Why sex evolved; no one knows (Introduction)

by David Turell , Friday, June 17, 2016, 14:44 (3080 days ago)

Simply splitting apart as bacteria do is complex enough, but much simpler than eukaryotic sex, which has lead to cells that are highly complex compared to bacteria: - https://aeon.co/ideas/sex-is-a-costly-molecular-kind-of-wizardry-why-evolve-it - "Sex is unknown in bacteria - the simplest and most ancient living cells on Earth - that reproduce by simply splitting into two. Evolving considerably later, eukaryotes are built of much larger and awfully complex cells, their insides full of organelles and membranous labyrinths buzzing with sophisticated molecular machinery and cargo-transport networks. Unlike bacteria, very few eukaryotic species revert to strict asexuality, and those that do seem to be relatively short-lived on the evolutionary timescale. Sex is costly, but it also appears to be essential for the long-term survival of complex life. - "Some of the most talented theorists have striven to understand why. Myriad explanations made their way into science journals and textbooks - from the earliest proposals that sex generates variation and speeds up adaptation, to mathematical models demonstrating that gene shuffling bolsters resistance to parasites and slows down the accumulation of hazardous genetic defects. But even with the overwhelming amount of attention the problem has received over the years, it is still considered unsolved. - *** - "But the past few decades have seen a paradigm shift in the understanding of how complex life came into being. All Archezoa turned out to be secondarily simplified versions of cells as complex as any other unicellular eukaryote that lives today. Genetic composition analyses showed that genes required for sexual reproduction permeate all eukaryotic groups, from animals and plants, to unicellular amoeba and even Giardia. The last common ancestor of all eukaryotes and the first truly complex cell was already sexual - which is why the ultimate solution to the evolutionary enigma of sex should be sought within the origin of eukaryotes themselves. - *** - "Around 2 billion years ago in a world ruled by microbes, a bacterial species formed a close symbiotic partnership with another simple cell - an archaeon. The interaction was so tight that bacterial symbionts eventually colonised the insides of archaea and were gradually transformed into mitochondria - the organelles of our cells specialising in energy production. The chimeric cell grew and expanded, using the genetic material of both partners and the newly available mitochondrial energy source to forge a cell of unparalleled complexity, inventing countless eukaryotic features along the way - including sex. - *** - "Lane is convinced that the pre-eukaryotic cell was rescued by its ability to fuse with similar cells - a skill that bacteria do not possess. Frequent cell unions brought several copies of the genome together, each from a different individual and each containing a different set of mutations or newly acquired symbiont genes. Cell fusions masked lethal defects and ensured that the host always possessed a full set of indispensable chromosomes and vital genes. Fusions facilitated gene mixing through recurrent recombination, stabilised the genome, and promoted its further expansion. - "Quite unexpectedly, the evolutionary puzzle of sex comes down to the origin of cell fusion, not just recombination, in the early stages of the transition to complex life. .... Critically, both hypotheses predict that the ability to fuse was a direct consequence of the bacterial invasion into the archaeal host, putting the origin of the first sexual trait well before the development of the eukaryotic cell was complete. - "Regardless of what the initial benefit of sexual cell fusion was, the repercussions were immense. Sex rescued the emerging eukaryotic cell when it was most vulnerable; without sex, the evolutionary transition would have plainly failed. While sex appears to be critical for the long-term survival of contemporary eukaryotes, it was not itself an invention of the eukaryotic cell. It is far more likely that the eukaryotic cell evolved only because sex - through cell fusion and recombination - was invented in ancestral chimeric lineages leading to the last common ancestor of all complex life." - Comment: Still lots of theory with n o solid answers. Margulies theory of fusion of bacteria to create mitochondria is obviously a key component. Eukaryotic cells are huge complex factories compared to bacteria. It is clear that combining genetic pools helped to create this complexity. Like the origin of life mystery, this event (sex) is just as mysterious. Why not consider God creating both giant steps?