diakinesis and metaphase I may be due to the presence of the unresolved recombina- tion intermediates, including those between more than two chromatids that are seen in yeast [29,30]. The fragmentation of univa- lent chromosomes at meiotic anaphase I in double dmc1/blap75 mutants might also suggest such interchomatid linkages (Arabi- dopsis dmc1 mutants have asynaptic meiosis [31]), although other explanations, such as the existence of unrepaired chromatid breaks, are equally likely. Interestingly, no later meiotic stages (second division) were seen in either study, implying meiotic arrest at the end of the first division. This phenotype contrasts strikingly with that seen in many Arabidopsis recombination mutants, such as rad50 or rad51, which complete the two meiotic divisions, not- withstanding severe chromosome fragmen- tation at leptotene [32,33]. 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Jessop L, Lichten M (2008) Mus81/Mms4 endonuclease and Sgs1 helicase collaborate to PLoS Genetics | www.plosgenetics.org 2 December 2008 | Volume 4 | Issue 12 | e1000306