeding depression but they do show heterosis.

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