The role of genomic structural variation in the genetic improvement of polyploid crops

Many of our major crop species are polyploids, containing more than one genome or set of chromosomes. Polyploid crops present unique challenges, including difficulties in genome assembly, in discriminating between multiple gene and sequence copies, and in genetic mapping, hindering use of genomic data for genetics and breeding. Polyploid genomes may also be more prone to containing structural variation, such as loss of gene copies or sequences(presence–absence variation) and the presence of genes or sequences in multiple copies(copynumber variation). Although the two main types of genomic structural variation commonly identified are presence–absence variation and copy-number variation, we propose that homeologous exchanges constitute a third major form of genomic structural variation in polyploids. Homeologous exchanges involve the replacement of one genomic segment by a similar copy from another genome or ancestrally duplicated region, and are known to be extremely common in polyploids. Detecting all kinds of genomic structural variation is challenging, but recent advances such as optical mapping and long-read sequencing offer potential strategies to help identify structural variants even in complex polyploid genomes. All three major types of genomic structural variation(presence–absence, copy-number, and homeologous exchange) are now known to influence phenotypes in crop plants, with examples of flowering time, frost tolerance, and adaptive and agronomic traits. In this review,we summarize the challenges of genome analysis in polyploid crops, describe the various types of genomic structural variation and the genomics technologies and data that can be used to detect them, and collate information produced to date related to the impact of genomic structural variation on crop phenotypes. We highlight the importance of genomic structural variation for the future genetic improvement of polyploid crops.

Dissection of a rapidly evolving wheat resistance gene cluster by long-read genome sequencing accelerated the cloning of Pm69

Gene cloning in repeat-rich polyploid genomes remains challenging.Here,we describe a strategy for overcoming major bottlenecks in cloning of the powdery mildew resistance gene(R-gene)Pm69 derived from tetraploid wild emmer wheat.A conventional positional cloning approach was not effective owing to suppressed recombination.Chromosome sorting was compromised by insufficient purity.A Pm69 physical map,constructed by assembling Oxford Nanopore Technology(ONT)long-read genome sequences,revealed a rapidly evolving nucleotide-binding leucine-rich repeat(NLR)R-gene cluster with structural variations.A single candidate NLR was identified by anchoring RNA sequencing reads from susceptible mutants to ONT contigs and was validated by virus-induced gene silencing.Pm69 is likely a newly evolved NLR and was discovered in only one location across the wild emmer wheat distribution range in Israel.Pm69 was successfully introgressed into cultivated wheat,and a diagnostic molecular marker was used to accelerate its deployment and pyramiding with other R-genes.