HO endonuclease-induced recombination in yeast meiosis resembles Spo 11-induced events

In meiosis, gene conversions are accompanied by higher levels of crossing over than in mitotic cells. To determine whether the special properties of meiotic recombination can be attributed to the way in which Spo11p creates double-strand breaks (DSBs) at special hot spots in Saccharomyces cerevisiae, we expressed the site-specific HO endonuclease in meiotic cells. We could therefore compare HO-induced recombination in a well-defined region both in mitosis and meiosis, as well as compare Spo11p- and HO-induced meiotic events. HO-induced gene conversions in meiosis were accompanied by crossovers at the same high level (52%) as Spo11p-induced events. Moreover, HO-induced crossovers were reduced 3-fold by a msh4Δ mutation that similarly affects Spo11p-promoted events. In a spo11Δ diploid, where the only DSB is made by HO, crossing over was significantly higher (27%) than in mitotic cells (≤7%). This single meiotic DSB failed to induce the formation of a synaptonemal complex. We also show that HO-induced gene conversion tract lengths are shorter in meiotic than in mitotic cells. We conclude that a hallmark of meiotic recombination, the production of crossovers, is independent of the nature of Spo11p-generated DSBs at special hotspots, but some functions of Spo11p are required in trans to achieve maximum crossing over.