The Synaptonemal Complex Protein ZYP1 Is Required for Imposition of Meiotic Crossovers in Barley

In many cereal crops, meiotic crossovers predominantly occur toward the ends ofchromosomes and 30 to 50% of genes rarely recombine. This limits the exploitation ofgenetic variation by plant breeding. Previous reports demonstrate that chiasmafrequency can be manipulated in plants by depletion of the synaptonemal complexprotein ZIPPER1 (ZYP1) but conflict as to the direction of change, with fewerchiasmata reported in Arabidopsis thaliana and more crossoversreported for rice (Oryza sativa). Here, we use RNA interference(RNAi) to reduce the amount of ZYP1 inbarley (Hordeum vulgare) to only 2 to 17% of normal zygotene levels.In the ZYP1^RNAi lines, fewer than half of the chromosomepairs formed bivalents at metaphase and many univalents were observed, leading tochromosome nondisjunction and semisterility. The number of chiasmata per cell wasreduced from 14 in control plants to three to four in the ZYP1-depleted lines,although the localization of residual chiasmata was not affected. DNA double-strandbreak formation appeared normal, but the recombination pathway was defective at laterstages. A meiotic time course revealed a 12-h delay in prophase I progression to thefirst labeled tetrads. Barley ZYP1 appears to function similarly to ZIP1/ZYP1 inyeast and Arabidopsis, with an opposite effect on crossover numberto ZEP1 in rice, another member of the Poaceae.