Destefanis Marialaura, Nagy Istvan, Rigney Brian, Bryan Glenn J, McLean Karen, Hein Ingo, Griffin Denis, Milbourne Dan
Crops, Environment and Land Use Programme, Teagasc, Oak Park, Carlow, Ireland.
Pesticides, Plant Health & Seed Testing Laboratories, Department of Agriculture, Food and the Marine, Backweston Campus, Celbridge, Co. Kildare, Ireland.
BMC Plant Biol. 2015 Oct 24;15:255. doi: 10.1186/s12870-015-0645-8.
In plant genomes, NB-LRR based resistance (R) genes tend to occur in clusters of variable size in a relatively small number of genomic regions. R-gene sequences mostly differentiate by accumulating point mutations and gene conversion events. Potato and tomato chromosome 4 harbours a syntenic R-gene locus (known as the R2 locus in potato) that has mainly been examined in central American/Mexican wild potato species on the basis of its contribution to resistance to late blight, caused by the oomycete pathogen Phytophthora infestans. Evidence to date indicates the occurrence of a fast evolutionary mode characterized by gene conversion events at the locus in these genotypes.
A physical map of the R2 locus was developed for three Solanum tuberosum genotypes and used to identify the tomato syntenic sequence. Functional annotation of the locus revealed the presence of numerous resistance gene homologs (RGHs) belonging to the R2 gene family (R2GHs) organized into a total of 4 discrete physical clusters, three of which were conserved across S. tuberosum and tomato. Phylogenetic analysis showed clear orthology/paralogy relationships between S. tuberosum R2GHs but not in R2GHs cloned from Solanum wild species. This study confirmed that, in contrast to the wild species R2GHs, which have evolved through extensive sequence exchanges between paralogs, gene conversion was not a major force for differentiation in S. tuberosum R2GHs, and orthology/paralogy relationships have been maintained via a slow accumulation of point mutations in these genotypes.
S. tuberosum and Solanum lycopersicum R2GHs evolved mostly through duplication and deletion events, followed by gradual accumulation of mutations. Conversely, widespread gene conversion is the major evolutionary force that has shaped the locus in Mexican wild potato species. We conclude that different selective forces shaped the evolution of the R2 locus in these lineages and that co-evolution with a pathogen steered selection on different evolutionary paths.
在植物基因组中，基于NB-LRR的抗性（R）基因倾向于在相对少数的基因组区域中以大小可变的簇状形式出现。R基因序列大多通过积累点突变和基因转换事件而分化。马铃薯和番茄的4号染色体含有一个同线R基因座（在马铃薯中称为R2基因座），主要基于其对由卵菌病原体致病疫霉引起的晚疫病抗性的贡献，在中美洲/墨西哥野生马铃薯物种中进行了研究。迄今为止的证据表明，在这些基因型中该基因座存在以基因转换事件为特征的快速进化模式。
为三种马铃薯基因型构建了R2基因座的物理图谱，并用于鉴定番茄的同线序列。该基因座的功能注释揭示了存在许多属于R2基因家族（R2GHs）的抗性基因同源物（RGHs），它们被组织成总共4个离散的物理簇，其中三个在马铃薯和番茄中是保守的。系统发育分析表明，马铃薯R2GHs之间存在明显的直系同源/旁系同源关系，但从茄属野生种克隆的R2GHs中不存在这种关系。本研究证实，与通过旁系同源物之间广泛的序列交换进化而来的野生种R2GHs不同，基因转换不是马铃薯R2GHs分化的主要力量，并且在这些基因型中，直系同源/旁系同源关系是通过点突变的缓慢积累来维持的。
马铃薯和番茄的R2GHs主要通过重复和缺失事件进化，随后是突变的逐渐积累。相反，广泛的基因转换是塑造墨西哥野生马铃薯物种中该基因座的主要进化力量。我们得出结论，不同的选择力量塑造了这些谱系中R2基因座的进化，并且与病原体的共同进化引导了不同进化路径上的选择。
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