Zip4/Spo22 Is Required for Class I CO Formation but Not for Synapsis Completion in Arabidopsis thaliana

Abstract

In budding yeast meiosis, the formation of class I interference-sensitive crossovers requires the ZMM proteins. These ZMM proteins are essential in forming a mature synaptonemal complex, and a subset of these (Zip2, Zip3, and Zip4) has been proposed to compose the core of synapsis initiation complexes (SICs). Zip4/Spo22 functions with Zip2 to promote polymerization of Zip1 along chromosomes, making it a crucial SIC component. In higher eukaryotes, synapsis and recombination have often been correlated, but it is totally unknown how these two processes are linked. In this study, we present the characterization of a higher eukaryote SIC component homologue: Arabidopsis AtZIP4. We show that mutations in AtZIP4 belong to the same epistasis group as Atmsh4 and eliminate approximately 85% of crossovers (COs). Furthermore, genetic analyses on two adjacent intervals of Chromosome I established that the remaining COs in Atzip4 do not show interference. Lastly, immunolocalization studies showed that polymerization of the central element of the synaptonemal complex is not affected in Atzip4 background, even if it may proceed from fewer sites compared to wild type. These results reveal that Zip4 function in class I CO formation is conserved from budding yeast to Arabidopsis. On the other hand, and contrary to the situation in yeast, mutation in AtZIP4 does not prevent synapsis, showing that both aspects of the Zip4 function (i.e., class I CO maturation and synapsis) can be uncoupled.

Great majority of recombination events in Arabidopsis are gene conversion events

“…The analysis of the two negative controls showed that the error rates for gene conversion identification to be 0-5% (SI Appendix, Table S4), consistent with PCR results in Table 2. Our analysis hence largely confirms the prior higher estimates based on immunolocalization data (14) and suggests that at a minimum 90% of recombination events are resolved as gene conversions.…”

“…This direct estimate, however, disagrees with indirect inferences. An immunolocalization study (14) suggests in excess of 200 recombination events per meiosis, whereas another (15) suggests a more modest 120-140. Assuming that these events mostly reflect non-CO recombination events, this suggests a considerable excess of gene conversion compared with CO.…”

ASY1 mediates AtDMC1-dependent interhomolog recombination during meiosis in Arabidopsis

“…In this study, up to ∼150 AtDMC1 foci were detected in the Col-0 wild type at mid-prophase I. This is somewhat less than the figure reported in a recent study using the Arabidopsis ecotype Ws, where >200 AtDMC1 were recorded (Chelysheva et al 2007). Nevertheless, in both cases the figures are considerably greater than the peak number of AtSPO11-1 foci (∼90) that we observed.…”

Cytological analysis of MRE11 protein during early meiotic prophase I in Arabidopsis and tomato

“…7). However, in Arabidopsis, the number of MRE11 foci exceeds the expected number of ENs (as estimated by Dmc1 foci, Chelysheva et al 2007), and in tomato, immunogold localization revealed that most of the gold label at all early prophase I stages is associated with AE/LEs, not ENs. Only about 10% of ENs label with gold regardless of whether the preparations are treated with DNase I, prepared using sucrose step gradients, or labeled with ultrasmall gold-conjugated antibodies, all procedures that should increase antibody access to ENs.…”