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A HIGH DENSITY MAP OF AN AVR-GENE CLUSTER IN PHYTOPHTHORA INFESTANS

T VAN DER LEE1, A TESTA2 and F GOVERS2

1Phytopathology, Graduate School Experimental Plant Sciences, Wageningen Agricultural University, Binnenhaven 9, 6709 PD, The Netherlands; 2Dipartimento di Arboricoltura, Botanica e Patologia Vegetale, via Universita 100, 80055 Portici ( Naples), Universita degli Studi di Napoli 'Federico II', Italy

Background and objectives
The heterothallic oomycetous plant pathogen Phytophthora infestans is the causal agent of potato late blight. Vertical resistance to late blight is based on a gene-for-gene interaction between pathogen and host. We expect this interaction to be the result of active defence responses of the host initiated upon recognition of a race-specific elicitor produced by the pathogen. We want to characterize the main fungal components of this race-specific incompatibility and set out to clone the avirulence (avr) genes of the pathogen.

Materials and methods
We choose a map-based cloning strategy, using the powerful AFLP DNA fingerprint technology [1] to deal with the relatively large genome size (250 Mb) of P. infestans. AFLP DNA fingerprints were generated from 83 F1 progeny from a cross between two Dutch P. infestans field isolates (80029x88133). Part of this progeny was also tested for virulence on a differential set of potato fines carrying different major R-genes.

Results and conclusions
In the F1 progeny there is segregation for virulence on potato plants carrying the Rl, R2, R3, R4, R10 and R11 resistance gene. We demonstrated that in this cross avirulence to the R3, R4, R10 and R11 is a dominant trait. Avr3 avrlo and avrll appeared to be closely linked and we positioned this avr gene cluster on linkage group VIll of the AFLP linkage map previously generated with the same progeny [2]. We pooled DNA from avirulent and virulent progeny and performed a bulked segregant analysis (BSA) to identify AFLP markers linked to the avr3, avrlo, avrll-cluster. We tested 25,000 AFLP fragments and identified 18 linked markers. Twelve of these markers co-segregate with the avr-cluster.

References
1. Vos P, Hogers R, Bleeker M et al., 1995. Nucleic Acids Research 23, 4407-4414. 2. Van der Lee T, De Witte IJ, Drenth A et al., 1997. Fungal Genetics and Biology 21, 278-291.