Plant Genome Structure

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Comparative Genome Mapping

Physical and Genetic Distances

Linkage Drag

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Linkage Drag

One goal of plant breeding is to introduce a gene from a donor parent to improve a cultivar for a specific trait. For example, a wild germplasm could be used as a source of disease resistance. At the same time the breeder does not want to carry any of the other genes from the wild germplasm that might reduce the a agronomic fitness of the cultivar. The backcross method of plant breeding is one manner in which the introduction of a specific gene is accomplished. One genetic feature though of backcross breeding is linkage drag. This refers to the reduction in fitness in a cultivar due to deleterious genes introduced along with the beneficial gene during backcrossing. Molecular makers offers a tool in which the amount of wild or alien DNA can be monitored during each backcross generation.

First, though, lets look at the amount of alien DNA that can be maintained after a backcrossing program. The accompanying figure handed out in class (from Young and Tanksley, TAG 77:353-359, 1989) shows the chromosomal region around the Tm-22 allele in a number of tomato cultivars. This allele was introduced from the wild tomato species L. peruvianum, and it provides resistance to tobacco mosaic virus. Large variation in the amount of introgressed DNA was observed. For example, Craigella-Tm-22 contains 51 cM of wild DNA, whereas Vendor-Tm-22 and Nova-Tm- 22 each contain about 8 cM of introgressed DNA.

Backcrossed lines could be developed rapidly in which only a small amount of foreign DNA is linked to the gene of interest. After the first backcross all lines with the gene of interest could be screened with an RFLP that is 1 cM away. Those lines in which a crossover occurred at this marker would be selected. Then those lines would be crossed to the recurrent parent. The progeny from these crosses would then be scored for a second marker 1 cM away on the other side of the gene. Again crossovers progeny could be selected. Thus, in only two generations the amount of wild DNA could be reduced to 2 cM. The key to this procedure is to have markers that are polymorphic between the two parents that are closely linked to the gene of interest.

Young and Tanklsley applied these principles (described in the above paper, in which the theory was presented) to the reduction of L. peruvianum DNA in Craigella-Tm-22. The authors were able to reduce the amount of DNA on one side of the Tm-22 allele from 47 to 7 cM in one generation by selecting for crossovers at the CD32A locus.

Copyright © 1998. Phillip McClean