Download 1 Sequence evolution of the disease resistance genes Rcr3 and

Sequence evolution of the disease resistance genes Rcr3 and Rin4 in wild tomato
(Lycopersicon peruvianum)
Diploma Thesis by Anja Hörger
Summary
The objective of my diploma thesis was to reveal evolutionary forces, which are
working on the two disease resistance genes Rcr3 and Rin4 in the wild tomato species
Lycopersicon peruvianum. Both genes are involved in different disease resistance
pathways. Knowledge of evolutionary mechanisms shaping these two genes will
contribute to the understanding of the evolution of disease resistance pathways in
plants. To reveal the evolutionary history of Rcr3 and Rin4 I amplified and sequenced
both genes and analyzed them with population genetic methods. As basic material I
used ten individuals of the L. peruvianum accession LA2744 and one individual of the
species Solanum lycopersicoides as outgroup.
Rcr3 revealed itself as putative paralogue within an old gene family, which
occurs in several Lycopersicon species. There is also evidence that Rcr3 was
duplicated within the genome of L. peruvianum again. The Rcr3 interacting guard
molecule Cf-2 is part of a gene family as well. Therefore, duplication events of both
interaction partners could be the consequence of specific coadaption of these two
genes.
The Rcr3 gene shows evidence of purifying selection. There is little nucleotide
diversity within the gene and expectations under neutrality were not met in almost all
kinds of neutrality tests I performed. These observations support the assumption that
Rcr3 is tightly integrated in its disease resistance network and therefore has to be
highly conserved. Additionally, the specific interaction between Rcr3 and Cf-2 should
contribute to purifying selection as well.
For the Rin4 gene I reported a very low level of nucleotide diversity as well.
Tests of neutrality could not be rejected significantly, but all results tended to show a
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limitation of the variation due to purifying selection. These findings are very likely
for the Rin4 gene as well. The Rin4 protein is involved in an extremely complex
signaling system of herbal disease resistance. It is known to interact with at least
seven - probably even more - different resistance-mediating factors. All these
interactions are realized in different ways. To keep all these ways of interaction
stable, a high level of conservation within the Rin4 gene is required.
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