Comparison of isozymes, rDNA spacer regions and MAT-2 DNA sequences as phylogenetic characters in the analysis of the Ceratocystis coerulescens complex

Abstract

The genus Ceratocystis sensu stricto includes species that are insect-vectored, plant pathogens, occurring mainly on angiosperm hosts. Species in the Ce. coerulescens complex form a morphologically distinct group that are pathogens on gymnosperms (Pinaceae) and angiosperms. In recent years, species in the Ce. coerulescens complex have been intensively studied based on their morphology and isozymes, as well as on their molecular characteristics. The aim of this study was to compare the phylogeny of these species inferred from the MAT-2 HMG box DNA sequences to the phylogenies inferred from partial rDNA sequences and isozyme data. Part of the MAT-2 HMG box of species in the Ce. coerulescens complex was amplified using PCR. The PCR fragments were sequenced and phylogenetically analysed using parsimony, with Ce. fimbriata as the outgroup taxon. Species in the Ce. coerulescens complex on gymnosperms formed a single clade, inferred to be a sister-taxon to the clade that included the species on angiosperm hosts, thus supporting the hypothesis that the adaptation to gymnosperms was a single evolutionary event in the history of Ceratocystis. The phylogeny inferred from the MAT-2 HMG box sequences was found to be similar to the phylogeny based on the internal transcribed spacer (ITS) regions of the rDNA. However, the MAT-2 HMG box sequences enabled the separation of Ce. laricicola from Ce. polonica, as well as the separation of the Ce. virescens isolates on Acer and Liriodendron from those on Fagus and Quercus. Similarly, isozyme data separated these morphologically similar sister species, but isozyme analysis did not appear to reflect a true phylogeny of the species in the Ce. coerulescens complex.

Key Words:

• Ascomycetes
• ITS
• HMG box
• Microascales