Loss of E3 ligase HvST1 function substantially increases recombination

During meiosis, genetic recombination occurs via repair of DNA double-strand breaks (DSBs) as crossovers (COs) resulting in the exchange of parental genetic material (De Muyt et al., 2009). Crossovers are important for chromosome segregation and shuffling genetic variation, but their number and distribution are tightly regulated (Zickler and Kleckner, 2015). In barley and other large genome cereals, recombination events are limited in number and mainly restricted to the ends of chromosomes (Mascher et al., 2017), constraining progress in plant breeding. Recent studies have highlighted subtle differences in meiotic progression (Higgins et al., 2012; Phillips et al., 2013) and the distribution of recombination events in barley compared to other plants (Colas et al., 2016; Colas et al., 2017; Colas et al 2019), indicating possible evolutionary divergence of the meiotic program in large genome crops. Here we identify a spontaneous loss of function mutation in the grass specific E3 ubiquitin ligase HvST1 (Sticky Telomeres 1) which results in semi-sterility in barley. We show that abnormal synapsis in the absence of HvST1 function increases overall recombination by up to 2.5-fold and that HvST1 is capable of ubiquitinating ASY1, a key component of the lateral elements of the synaptonemal complex. Our findings shed light on an evolutionarily divergent pathway regulating synapsis and recombination in cereals. This natural loss of function variant presents new opportunities for the modulation of recombination in large genome cereals.