Programmed cell death — apoptosis — is a universal phenomenon among multicellular organisms, and is especially important during development. Genetically orchestrated mechanisms of cell death have also been found in single-celled protists and yeast. Writing in Fungal Genetics and Biology (39, 82–93; 2003. doi: 10.1016/S1087-1845(03)00024-0), Benjamin Lu and colleagues describe a remarkably simple version of apoptosis in the ink-cap mushroom Coprinus cinereus (pictured).
Lu et al. studied apoptosis in mutant strains of the fungus that have defects in spore formation. Spores are formed by meiosis, which is the same type of cell division that halves the number of chromosomes in the egg and sperm cells of animals. In the mushroom, meiosis occurs in a synchronous fashion, sweeping across the gill surfaces underneath the cap, reconfiguring and sorting the chromosomes within 10 million spore-producing cells called basidia (inset). Mutants of C. cinereus called 'white-caps' are infertile: their basidia show defects at various points in the cell cycle, and the ashen hue of their delicate umbrellas is caused by the resulting failure to form black-pigmented spores.
It turns out that, in the mutants, basidia that experience problems at the beginning of meiosis (prophase I) undergo mass apoptosis, showing the classical apoptotic hallmark of DNA fragmentation. Lu and colleagues' experiments suggest that apoptosis is triggered at a single checkpoint in the mushroom cell cycle. This contrasts with the situation in the mouse, where the switch that activates apoptosis can be tripped at many steps throughout meiosis, implying that there is an almost continuous molecular assessment of the viability of the gamete-forming cells. Not surprisingly, it seems that mushrooms lack some of the developmental sophistication of mammals.
But why should apoptosis occur in mushrooms at all? Resource conservation is a likely explanation. Mushrooms disperse astonishing numbers of airborne spores, but there is an infinitesimal chance that any individual spore will reach a suitable patch of ground, survive, out-compete the resident microbes, grow and one day find a mate. So by aborting any basidia that have mishaps early in meiosis, the fungus conserves resources for healthy cells that will succeed in producing viable spores.
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Money, N. Suicidal mushroom cells. Nature 423, 26 (2003). https://doi.org/10.1038/423026b