Most complex organisms engage in a strange bit of genomic math at some point in their lives: To multiply, they subtract and then add. That is, to reproduce through the process of meiosis they create specialized sex cells, or gametes, with half the usual number of chromosomes; they then merge pairs of those gametes to create new individuals with a full, unique genome. Sexual reproduction is nearly ubiquitous among eukaryotes — organisms from kelp to koalas that have a membrane-bound nucleus and organelles. We animals merge a sperm and an egg; mushrooms sprout from the underground collision of fungal threads; pollen sends tubes racing through floral tissues to join ovules, creating fruit and seeds where they meet.
Yet fusing cells like this runs contrary to the normal cellular life cycle. Cells divide into two through mitosis to reproduce asexually, but otherwise they mostly guard against major disruptions of their integrity, which could snuff out their lineage.
Why this hazardous arrangement caught on in evolution has been the subject of intense study; it’s generally presumed that sexual recombination helps to keep a species’ genome healthy and diverse enough to offset the risks. But researchers have also been piecing together molecular clues about how it evolved.
Now research recently shared on the biorxiv.org preprint server suggests that some cells had the power to fuse their membranes far back in their evolutionary history. The molecular machinery that makes this part of sexual reproduction possible may have existed more than 2 billion years ago in the simple prokaryotic cells called archaea, perhaps as much as a billion years before eukaryotes and sex evolved. But the new findings also hint at an explanation for why this kind of cell fusion for sexual reproduction didn’t appear earlier in life’s history, when it seemingly could have. [Continue reading…]