Variation in genome size and ploidy levels in plant pathogenic oomycetes, particularly the downy mildews.
L. BERTIER (1), J. Gil (1), S. Reyes-Chin-Wo (1), K. Fletcher (1), R. Michelmore (1) (1) UC Davis Genome Center, U.S.A.

 The oomycetes are important plant pathogens. They resemble fungi in their macroscopic appearance and lifestyle but have very different evolutionary origins and genetics. Their diploid vegetative lifestyle as well as infamous genomic instability and flexibility make them challenging to study.  There is accumulating evidence for polyploidy in Phytophthora spp.  Allopolyploidy can be a consequence of interspecific hybridization between closely related species that have diverged on different hosts or in different geographic regions.  Autopolyploids can arise within a single species as a consequence of either duplication of the genome or fusion of dissimilar genomes. The absence of sexual reproduction for extended periods of time may reduce selection pressure on genes important in meiosis, leading to a higher frequency of unreduced gametes, which can give rise to polyploid progeny. Polyploidy has not been studied in the obligately biotrophic downy mildews. We are surveying genome size and ploidy variation in the downy mildews using flow cytometry.  We have been able to get reliable genome size estimates from fresh field samples. Comparison of these estimates with assembly sizes from whole genome sequencing efforts point to polyploidy in isolates of Bremia lactucae, Peronospora effusa and P. schachtii from California that cause downy mildew diseases of lettuce, spinach and swiss chard, respectively. Allele frequencies at polymorphic SNPs are being investigated to determine if these are likely autopolyploids or allopolyploids. 

Abstract Number: C2-6, P11-392
Session Type: Concurrent