During my Masters degree at the University of Pretoria (South Africa) I worked on Fusarium circinatum, an important pathogen of susceptible Pinus species. Accessory chromosomes have been identified in numerous fungal pathogens, including F. circinatum. These chromosomes are associated with pathogenicity in several fungal species. The primary focus of my Masters was to characterize the accessory chromosome of F. circinatum. The chromosome was sequenced and compared to the available sequences for the accessory chromosomes of Fusarium fujikuroi and Fusarium temperatum. The chromosomes varied in terms of length and genetic content. The chromosomes of F. circinatum and F. temperatum shared sequence similarity suggesting that the chromosomes emerged from the core genome. The second part of the study aimed to determine the distribution of the accessory chromosome by screening a global population of F. circinatum isolates, as well as to investigate how the chromosome is inherited following sexual reproduction. The isolates from the global population exhibited great variation in the length of the accessory chromosome, but no naturally occurring isolate lacking the chromosome was identified. However, the progeny of experimental crosses frequently lose the accessory chromosome, suggesting that it is not require for growth under laboratory conditions. Meiosis may be the mechanism whereby the chromosome can be lost.
During my PhD I will be investigating the mechanisms whereby chromosome polymorphisms are generated. Zymoseptoria tritici has eight accessory chromosomes and is therefore the perfect candidate for elucidating the mechanisms whereby chromosome rearrangements occur. What regions of the accessory chromosomes are typically lost? Are these chromosomes unstable due to the absence of telomeric sequences? Where did accessory chromosomes originate, from the core genome, via horizontal transfer or have they always been present?