Sex without crossing over in the yeast Saccharomycodes ludwigii

Background Intermixing of genomes through meiotic reassortment and recombination of homologous chromosomes is a unifying theme of sexual reproduction in eukaryotic organisms and is considered crucial for their adaptive evolution. Previous studies of the budding yeast species Saccharomycodes ludwigii suggested that meiotic crossing over might be absent from its sexual life cycle, which is predominated by fertilization within the meiotic tetrad. Results We demonstrate that recombination is extremely suppressed during meiosis in Sd. ludwigii. DNA double-strand break formation by the conserved transesterase Spo11, processing and repair involving interhomolog interactions are required for normal meiosis but do not lead to crossing over. Although the species has retained an intact meiotic gene repertoire, genetic and population analyses suggest the exceptionally rare occurrence of meiotic crossovers in its genome. A strong AT bias of spontaneous mutations and the absence of recombination are likely responsible for its unusually low genomic GC level. Conclusions Sd. ludwigii has followed a unique evolutionary trajectory that possibly derives fitness benefits from the combination of frequent mating between products of the same meiotic event with the extreme suppression of meiotic recombination. This life style ensures preservation of heterozygosity throughout its genome and may enable the species to adapt to its environment and survive with only minimal levels of rare meiotic recombination. We propose Sd. ludwigii as an excellent natural forum for the study of genome evolution and recombination rates

Dissection of quantitative trait loci in the Lachancea waltii yeast species highlights major hotspots

Dissecting the genetic basis of complex trait remains a real challenge. The budding yeast Saccharomyces cerevisiae has become a model organism for studying quantitative traits, successfully increasing our knowledge in many aspects. However, the exploration of the genotype-phenotype relationship in non-model yeast species could provide a deeper insight into the genetic basis of complex traits. Here, we have studied this relationship in the Lachancea waltii species which diverged from the S. cerevisiae lineage prior to the whole-genome duplication. By performing linkage mapping analyses in this species, we identified 86 quantitative trait loci (QTL) impacting the growth in a large number of conditions. The distribution of these loci across the genome has revealed two major QTL hotspots. A first hotspot corresponds to a general growth QTL, impacting a wide range of conditions. By contrast, the second hotspot highlighted a trade-off with a disadvantageous allele for drug-free conditions which proved to be advantageous in the presence of several drugs. Finally, a comparison of the detected QTL in L. waltii with those which had been previously identified for the same trait in a closely related species, Lachancea kluyveri was performed. This analysis clearly showed the absence of shared QTL across these species. Altogether, our results represent a first step toward the exploration of the genetic architecture of quantitative trait across different yeast species

Lessons from the meiotic recombination landscape of the ZMM deficient budding yeast Lachancea waltii

Meiotic recombination has been deeply characterized in a few model species only, notably in the budding yeast Saccharomyces cerevisiae. Interestingly, most members of the ZMM pathway that implements meiotic crossover interference in S. cerevisiae have been lost in Lachancea yeast species after the divergence of Lachancea kluyveri from the rest of the clade. This suggests major differences in the control of crossover distribution. After investigating meiosis in L. kluyveri, we determined the meiotic recombination landscape of Lachancea waltii and identified several characteristics that should help understand better the underlying mechanisms. Such characteristics include systematic regions of loss of heterozygosity (LOH) in L. waltii hybrids, compatible with dysregulated Spo11-mediated DNA double strand breaks (DSB) independently of meiosis. They include a higher recombination rate in L. waltii than in L. kluyveri despite the lack of multiple ZMM pro-crossover factors. L. waltii exhibits an elevated frequency of zero-crossover bivalents as L. kluyveri but opposite to S. cerevisiae. L. waltii gene conversion tracts lengths are comparable to those observed in S. cerevisiae and shorter than in L. kluyveri despite the lack of Mlh2, a factor limiting conversion tracts size in S. cerevisiae. L. waltii recombination hotspots are not shared with either S. cerevisiae or L. kluyveri, showing that meiotic recombination hotspots can evolve at a rather limited evolutionary scale within budding yeasts. Finally, in line with the loss of several ZMM genes, we found only residual crossover interference in L. waltii likely coming from the modest interference existing between recombination precursors.Significance statement Studying non-model species is relevant to understand better biological processes by shedding light on their evolutionary variations. Here we chose the non-model budding yeast Lachancea waltii to study meiotic recombination. In sexually reproducing organisms, meiotic recombination shuffles parental genetic combinations notably by crossovers that cluster in hotspots at the population level. We found remarkable variations compared to both the canonical Saccharomyces cerevisiae model and another close relative Lachancea kluyveri. Such variations notably include the loss in L. waltii of a layer of regulation of crossover distribution that is otherwise conserved in budding yeasts and mammals. They also include the lack of conservation of crossover hotspots across the Lachancea species while crossover hotspots are remarkably stable across the Saccharomyces species

