Plant Genome Structure

Evolutionary Relatedness

Comparative Genome Mapping

Physical and Genetic Distances

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Plant Genome Structure

The genomes of all eukaryotic species consist of single-copy, middle repetitive and high copy number sequences. To gain an understanding of the percentages of each of the classes within any particular species, a group of random clones can be hybridized to blots of plant DNA. One such study was performed by Zamir and Tanksley (MGG, 1988, 213:254-261). They hybridized 50 random genomic clones to tomato DNA. Washing was performed at two stringencies. The following are the results with regards to copy number.

Clone Class Low Stringency Wash1 High Stringency Wash2
Single copy clones 44% 78%
Multiple copy clones 46% 18%
Repetitive clones 10% 4%
11.0X SSC, 65 C;<80% homology
20.05X SSC, 65 C;>98% homology

The table shows that hybridization stringency has a significant effect upon the number of sequences to which a sequence hybridizes. At the higher stringencies, most of the clone recognized only a a single copy within the genome. What this also shows is that the tomato genome contains many sequences that are about 80% homologous, but fewer sequences that are highly homologous. Clearly similar sequences must have diverged by some mechanism during the evolution of the species.

The authors also hybridized these 50 clones to filters containing tomato, related tomato species, eight Solanum species and one member of the Curcurbitaceae family. As a control,single copy tomato cDNA clones were also hybridized to these clones. At moderate stringency, the cDNAs hybridized to all the tomato species, and most hybridized to the Solanum species. 80% of the clones hybridized to the related Curcurbitaceae species. In contrast, only 50% of the random clones hybridized to related tomato and Solanum species, and only 10 % hybridized to the Curcurbitaceae species.

The principal conclusion that can be drawn from these hybridizations is that the random sequences (as represented by the random genomic clones) are evolving faster than the single-copy sequences. Why? At the higher stringency, homology to distant related species was reduced. Therefore these sequences have undergone greater divergence. This evidenced by the fact that 0/5 of the repetitive sequences hybridized to tobacco , whereas 20/45 of the single and multiple copy clones hybridized to tobacco.

Copyright © 1998. Phillip McClean