Michelmore R W, Meyers B C
Department of Vegetable Crops, University of California, Davis, California 95616, USA.
Genome Res. 1998 Nov;8(11):1113-30. doi: 10.1101/gr.8.11.1113.
Classical genetic and molecular data show that genes determining disease resistance in plants are frequently clustered in the genome. Genes for resistance (R genes) to diverse pathogens cloned from several species encode proteins that have motifs in common. These motifs indicate that R genes are part of signal-transduction systems. Most of these R genes encode a leucine-rich repeat (LRR) region. Sequences encoding putative solvent-exposed residues in this region are hypervariable and have elevated ratios of nonsynonymous to synonymous substitutions; this suggests that they have evolved to detect variation in pathogen-derived ligands. Generation of new resistance specificities previously had been thought to involve frequent unequal crossing-over and gene conversions. However, comparisons between resistance haplotypes reveal that orthologs are more similar than paralogs implying a low rate of sequence homogenization from unequal crossing-over and gene conversion. We propose a new model adapted and expanded from one proposed for the evolution of vertebrate major histocompatibility complex and immunoglobulin gene families. Our model emphasizes divergent selection acting on arrays of solvent-exposed residues in the LRR resulting in evolution of individual R genes within a haplotype. Intergenic unequal crossing-over and gene conversions are important but are not the primary mechanisms generating variation.
经典遗传学和分子数据表明，决定植物抗病性的基因在基因组中常常成簇分布。从多个物种中克隆出的针对不同病原体的抗性基因（R基因）编码的蛋白质具有共同的基序。这些基序表明R基因是信号转导系统的一部分。这些R基因大多编码富含亮氨酸重复序列（LRR）区域。该区域中编码假定溶剂暴露残基的序列高度可变，非同义替换与同义替换的比率升高；这表明它们已经进化到能够检测病原体衍生配体的变异。以前认为新抗性特异性的产生涉及频繁的不等交换和基因转换。然而，抗性单倍型之间的比较表明，直系同源物比旁系同源物更相似，这意味着不等交换和基因转换导致的序列同质化速率较低。我们提出了一个新模型，该模型改编并扩展自一个针对脊椎动物主要组织相容性复合体和免疫球蛋白基因家族进化提出的模型。我们的模型强调对LRR中溶剂暴露残基阵列的分歧选择，导致单倍型内单个R基因的进化。基因间不等交换和基因转换很重要，但不是产生变异的主要机制。
