title: Variation in crossover frequencies perturb crossover assurance without affecting meiotic chromosome segregation in Saccharomyces cerevisiae.
abstract: The segregation of homologous chromosomes during the Meiosis I division requires an obligate crossover per homolog pair (crossover assurance). In Saccharomyces cerevisiae and mammals, Msh4 and Msh5 proteins stabilize Holliday junctions and its progenitors to facilitate crossing over. S. cerevisiae msh4/5 hypomorphs that reduce crossover levels up to twofold at specific loci on chromosomes VII, VIII, and XV without affecting homolog segregation were identified recently. We use the msh4-R676W hypomorph to ask if the obligate crossover is insulated from variation in crossover frequencies, using a S. cerevisiae S288c/YJM789 hybrid to map recombination genome-wide. The msh4-R676W hypomorph made on average 64 crossovers per meiosis compared to 94 made in wild type and 49 in the msh4Δ mutant confirming the defect seen at individual loci on a genome-wide scale. Crossover reductions in msh4-R676W and msh4Δ were significant across chromosomes regardless of size, unlike previous observations made at specific loci. The msh4-R676W hypomorph showed reduced crossover interference. Although crossover reduction in msh4-R676W is modest, 42% of the four viable spore tetrads showed nonexchange chromosomes. These results, along with modeling of crossover distribution, suggest the significant reduction in crossovers across chromosomes and the loss of interference compromises the obligate crossover in the msh4 hypomorph. The high spore viability of the msh4 hypomorph is maintained by efficient segregation of the natural nonexchange chromosomes. Our results suggest that variation in crossover frequencies can compromise the obligate crossover and also support a mechanistic role for interference in obligate crossover formation.
authors.affiliation: School of Biology, Indian Institute of Science Education and Research, Thiruvananthapuram, Trivandrum 695016, India.
authors.forename: Gurukripa N
authors.initials: GN
authors.lastname: Krishnaprasad
authors.affiliation: School of Biology, Indian Institute of Science Education and Research, Thiruvananthapuram, Trivandrum 695016, India.
authors.forename: Mayakonda T
authors.initials: MT
authors.lastname: Anand
authors.affiliation: European Molecular Biology Laboratory, Genome Biology Unit, 69117, Heidelberg, Germany.
authors.forename: Gen
authors.initials: G
authors.lastname: Lin
authors.affiliation: European Molecular Biology Laboratory, Genome Biology Unit, 69117, Heidelberg, Germany.
authors.forename: Manu M
authors.initials: MM
authors.lastname: Tekkedil
authors.affiliation: European Molecular Biology Laboratory, Genome Biology Unit, 69117, Heidelberg, Germany Department of Genetics, Stanford University, Stanford, California 94305 Stanford Genome Technology Center, Palo Alto, California 94304.
authors.forename: Lars M
authors.initials: LM
authors.lastname: Steinmetz
authors.affiliation: School of Biology, Indian Institute of Science Education and Research, Thiruvananthapuram, Trivandrum 695016, India nishantkt@iisertvm.ac.in.
authors.forename: Koodali T
authors.initials: KT
authors.lastname: Nishant
mesh_terms: D020090:Chromosome Segregation; D015825:Chromosomes, Fungal; D003434:Crossing Over, Genetic; D014644:Genetic Variation; D059014:High-Throughput Nucleotide Sequencing; D008540:Meiosis; D050296:Microbial Viability / Q000235:genetics; D009154:Mutation; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics; D013172:Spores, Fungal
chemical_list: D029701:Saccharomyces cerevisiae Proteins
keywords: crossover assurance; genome-wide recombination map; nonexchange chromosomes; obligate crossover; whole-genome sequencing