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b>SMALL POPULATIONS OF PHYTOPHTHORA INFESTANS IN NORTH WALES ARE HIGHLY VARIABLE: IS SEX INVOLVED?
SMALL POPULATIONS OF PHYTOPHTHORA INFESTANS IN NORTH WALES ARE HIGHLY VARIABLE: IS SEX INVOLVED?

DS SHAW, ND PIPE, K HANSON, CJ GLIDDON and RC SHATTOCK
School of Biological Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, UK.

Background and objectives
In the UK, the old Al clonal genotype of P. infestans (equivalent to US-I) was replaced by new genotypes including some of A2 mating type around 15 years ago. Extensive sampling across the UK has revealed that the frequency of sites with the A2 mating type has remained low, at about 4%[1]. In this study, several small populations around Bangor, North Wales were intensively sampled over several seasons to determine their genetic diversity.

Materials and methods
Over a single blight epidemic in 1996, 450 isolates were collected from 5 main sites and 8 minor sites up to 35 km apart. Isolates were characterised for mating type; total DNA was extracted and mtDNA haplotype and multi-locus RG57 DNA fingerprints were determined. One private-garden site was repeatedly sampled over 3 years (1995-1997).

Results and conclusions
In 1996, the frequency of A2 and self-fertile isolates across all sites was 15% and 12%, respectively. A2 was detected at 5/13 sites. A total of 32 different RG57 fingerprints were identified. However, one fingerprint predominated (40.6% of isolates) and was detected at 9 sites; this same phenotype was mostly of Al mating type but also occurred as A2 and self-fertile mating types (4.2%). Two other clones accounted for 30% of the megapopulation, each occurring at 7 sites; the rest of the population comprised 29 phenotypes at frequencies of <5%. All sites were highly variable showing between 6-12 RG57 fingerprints. The frequency of A2 mating type present at individual sites was not related to the number of phenotypes detected. Site A was composed of 46.2% A2, yet showed 12 fingerprints, site D consisted of 97.4% Al and only 2.6% self-fertile isolates yet showed 15 different RG57 fingerprint patterns. Each site contained 2-9 unique phenotypes, not detected at other sites.

The private-garden site, sampled over three consecutive years, showed varying frequencies of A2 mating type; 1995: 3/58, 1996: 50/158 and in 1997 A2 was not detected from 76 isolates. The level of genetic diversity based on the RG57 fingerprint remained high: 1995- 15 phenotypes, 1996- 12 phenotypes, 1997- 11 phenotypes. Each season, only low numbers of phenotypes from the previous year were detected. In 1996 only 3/15 phenotypes survived and in 1997 only 4/12 phenotypes survived.

Intensive sampling of P. infestans from gardens and small commercial crops has revealed higher frequencies of the A2 mating type than expected from extensive sampling throughout the UK. The presence of the A2 mating type in populations may be providing localised opportunity for sexually generated variation. We hope to determine how variation is generated and maintained using allele frequencies of codominant, microsatellite markers [2].

References
1. Day JD, Shattock RS, Shaw DS, 1998. Proceedings of the 7th International Congress of Plant Pathology.
2. Pipe ND, Gliddon CJ, Shattock RC, Shaw, DS, 1998. Proceedings of the 7th International Congress of Plant Pathology.