Selection arena in Aspergillus nidulans.

The selection arena hypothesis states that overproduction of zygotes--a widespread phenomenon in animals and plants--can be explained as a mechanism of progeny choice. As a similar mechanism, the ascomycetous fungus Aspergillus nidulans may overproduce dikaryotic fruit initials, hereafter called dikaryons. Then, progeny choice might involve selection on which of these dikaryons will thrive to produce thousands of zygotes. These zygotes each produce eight sexual spores which together fill up one fruiting body. In this study, we test the selection arena hypothesis in this homothallic fungus that produces both sexual and asexual spores. We analyzed two mitochondrial and 15 auxotrophic mutations for consequences on sexual and asexual reproduction. We found that many of these mutations confer sexual self-sterility as a pleiotropic effect under conditions of normal asexual spore production. This confirms an important prediction of the selection arena, namely that dikaryons carrying a (slightly) deleterious mutation are not able to proliferate and produce sexual spores. The selection arena ensures that reproductive energy is invested mainly in dikaryons and thus sexual spores of good genetic quality.