Evolutionary history of the impala transposon in Fusarium oxysporum

Impala is an active DNA transposon family that was first identified in a st rain of Fusarium oxysporum pathogenic to melon. The 10 copies present in th is strain define three subfamilies that differ by about 20% at the nucleoti de level. This high level of polymorphism suggests the existence of an ance stral polymorphism associated with vertical transmission and/or the introdu ction of some subfamilies by horizontal transfer from another species. To g ain insights into the molecular evolution of this family, impala distributi on was investigated in strains with various host specificities by Southern blot, PCR, and sequencing. Detection of impala elements in most of the F. o xysporum strains tested indicates that impala is an ancient component of th e F. oxysporum genome. Subfamily-specific amplifications and sequence and p hylogenetic analyses revealed five subfamilies, several of which can be fou nd within the same genome. This supports the hypothesis of an ancestral pol ymorphism followed by vertical transmission and independent evolution in th e host-specific forms. Highly similar elements showing unique features (int ernal deletions, high rates of CG-to-TA transitions) or being present at th e same genomic location were identified in several strains with different h ost specificities, raising questions about the phylogenetic relationships o f these strains. A phylogenetic analysis pet-formed by sequencing a portion of the EF1 alpha gene showed in most cases a correlation between the prese nce of a particular element and a close genetic relationship. All of these data provide important information on the evolutionary origin of this eleme nt and reveal its potential as a valuable tool for tracing populations.