Reconstruction of ancestral chromosome architecture and gene repertoire reveals principles of genome evolution in a model yeast genus.

Reconstructing genome history is complex but necessary to reveal quantitative principles governing genome evolution. Such reconstruction requires recapitulating into a single evolutionary framework the evolution of genome architecture and gene repertoire. Here, we reconstructed the genome history of the genus Lachancea that appeared to cover a continuous evolutionary range from closely related to more diverged yeast species. Our approach integrated the generation of a high-quality genome data set; the development of AnChro, a new algorithm for reconstructing ancestral genome architecture; and a comprehensive analysis of gene repertoire evolution. We found that the ancestral genome of the genus Lachancea contained eight chromosomes and about 5173 protein-coding genes. Moreover, we characterized 24 horizontal gene transfers and 159 putative gene creation events that punctuated species diversification. We retraced all chromosomal rearrangements, including gene losses, gene duplications, chromosomal inversions and translocations at single gene resolution. Gene duplications outnumbered losses and balanced rearrangements with 1503, 929, and 423 events, respectively. Gene content variations between extant species are mainly driven by differential gene losses, while gene duplications remained globally constant in all lineages. Remarkably, we discovered that balanced chromosomal rearrangements could be responsible for up to 14% of all gene losses by disrupting genes at their breakpoints. Finally, we found that nonsynonymous substitutions reached fixation at a coordinated pace with chromosomal inversions, translocations, and duplications, but not deletions. Overall, we provide a granular view of genome evolution within an entire eukaryotic genus, linking gene content, chromosome rearrangements, and protein divergence into a single evolutionary framework.

The yeast Fun30 and human SMARCAD1 chromatin remodellers promote DNA end resection.

Several homology-dependent pathways can repair potentially lethal DNA double-strand breaks (DSBs). The first step common to all homologous recombination reactions is the 5'-3' degradation of DSB ends that yields the 3' single-stranded DNA required for the loading of checkpoint and recombination proteins. In yeast, the Mre11-Rad50-Xrs2 complex (Xrs2 is known as NBN or NBS1 in humans) and Sae2 (known as RBBP8 or CTIP in humans) initiate end resection, whereas long-range resection depends on the exonuclease Exo1, or the helicase-topoisomerase complex Sgs1-Top3-Rmi1 together with the endonuclease Dna2 (refs 1-6). DSBs occur in the context of chromatin, but how the resection machinery navigates through nucleosomal DNA is a process that is not well understood. Here we show that the yeast Saccharomyces cerevisiae Fun30 protein and its human counterpart SMARCAD1 (ref. 8), two poorly characterized ATP-dependent chromatin remodellers of the Snf2 ATPase family, are directly involved in the DSB response. Fun30 physically associates with DSB ends and directly promotes both Exo1- and Sgs1-dependent end resection through a mechanism involving its ATPase activity. The function of Fun30 in resection facilitates the repair of camptothecin-induced DNA lesions, although it becomes dispensable when Exo1 is ectopically overexpressed. Interestingly, SMARCAD1 is also recruited to DSBs, and the kinetics of recruitment is similar to that of EXO1. The loss of SMARCAD1 impairs end resection and recombinational DNA repair, and renders cells hypersensitive to DNA damage resulting from camptothecin or poly(ADP-ribose) polymerase inhibitor treatments. These findings unveil an evolutionarily conserved role for the Fun30 and SMARCAD1 chromatin remodellers in controlling end resection, homologous recombination and genome stability in the context of chromatin.