New Populations of Sclerotinia sclerotiorum from Lettuce in California and Peas and Lentils in Washington

Abstract

Four populations of Sclerotinia sclerotiorum in North America were inferred previously, based on analyses of both rapidly evolving markers (DNA fingerprint and mycelial compatiblity), and multilocus DNA sequence spanning the range between fast and slow evolution. Each population was defined as an interbreeding unit of conspecific individuals sharing a common recent ancestor and arising in a unique evolutionary event. The present study applies this standard to extend characterization of S. sclerotiorum populations to the Western United States. Isolates of S. sclerotiorum (N = 294) were determined to represent three genetically differentiated populations: California (CA, lettuce), Washington (WA, pea/lentil), and Ontario (ON, lettuce). CA was the most diverse population yet sampled in North America. Clonality was detected in ON and WA. No DNA fingerprints were common among the populations. The index of association (I(A)), based on fingerprint, was closer to zero (0) for CA than it was for the other populations. High diversity and lack of association of markers in California are consistent either with genetic exchange and recombination, or with large population size and high standing genetic variation. Intra- and interlocus conflict among three DNA sequence loci was consistent with recombination. The coalescent IGS genealogy confirmed subdivision and showed CA to be older than WA or ON. The Nearest Neighbor statistic on combined data confirmed subdivision among all present and previously defined populations. All isolates had both MAT1-1 and MAT1-2, consistent with uniform homothallism.

Mycelial compatibility and aggressiveness of Sclerotinia sclerotiorum isolates from Brazil and the United States

“…MCGs identified Set 1 18 12 BR1; BR2, BR3, and BR5; BR4 and BR8; BR6; BR7; BR9; US1, US3, US4, and US7; US2; US5; US6; US8; and US9 Set 2 17 4 BR9 and BR23; BR10-BR21 and BR24; BR22; and US1 accumulate mutations and generate new genotypes, as reported elsewhere (Atallah et al, 2004;Kohli & Kohn, 1998;Malvárez et al, 2007;Wu & Subbarao, 2006). In experiment two, 81% of the isolates from Brazil belonged to a single MCG, even though these isolates were sampled over an extensive area with the furthest isolate separation of about 2,000 km.…”

“…The first report of S. sclerotiorum on soybean in the USA was from Illinois in 1946 (Chamberlain, 1951). Studies on DNA fingerprints and MCGs have shown that S. sclerotiorum populations show a higher diversity on a local scale, where boundaries have been established by genetic or phylogenetic means (Malvárez et al, 2007). Thus, the high diversity in the isolates found in that Brazilian field might indicate that population boundaries were present, favoring some level of outcrossing in addition to the probable intra-clonal variation, since each clone can Table 2.…”

Case Study: Sclerotinia sclerotiorum: Genetic Diversity and Disease Control

“…Over a period of 20 years, population studies on S. sclerotiorum carried out in the Unites States and Canada have revealed a predominantly clonal mode of reproduction (Kohn et al, 1991;Kohli et al, 1995;Cubeta et al, 1997;Kohli and Kohn, 1998;Winton et al, 2006), with some evidence of outcrossing contributing to the population structure in a few temperate regions of North America (Atallah et al, 2004;Malvárez et al, 2007), as previously described in item 3.1.…”

“…MCGs or microsatellite markers have not been associated with specific virulence characteristics or ecological adaptations of the pathogen, and lack of variation in virulence among isolates from defined geographic areas has been reported in a number of studies on agricultural populations (Kohn et al, 1991;Kohli et al, 1992;Cubeta et al, 1997;Atallah et al, 2004;Auclair et al, 2004;Kull et al, 2004;Sexton and Howlett, 2004;Sun et al, 2005;Malvárez et al, 2007;Litholdo Júnior et al, 2011). Sexton and Howlett (2004) reported that among 44 isolates tested for virulence, the only two ones that caused relatively small leaf lesions were originally cultured from stem lesions rather than petals.…”

Best Practices for Population Genetic Analyses

“…Markers with high mutation rates such as microsatellites (also known as simple sequence repeats or SSRs) provide insights into recent divergence (Atallah et al 2010;Baumgartner et al 2010;Berbegal et al 2013;Dunn et al 2014;Dutech et al 2010;Goss et al 2009b), whereas mitochondrial, nuclear or other sequence loci provide inferences about the more distant evolutionary history given the slower mutation rates Goss et al 2009a;Malvarez et al 2007;Schoebel et al 2014;Stukenbrock et al 2007).…”