In trans poson-tagged lines of Arabidopsis, we found a mutant that was defective in meiotic chromosome segregation. This mutant, named atzip4-4, was due to a novel mutant allele of AtZIP4, which has sequence similarity to yeast ZIP4/SPO22, which codes a ZMM protein that is a proposed unit of the synapsis initiation complex. The chiasma distribution in atzip4-4 differed from that in the wild-type, involved in a deficiency of interfering crossovers in the mutant genome. On the other hand, FISH staining of loci on two independent chromosomes in mutant meiocytes indicated that homologous chromosome pairing to synapse progresses normally until the pachytene stage, yet homologous chromosomes often separated abruptly at diplotene and diakinesis. These results suggest that AtZIP4 plays an important role in normal crossover formation and meiotic chromosome segregation, but not in homolog search. The relationship of AtZIP4 and other related proteins in meiotic events is discussed and compared with that in yeast. (c) 2008 Elsevier Inc. All rights reserved.