Phylogenetic relationships of necrogenic Erwinia and Brenneria species as revealed by glyceraldehyde-3-phosphate dehydrogenase gene sequences

Recent examination of the relationships of the dry necrosis-inducing (necro genic) erwinias using 16S rDNA sequences demonstrated that these bacteria c omprise a polyphyletic group and, therefore, have been subdivided into thre e distinct genera, Erwinia, Brenneria and Pectobacterium, with the classica l 'amylovora' group species now being distributed nearly evenly among the f irst two. To further assess the molecular evolutionary relationships betwee n current necrogenic Erwinia and Brenneria species, as well as between thes e genera and the exclusively soft-rotting genus Pectobacterium, the glycera ldehyde-3-phosphate dehydrogenase (gapDH) genes from 57 Erwinia and Brenner ia isolates along with Pectobacterium type strains were PCR-amplified, sequ enced and subjected to phylogenetic analysis. Pairwise alignments of cloned gapDH genes revealed remarkably high interspecies genetic diversity among necrogenic isolates, Four evolutionary clades of necrogenic species were de scribed that assorted more closely to known soft-rotting species than to ea ch other. Interclade comparisons of gapDH nucleotide sequences revealed as much genetic divergence between these four necrogenic clades as existed bet ween necrogenic and soft-rotting clades, An examination of the phylogenetic utility of the gapDH gene in light of current 16S rDNA clustering of these species revealed varying levels of taxonomic congruence between these gene s for the structure of Erwinia, Brenneria and Pectobacterium. These analyse s suggest that, while gapDH possesses sufficient genetic variation to fully differentiate Erwinia and Brenneria species, the gene may not accurately r eflect interspecies taxonomic relatedness among all three phytopathogenic g enera.