Chimeric Sex-Determining Chromosomal Regions and Dysregulation of Cell-Type Identity in a Sterile Zygosaccharomyces Allodiploid Yeast
Allodiploidization is a fundamental yet evolutionarily poorly characterized event, which impacts genome evolution and heredity, controlling organismal development and polyploid cell-types. In this study, we investigated the sex determination system in the allodiploid and sterile ATCC 42981 yeast, a member of the Zygosaccharomyces rouxii species complex, and used it to study how a chimeric mating-type gene repertoire contributes to hybrid reproductive isolation. We found that ATCC 42981 has 7 MAT-like (MTL) loci, 3 of which encode α-idiomorph and 4 encode a-idiomorph. Two phylogenetically divergent MAT expression loci were identified on different chromosomes, accounting for a hybrid a/α genotype. Furthermore, extra a-idimorph-encoding loci (termed MTLa copies 1 to 3) were recognized, which shared the same MATa1 ORFs but diverged for MATa2 genes. Each MAT expression locus was linked to a HML silent cassette, while the corresponding HMR loci were located on another chromosome. Two putative parental sex chromosome pairs contributed to this unusual genomic architecture: one came from an as-yet-undescribed taxon, which has the NCYC 3042 strain as a unique representative, while the other did not match any MAT-HML and HMR organizations previously described in Z. rouxii species. This chimeric rearrangement produces two copies of the HO gene, which encode for putatively functional endonucleases essential for mating-type switching. Although both a and α coding sequences, which are required to obtain a functional cell-type a1-α2 regulator, were present in the allodiploid ATCC 42981 genome, the transcriptional circuit, which regulates entry into meiosis in response to meiosis-inducing salt stress, appeared to be turned off. Furthermore, haploid and α-specific genes, such as MATα1 and HO, were observed to be actively transcribed and up-regulated under hypersaline stress. Overall, these evidences demonstrate that ATCC 42981 is unable to repress haploid α-specific genes and to activate meiosis in response to stress. We argue that sequence divergence within the chimeric a1-α2 heterodimer could be involved in the generation of negative epistasis, contributing to the allodiploid sterility and the dysregulation of cell identity.
[…] Sequences were assembled and edited using DNAStar Software (DNASTAR, Inc. Madison, Wisconsin USA). Multiple nucleotide and amino acid sequence alignments were performed using Clustal W2 . Searches for nucleotide and protein sequence homologs were carried out in the GenBank database with Blastn and Blastp algorithms, respectively . Phylogenetic analysis was conducted on aa sequences using MEGA6 . The phylogenetic relationship was inferred using the neighbour-joining (NJ) method. Support percentages for the nodes of NJ-trees were computed using bootstrapping analysis with 1,000 replications and were shown next to the branches when ≥ 60%. For domain identification, Pfam-searches  were run on http://pfam.sanger.ac.uk/. Structure predictions were obtained with Jpred3  and validated according to Martin et al. . Sequences were submitted to the EMBL/GenBank databases under the accession numbers from KT598024 to KT598027 and from KT694298 to KT6942302. […]