Abstract
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
msh4
Δ mutation that similarly affects Spo11p-promoted
events. In a
spo11
Δ 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.