Coprinus cinereus Mer3 is required for synaptonemal complex formation during meiosis

Mer3 is an evolutionarily conserved DNA helicase that has crucial roles in meiotic recombination and crossover formation. We have identified the MER3 homolog in Coprinus cinereus (Ccmer3) and show that it is expressed in zygotene and pachytene meiocytes. Immunostaining analysis indicated that CcMer3 was localized on chromosomes at zygotene and pachytene and CcMer3 foci were more frequent on paired than unpaired chromosomes. We generated a C. cinereus mer3 mutant (#1) and found that it showed abnormal meiosis progression and underwent apoptosis after prophase I. Basidiospore production in #1 was reduced to 0.8% of the wild-type level; the spores showed slower germination at 25A degrees C but were similar to the wild type at 37A degrees C. Electron microscopic analysis of chromosome spreads revealed that axial elements were formed in the mutant but that synapsis was defective, resulting in a reduction in spore production. Our results demonstrate that CcMer3 is required for synaptonemal complex formation after axial elements align and is thus essential for homologous synapsis.