Academic journal article Genetics

Intraspecific Genetic Variations, Fitness Cost and Benefit of RPW8, A Disease Resistance Locus in Arabidopsis Thaliana

Academic journal article Genetics

Intraspecific Genetic Variations, Fitness Cost and Benefit of RPW8, A Disease Resistance Locus in Arabidopsis Thaliana

Article excerpt

ABSTRACT

The RPW8 locus of Arabidopsis thaliana confers broad-spectrum resistance to powdery mildew pathogens. In many A. thaliana accessions, this locus contains two homologous genes, RPW8.1 and RPW8.2. In some susceptible accessions, however, these two genes are replaced by HR4, a homolog of RPW8.1. Here, we show that RPW8.2 from A. lyrata conferred powdery mildew resistance in A. thaliana, suggesting that RPW8.2 might have gained the resistance function before the speciation of A. thaliana and A. lyrata. To investigate how RPW8 has been maintained in A. thaliana, we examined the nucleotide sequence polymorphisms in RPW8 from 51 A. thaliana accessions, related disease reaction phenotypes to the evolutionary history of RPW8.1 and RPW8.2, and identified mutations that confer phenotypic variations. The average nucleotide diversities were high at RPW8.1 and RPW8.2, showing no sign of selective sweep. Moreover, we found that expression of RPW8 incurs fitness benefits and costs on A. thaliana in the presence and absence of the pathogens, respectively. Our results suggest that polymorphisms at the RPW8 locus in A. thaliana may have been maintained by complex selective forces, including those from the fitness benefits and costs both associated with RPW8.

DURING the long-time conflict between plants and potential pathogens, plants have evolved disease resistance (R) genes to detect the invasion of infectious pathogens and trigger effective defenses (CHISHOLM et al. 2006). In the past 15 years, .60 plant R genes have been isolated (XIAO 2006), of which the majority encode nucleotide-binding site (NBS) and leucine-richrepeat (LRR) domains. The NBS-LRR genes constitute the largest R gene class and are abundant and ubiquitously expressed in all higher plants (DANGL and JONES 2001; MCHALE et al. 2006). A less frequent class of R genes comprises members of extracellular (e) LRR-containing receptor-like proteins (eLRR-RLPs) (JONES et al. 1994; HAMMOND-KOSACK and JONES 1997) and receptor-like kinases (eLRR-RLKs) (SONG et al. 1995; SUN et al. 2004). These two classes of LRR-containing R proteins are thought to be intracellular or cell-surface receptors that detect pathogen-derived virulence proteins (referred to as Avr effectors if recognized by R proteins) through direct or indirect interaction (DANGL and JONES 2001). The remaining characterized R genes encode proteins that either resemble the overall structure or a domain of the above two classes with some degree of structural variations, or have a novel protein structure that does not show significant homology to any other R proteins (XIAO 2006). Therefore, in terms of protein structures, they are atypical R genes in contrast to the typical LRRencoding R genes.

The evolution and maintenance of plant R genes has become a research focus in recent years. Different mechanisms for sequence evolution have been documented for R genes (MICHELMORE and MEYERS 1998; MEYERS et al. 2005). However, the type and strength of selection acting on specific R genes is not well characterized. In conjunction with the recent advances in understanding of the molecular mechanisms of R-Avr interaction, several recent evolutionary analyses suggest that the mode of R-Avr recognition may profoundly influence the patterns of R-Avr coevolution (DANGL and MCDOWELL 2006; DODDS et al. 2006).

While the simplest "arms-race" model used for describing the coevolution between plants and pathogens predicts directional selection or selective sweeps, a recent genomewide survey of R gene polymorphisms in Arabidopsis did not detect convincing evidence for a recent selective sweep for any of the R genes analyzed (BAKKER et al. 2006). For some R genes in Arabidopsis, balancing selection appears to play a central role in molecular evolution (STAHL et al. 1999; TIAN et al. 2002; SHEN et al. 2006). For example, Arabidopsis RPM1 and RPS5, whose protein products detect their Avr proteins indirectly by association with the host target proteins of the Avr effectors (MACKEY et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.