Fusion: a tale of recombination in an asexual fungus: The role of nuclear dynamics and hyphal fusion in horizontal chromosome transfer in Fusarium oxysporum

Recent studies have shown that not only meiotic recombination is responsible for the fast evolution of fungal pathogens. In the asexual fungus F. oxysporum (Fo) the "fast" evolving part of the genome is organized into small chromosomes and one such chromosome houses all effector genes and is referred to as the "pathogenicity" chromosome. This pathogenicity chromosome can be horizontally transferred to a non-pathogenic strain, conferring pathogenicity. Here we use Fo as a model organism to address how horizontal transfer of whole chromosomes is accomplished.
First we investigated nuclear dynamics and mitotic patterns in different strains of Fo. We established that Fo undergoes a developmental transition between uninucleate and multinucleate states.
Next we studied the role of heterokaryon formation for horizontal chromosome transfer. We demonstrated that the formation of viable heterokaryons apparently lead to nuclear fusion and upon colony formation, the hybrid offspring is subject to progressive and gradual genome rearrangement, leaving offspring with the genomic background of one parent, with the addition of one or two chromosomes from the second parent, including the pathogenicity chromosome.
Finally, we researched the role of "suppressor of fusion" (SUF), a putative transcription factor, in the regulation of heterokaryon formation in Fo and identified a novel function for this class of transcription factors as a nutrient-dependent regulator of anastomosis. From our studies we conclude that horizontal transfer of chromosomes is the product of asexual recombination followed by directed chromosome elimination from which "transferrable" chromosomes can escape.