Degree of Inbreeding Depression
The various plant species differ considerably in their responses to inbreeding. Inbreeding depression may range from very high to very low or may even be absent into the following four broad categories.
1) High inbreeding depression,
2) Moderate inbreeding depression,
3) Low inbreeding depression, and
4) Absence of inbreeding depression.
High Inbreeding Depression:
Several plant species, E. g alfalfa (M. sativa) carrot (D. carota) , hayfield, tarweed etc show very high inbreeding depression. A large proportion of plants produced by selfing shows lethal characteristics and do not survive. The loss in vigour and fertility is so great that very few lines can be maintained after 3 to 4 generation of inbreeding. The line shows greatly reduced yields, generally less than 25 percent of the yield of open – pollinated varieties.
Moderate Inbreeding Depression:
Many crops species, such as maize, jowar, bajara etc. shows moderate inbreeding depression. Many lethal and sublethal types appear in the selfed progeny, but a substantial proportion of the population can be maintained under self- pollination. There is appreciable reduction in fertility and many line reproduce so poorly that they are lost. However, a large number of inbred lines can be obtained, which yield upto 50 percent of the open- pollinated varieties.
Low Inbreeding Depression:
Several crop plants, E. g onion (A. cepa), many cucurbits, rye (S. cereale), sunflower (Hannus) , hemp etc show only a small degree of inbreeding depression. Only a small proportion of the plants show lethal or subvital characteristics. The loss in vigour and fertility is small; rarely a line cannot be maintained due to poor fertility. The reduction in yield due to inbreeding is small or absent. Some of the inbreds lines may yields as much as the open pollinated varieties from which they were developed.
Lack of inbreeding Depression:
The self- pollinated species do not show inbreeding depression although they do not show heterosis. It is because their species reproduce by self – fertilization and as a result, have developed homozygous balance. In cost of the cross- pollinated species exhibit heterozygous balance.