Jm. Mcdowell et al., INTRAGENIC RECOMBINATION AND DIVERSIFYING SELECTION CONTRIBUTE TO THEEVOLUTION OF DOWNY MILDEW RESISTANCE AT THE RPP8 LOCUS OF ARABIDOPSIS, The Plant cell, 10(11), 1998, pp. 1861-1874
Pathogen resistance (R) genes of the NBS-LRR class (for nucleotide bin
ding site and leucine-rich repeat) are found in many plant species and
confer resistance to a diverse spectrum of pathogens. Little is known
about the mechanisms that drive NBS-LRR gene evolution in the host-pa
thogen arms race. We cloned the RPP8 gene (for resistance to Peronospo
ra parasitica) and compared the structure of alleles at this locus in
resistant Landsberg erecta (Ler-0) and susceptible Columbia (Col-0) ac
cessions. RPP8-Ler encodes an NBS-LRR protein with a putative N-termin
al leucine zipper and is more closely related to previously cloned R g
enes that confer resistance to bacterial pathogens than it is to other
known RPP genes. The RPP8 haplotype in Ler-0 contains the functional
RPP8-Ler gene and a nonfunctional homolog, RPH8A. In contrast, the rpp
8 locus in Col-0 contains a single chimeric gene, which was likely der
ived from unequal crossing over between RPP8-Ler and RPH8A ancestors w
ithin a Ler-like haplotype. Sequence divergence among RPP8 family memb
ers has been accelerated by positive selection on the putative ligand
binding region in the LRRs. These observations indicate that NBS-LRR m
olecular evolution is driven by the same mechanisms that promote rapid
sequence diversification among other genes involved in non-self-recog
nition.