A novel mode of chromosomal evolution peculiar to filamentous Ascomycete fungi

These analyses identified a novel form of evolution in which genes are conserved within homologous chromosomes, but with randomized orders and orientations. This mode of evolution is designated mesosynteny, to differentiate it from micro- and macrosynteny seen in other organisms. Mesosynteny is an alternative evolutionary pathway very different from macrosyntenic conservation. Surprisingly, mesosynteny was not found in all fungal groups. Instead, mesosynteny appears to be restricted to filamentous Ascomycetes and was most striking between species in the Dothideomycetes.The existence of mesosynteny between relatively distantly related Ascomycetes could be explained by a high frequency of chromosomal inversions, but translocations must be extremely rare. The mechanism for this phenomenon is not known, but presumably involves generation of frequent inversions during meiosis.The evolutionary history of organisms, as revealed by comparisons of genome sequences, is of the greatest biological significance and interest. The current explosion in the number of genome assemblies of species within the same class, order and genus is allowing the whole-genome interrelationships between organisms to be examined in ever greater detail. There is a long history of comparisons of individual orthologous gene sequences and these have revolutionized our understanding of phylogenetic relationships [1]. A more complete understanding of both the mechanism and results of evolution can be obtained by comparing entire genomes [2]. These comparisons have refined the concept of synteny. This term is used loosely by many authors. Originally it was used in cytogenetics to describe two or more loci that are located on the same chromosome. As DNA sequencing and comparative genomics became commonplace, the term synteny acquired the additional property of co-linearity; i.e. the conservation of gene order and orientation. In this study we refer to synteny in the original cytogenetic sense and describe