LeptoLife: deciphering the life and pathogenicity cycle of a fungal phytopathogen using metatranscriptomics

Leptosphaeria maculans, the agent of stem canker of oilseed rape (OSR, Brassica napus) displays an unusually complex pathogenic cycle and interactions with its host plant. Its life cycle encompasses a saprophytic stage on stem debris, during which sexual reproduction takes place, and contrasted pathogenicity programs including two biotrophic stages and two necrotrophic stages. One of the biotrophic stages consists of an exceptionally long endophytic life within the apoplast of plant tissues during which no symptoms are expressed and no apparent damages are caused to the plant. In addition, a closely related species, L. biglobosa, has a very similar pathogenic strategy and may interact with L. maculans within the plant. To have a complete insight into the different programs set up by the fungus to complete its life cycle, we undertook an extensive RNAseq analysis of the fungallife in isolated conditions or in multiple combinations of interactions with the plant or other fungi/microbes. A total of 420 biological samples corresponding to 11 distinct stages of the life/pathogenic cycle (and numerous time points) were obtained and submitted to RNAseq sequencing. The data are in the process of being analysed but already shade light on unexpected interaction between L. maculans and L. biglobosa in planta

Analyse de l'interaction entre les membres d'un complexe d'espèces pathogènes du colza (Leptosphaeria maculans et L.biglobosa) par une approche transcriptomique

International audienc

Identification et analyse fonctionnelle des effecteurs tardifs impliqués dans la colonisation systémique du colza par Leptosphaeria maculans

National audienc

Status of ZMM proteins in <i>L</i>. <i>waltii</i> in reference to <i>S</i>. <i>cerevisiae</i> and <i>L</i>. <i>kluyveri</i>.

(XLSX)</p

De novo assembly and annotation of three Leptosphaeria genomes using Oxford Nanopore MinION sequencing

Leptosphaeria maculans and Leptosphaeria biglobosa are ascomycete phytopathogens of Brassica napus (oilseed rape, canola). Here we report the complete sequence of three Leptosphaeria genomes (L. maculans JN3, L. maculans Nz-T4 and L. biglobosa G12-14). Nz-T4 and G12-14 genome assemblies were generated de novo and the reference JN3 genome assembly was improved using Oxford Nanopore MinION reads. The new assembly of L. biglobosa showed the existence of AT rich regions and pointed to a genome compartmentalization previously unsuspected following Illumina sequencing. Moreover nanopore sequencing allowed us to generate a chromosome-level assembly for the L. maculans reference isolate, JN3. The genome annotation was supported by integrating conserved proteins and RNA sequencing from Leptosphaeria-infected samples. The newly produced high-quality assemblies and annotations of those three Leptosphaeria genomes will allow further studies, notably focused on the tripartite interaction between L. maculans, L. biglobosa and oilseed rape. The discovery of as yet unknown effectors will notably allow progress in B. napus breeding towards L. maculans resistance