Inheritance and mapping of 11 avirulence genes in Phytophthora sojae.

Two new crosses involving four races (races 7, 16, 17, and 25) of the soybean root and stem rot pathogen Phytophthora sojae were established (7/16 cross; 17/25 cross). An F2 population derived from each cross was used to determine the genetic basis of avirulence towards 11 different resistance genes in soybean. Avirulence was found to be dominant and determined by a single locus for Avr1b, 1d, 1k, 3b, 4, and 6, as expected for a simple gene-for-gene model. We also observed several cases of segregation, inconsistent with a single dominant gene being solely responsible for avirulence, which suggests that the genetic background of the different crosses can affect avirulence. Avr4 and 6 cosegregated in both the 7/16 and 17/25 crosses and, in the 7/16 cross, Avr1b and 1k were closely linked. Information from segregating RAPD, RFLP, and AFLP markers screened on F2 progeny from the two new crosses and two crosses described previously (a total of 212 F2 individuals, 53 from each cross) were used to construct an integrated genetic linkage map of P. sojae. This revised genetic linkage map consists of 386 markers comprising 35 RFLP, 236 RAPD, and 105 AFLP markers, as well as 10 avirulence genes. The map is composed of 21 major linkage groups and seven minor linkage groups covering a total map distance of 1640.4cM.

Physical mapping across an avirulence locus of Phytophthora infestans using a highly representative, large-insert bacterial artificial chromosome library.

The oomycete plant pathogen Phytophthora infestans is the causal agent of late blight, one of the most devastating diseases of potato worldwide. As part of efforts to clone avirulence (Avr) genes and pathogenicity factors from P. infestans, we have constructed a bacterial artificial chromosome (BAC) library from an isolate containing six Avr genes. The BAC library comprises clones with an average insert size of 98 kb and represents an estimated 10 genome equivalents. A three-dimensional pooling strategy was developed to screen the BAC library for amplified fragment length polymorphism (AFLP) markers, as this type of marker has been extensively used in construction of a P. infestans genetic map. Multiple positive clones were identified for each AFLP marker tested. The pools were used to construct a contig of 11 BAC clones in a region of the P. infestans genome containing a cluster of three avirulence genes. The BAC contig is predicted to encompass the Avr11 locus but mapping of the BAC ends will be required to determine if the Avr3 and Avr10 loci are also present in the BAC contig. These results are an important step towards the positional cloning of avirulence genes from P. infestans, and the BAC library represents a valuable resource for largescale studies of oomycete genome organisation and gene content.

Phytophthora infestans enters the genomics era.

UNLABELLED: summary Phytophthora infestans, cause of late-blight, is the most devastating disease of potato world-wide. Recent years have seen a dramatic intensification in molecular biological studies of P. infestans, including the development of novel tools for transformation and gene silencing and the resources for genetical, transcriptional and physical mapping of the genome. This review will focus on the increasing efforts to use these resources to discover the genetic bases of pathogenicity, avirulence and host-specificity. TAXONOMY: Phytophthora infestans (Mont.) de Bary-Kingdom Chromista, Phylum Oomycota, Order Peronosporales, Family Peronosporaceae, Genus Phytophthora, of which it is the type species. HOST RANGE: Infects a wide range of solanaceous species. Economically important hosts are potato, tomato, eggplant and some other South American hosts (tree tomato and pear melon) on which it causes late blight. Disease symptoms: Infected foliage is initially yellow, becomes water soaked and eventually blackens. Leaf symptoms comprise purple-black or brown-black lesions at the leaf tip, later spreading across the leaf to the stem. Whitish masses of sporangia develop on the underside of the leaf. Tubers become infected later in the season and, in the early stages, consist of slightly brown or purple blotches on the skin. In damp soils the tuber decays rapidly before harvest. Tuber infection is quickly followed by secondary fungal or bacterial infection known as 'wet rot'. Useful web sites:http://www.ncgr.org/pgc/; http://www.oardc.ohio-state.edu/phytophthora/.

Phytophthora sojae avirulence genes, RAPD, and RFLP markers used to construct a detailed genetic linkage map.

Two crosses between different races of Phytophthora sojae were established using one race as a common parent in both crosses. F2 populations comprising over 200 individuals were generated for each cross. A subset of 53 F2 individuals from each cross was selected at random for genetic analysis of virulence/avirulence and molecular markers, and finally the construction of a detailed genetic linkage map. The linkage map developed for P. sojae is based on a total of 257 markers (22 RFLP, 228 RAPD, and 7 avirulence genes). The linkage map comprises 10 major and 12 minor linkage groups covering a total of 830.5 cM. Close linkage was observed between Avr4 and Avr6 (0.0 cM), Avr1b and Avr1k (0.0 cM), and Avr3a and Avr5 (4.6 cM). Coupling phase linkage of RFLP and RAPD markers to all seven avirulence genes was identified at the minimum and maximum distances of 0.0 and 14.7 cM, respectively.

Evidence for outcrossing in Phytophthora sojae and linkage of a DNA marker to two avirulence genes.

Two genetically different isolates of the homothallic Oomycete, Phytophthora sojae, were demonstrated to outcross and form hybrid oospores after co-culturing in vitro. Random amplified polymorphic DNA (RAPD) markers revealed ten hybrids among 354 oospores analysed. One F1 hybrid was allowed to self fertilise and produce an F2 population of 247 individuals. Among 53 F2 individuals, selected at random, 18 polymorphic RAPD markers were observed to segregate at near 3:1 Mendelian ratios, consistent with segregation for dominant alleles at single loci. Segregation of virulence against soybean resistance genes Rps1a, 3a, and 5 revealed that the avirulence genes Avrla, 3a and 5 were dominant to virulence. Avirulence against these three resistance genes appeared to be conditioned by one locus for Avr1a and two independent, complementary dominant loci for both Avr3a and Avr5. Segregation of virulence against Rps6 was in the ratio of 1:2:1 (avirulent:mixed reaction:virulent), suggesting a semi-dominant allele at a single locus. Two avirulence genes and one RAPD marker formed one linkage group, in the order Avr3a, OPH4-1, Avr5, each separated by approximately 5 cM. Our results confirm that outcrossing occurred between the parental isolates, and that sexual recombination under field conditions may play an important role in generating and maintaining genetic diversity in populations of P. sojae.