Pyramiding of Disease Resistance Genes in Vegetable Crops
Authors: Nangsol D.Bhutia, Karma L. Bhutia

Pyramiding entails stacking multiple genes leading to the simultaneous expression of more than one gene in a variety to develop durable resistance expression. Gene pyramiding is gaining considerable importance as it would improve the efficiency of plant breeding leading to the development of genetic stocks and precise development of broad spectrum resistance capabilities. Pyramiding multiple genes is achieved by crossing parental lines with complementary genes and selecting the desired recombinants from among the progeny populations. A pyramid could be constructed with major genes, minor genes, effective genes, race-specific genes, non race-specific genes, or any other type of host gene that confers resistance

Marker-assisted backcross based gene pyramiding

Markers can aid selection on target alleles whose effects are difficult to observe phenotypically. Examples include recessive genes, multiple disease resistance gene pyramids combined in one genotype (where they can epistatically mask each other’s effects), alleles that are not expressed in the selection environments (e.g., genes conferring resistance to a disease that is not regularly present in environments), etc.

Markers can be used to select for rare progeny in which recombination near the target gene have produced chromosomes that contain the target allele and as little possible surrounding DNA from the donor parent. Markers can be used to select rare progeny that are the result of recombination near the target gene, thus minimizing the effects of linkage drag.

In general, the marker assisted backcross based gene pyramiding can be performed in three strategies. In the first method, the recurrent parent is crossed with donor parent to produce the F1 hybrid and backcrossed up to third backcross generation to produce the improved recurrent parent. This improved recurrent parent is then crossed with other donor parent to pyramid multiple genes. This strategy is less acceptable as it is time taking but pyramiding is very precise as it involve one gene at one time.

In the second strategy, the recurrent parent is crossed with donor parents to get the F1 hybrids which are then intercrossed to produce improved F1 .This improved F1 is then backcrossed with the recurrent parent to get the improved recurrent parent. As such, the pyramiding is done in the pedigree step itself. However, when the donor parents are different, this method is less likely to be used because there is chance that the pyramided gene may be lost in the process. The third strategy is an amalgamation of the first two which involve simultaneous crossing of recurrent parent with many donor parents and then backcrossing them up to the BC3 generation. The backcross populations with the individual gene are then intercrossed with each other to get the pyramided lines. This is the most acceptable way as in this method not only time is reduced but fixation of genes is fully assured.

Gene pyramiding in vegetables



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