Simple sequence repeat markers distinguish among morphotypes of Sphaeropsis sapinea

Sphaeropsis sapinea is a fungal endophyte of Pinus spp. that can cause disease following predisposition of trees by biotic or abiotic stresses. Four morphotypes of S. sapinea have been described from within the natural range of the fungus, while only one morphotype has been identified on exotic pines in the Southern Hemisphere. The aim of this study was to develop robust polymorphic markers that could be used in both taxonomic and population studies. Inter-short-sequence-repeat primers containing microsatellite sequences and degenerate anchors at the 5' end were used to target microsatellite-rich areas in an S. sapinea isolate. PCR amplification using an annealing temperature of 49 degrees C resulted in profiles containing 5 to 10 bands. These bands were cloned and sequenced, and new short-sequence-repeat (SSR) primer pairs were designed that flanked microsatellite-rich regions. Eleven polymorphic SSR markers were tested on 40 isolates of S. sapinea representing different morphotypes as well as on 2 isolates of the closely related species Botryosphaeria obtusa. The putative I morphotype was found to be identical to B. obtusa. Otherwise, the markers clearly distinguished the remaining three morphotypes and, furthermore, showed that the C morphotype was more closely related to the A than the B morphotype. The B morphotype was the most genetically diverse, and the isolates could be further divided based on their geographic origins. Sequencing of different alleles from each locus showed that the most polymorphic markers had mutations within a microsatellite sequence.