February 18th, 2019


sexless surprise

When you look at the multicellular part of the tree of life, almost everything reproduces sexually. Not all the time -- some plants can self-fertilize, many plants can spread vegetatively, some animals are optionally parthenogenetic. But almost everything has sex as an option. Not all: there are some animal species that only reproduce by parthenogenesis. But they're all twigs on the tree of life, not lush branches, suggesting that this approach to reproduction doesn't last long. Why not? That touches on the question of why sex evolved in the first place, but a rather plausible answer is that it helps protect against parasites and germs, by mixing things up. Asexual reproduction looks like a good short-term genetic bet for the parent -- 100% of genes pass on! -- but yields a population of clones that can be scythed through by the parasite that figures out the key.

Bacteria and archaea evolve fast enough to keep up with each other and with viruses, perhaps... and, also, they have their own forms of gene transfer: conjugation (like sex), or transformation (uptake of plasmids, say.) (A side note: modern GMOs are thus less unnatural than you might think; genes jump around, even between multicellular animals, and GMOs are made via 'natural' techniques.)

There is one big exception to the "all twigs" statement: the bdelloid rotifers, a clade of 450+ species that have apparently been asexual for 25 million years. How do they pull it off? I'd thought maybe their cuticles were tough enough that they thoroughly kept out viruses and such, unlike anything else. But The Tangled Tree by David Quammen gave a better explanation. As freshwater plankton, they've evolved to survive drying out and being rehydrated. And it's not that they're really good at preserving their DNA through such stages; rather, they're decent at repairing the damage after rehydration. 'Decent' meaning that in the process they may incorporate foreign bits of DNA.

...they found at least twenty-two genes from non-bdelloid creatures, genes that must have arrived by horizontal transfer. Some of those were bacterial genes, some were fungal. One gene had come from a plant. At least a few of those genes were still functional, producing enzymes or other products useful to the animal. Later work on the same rotifer suggested that 8 percent of its genes had been acquired by horizontal transfer from bacteria or other dissimilar creatures. A team of researchers based mostly in England looked at four other species of bdelloids and also found “many hundreds” of foreign genes. Some of the imports had been ensconced in bdelloid genomes for a long time, since before the group diversified, while some were unique to each individual species, and therefore more recently acquired. This implied that horizontal gene transfer is an ancient phenomenon among bdelloid rotifers, and that it’s still occurring.

...biologists suspect that such drying-and-rehydrating stresses cause bdelloid DNA to fracture and leave cell membranes leaky. Given that they’re surrounded in their environments by living bacteria and fungi, plus naked DNA remnants from dead microbes, the porous membranes and fracturing could make it easy for alien DNA to enter even the nuclei of bdelloid cells and to get incorporated into bdelloid genomes as they repair themselves. Let me say that again: broken DNA, as a cell fixes it, using ambient materials, may include bits that weren’t part of the original. If that mended DNA happens to be in cells of the germ line, the changes will be heritable. Baby rotifers will get them and, when the babies mature, pass the changes along to their own daughters. Thus a bacterial or fungal gene can become part of the genome of a lineage of animals.

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