Abstract
SummaryCrossover formation during meiosis in plants is required for proper chromosome segregation and is essential for crop breeding as it allows an (optimal) combination of traits by mixing parental alleles on each chromosome. Crossover formation commences with the production of a large number of DNA double‐strand breaks, of which only a few result in crossovers. A small number of genes, which drive the resolution of DNA crossover intermediate structures towards non‐crossovers, have been identified in Arabidopisis thaliana. In order to explore the potential of modification of these genes in interspecific hybrids between crops and their wild relatives towards increased production of crossovers, we have used CRISPR/Cas9‐mutagenesis in an interspecific tomato hybrid to knockout RecQ4. A biallelic recq4 mutant was obtained in the F1 hybrid of Solanum lycopersicum and S. pimpinellifolium. Compared with the wild‐type F1 hybrid, the F1 recq4 mutant was shown to have a significant increase in crossovers: a 1.53‐fold increase when directly observing ring bivalents in male meiocytes microscopically and a 1.8‐fold extension of the genetic map when measured by analysing SNP markers in the progeny (F2) plants. This is one of the first demonstrations of increasing crossover frequency in interspecific hybrids by manipulating genes in crossover intermediate resolution pathways and the first to do so by directed mutagenesis.Significance statementIncreasing crossover frequency during meiosis can speed up or simplify crop breeding that relies on meiotic crossovers to introduce favourable alleles controlling important traits from wild relatives into crops. Here we show for the first time that knocking out an inhibitor of crossovers in an interspecific hybrid between tomato and its relative wild species using CRISPR/Cas9‐mutagenesis results in increased recombination between the two genomes.
Highlights
Meiotic recombination is a crucial event in sexual reproduction
CRISPR/Cas9-mutagenesis and selection of a biallelic recq4 null mutation in an interspecific hybrid In order to examine the effect of RECQ4 on crossover frequency in an interspecific hybrid, we introduced two mutant RECQ4 alleles in the F1 hybrid S. lycopersicum
The results indicate that RECQ4 mutation by CRISPR/Cas9 increases the recombination frequency in an interspecific hybrid between tomato and S. pimpinellifolium
Summary
Meiotic recombination is a crucial event in sexual reproduction. In the first place, the formation of crossovers is essential for proper chromosome segregation during gamete formation. The reciprocal exchange between homologous chromosomes enables the reshuffling of parental genetic information and transfer of the recombined material to the generation. This makes meiotic crossover formation an important target in crop breeding. Meiosis starts with the formation of a large number of DNA double-strand breaks (DSB), most of which are repaired in non-crossover events and do not result in recombinant chromosomes. Especially in plants, only a few (one to three) DSBs per chromosome are processed into actual crossovers (Lambing et al, 2017; Mercier et al, 2015). DSB DNA repair via homologous chromosomes occurs through joint molecules that are processed into crossovers or, more frequently into non-crossovers. The number of class I crossovers is limited by interference, the phenomenon that prevents the occurrence of two crossovers in close proximity (Mercier et al, 2005)
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