Evolutionary history of the impala transposon in Fusarium oxysporum

Impala is an active DNA transposon family that was first identified in a strain of Fusarium oxysporum pathogenic to melon. The 10 copies present in this strain define three subfamilies that differ by about 20% at the nucleotide level. This high level of polymorphism suggests the existence of an ancestral polymorphism associated with vertical transmission and/or the introduction of some subfamilies by horizontal transfer from another species. To gain insights into the molecular evolution of this family, impala distribution was investigated in strains with various host specificities by Southern blot, PCR, and sequencing. Detection of impala elements in most of the F. oxysporum strains tested indicates that impala is an ancient component of the F. oxysporum genome. Subfamily-specific amplifications and sequence and phylogenetic analyses revealed five subfamilies, several of which can be found within the same genome. This supports the hypothesis of an ancestral polymorphism followed by vertical transmission and independent evolution in the host-specific forms. Highly similar elements showing unique features (internal deletions, high rates of CG-to-TA transitions) or being present at the same genomic location were identified in several strains with different host specificities, raising questions about the phylogenetic relationships of these strains. A phylogenetic analysis performed by sequencing a portion of the EF1alpha gene showed in most cases a correlation between the presence of a particular element and a close genetic relationship. All of these data provide important information on the evolutionary origin of this element and reveal its potential as a valuable tool for tracing populations.