Population genetic analyses of Verticillium dahliae from lettuce indicates regular introduction of novel genotypes
S. GURUNG (1), D. P. G. Short (1), K. V. Subbarao (1). (1) University of California-Davis, Salinas, CA, U.S.A.
<i>Verticillum dahliae</i> has caused serious yield losses in lettuce since 1995 when the disease was first described on this host. The causes of this sudden host range expansion of <i>V. dahliae</i> into lettuce has become an intense area of research over many years. We genotyped 13 microsatellite loci of 283 <i>V. dahliae</i> isolates sampled during surveys over the past two decades. The data showed low levels of genetic variation. Comparative analyses of older collections (1995-2005, <i>n</i>=69) and more recent collections (2006-2012, <i>n</i>=214) of <i>V. dahliae</i> populations indicated that some genotypes shared by isolates collected in 1995 are still present in current populations. More importantly, new genotypes have been introduced every year. Bayseian model-based population structure analyses designated three clusters; two of which contained isolates from both newer and older collections, whereas one cluster comprised isolates collected between 2009 and 2011. Analysis of pathogenic race structure using race-specific primers showed 66% of isolates as race 1 and 34% as race 2, and all of the isolates contained the MAT1-2 idiomorph. This study indicates that the increase in the genetic variation may be attributed to continuous inflow of novel genotypes primarily from infected spinach seeds planted extensively in coastal California and a few other sources. This study provides the contemporary population genetic structure of <i>V. dahliae</i> in coastal CA.
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