{"id":1247,"date":"2024-09-15T17:51:31","date_gmt":"2024-09-15T17:51:31","guid":{"rendered":"https:\/\/joshmitteldorf.peachpuff-wolverine-566518.hostingersite.com\/?p=1247"},"modified":"2024-09-26T19:37:23","modified_gmt":"2024-09-26T19:37:23","slug":"sex-and-the-single-paramecium","status":"publish","type":"post","link":"https:\/\/scienceblog.com\/joshmitteldorf\/2024\/09\/15\/sex-and-the-single-paramecium\/","title":{"rendered":"Sex and the Single Paramecium"},"content":{"rendered":"<p>In Darwinian terms, sex is good for the community, bad for the individual. If this statement doesn\u2019t make sense to you, perhaps you are thinking that sex is a part of reproduction, and reproduction is the very definition of Darwinian fitness.<\/p>\n<p>But sex is the sharing of genes. Reproduction is the process of creating new individuals. There is no logically necessary link between the two. In most higher organisms, evolution has linked gene-sharing to reproduction as a way to force individuals to share genes.<\/p>\n<div class=\"pullquote\">\n<blockquote><p>In most higher organisms, evolution has linked gene-sharing to<br \/>\nreproduction as a way to force individuals to share genes.<\/p><\/blockquote>\n<\/div>\n<p>Which brings me back to the opening statement: By neo-Darwinian standards, the ideal fitness strategy would be to clone oneself endlessly. But this leads to a collapse of genetic diversity and a fragile population. Sharing genes is essential for a healthy population, but it is only a distraction in pursuit of \u201cindividual fitness\u201d. If I clone myself, 100% of my genes are transferred to the next generation. If I reproduce sexually, only 50% of my genes get into the next generation. And if I have a special combination of genes that make me extra-fit, that combination is sure to be broken up in the mixing process that is sex.<\/p>\n<p>The strongest clue we have that evolution cares about populations and not just individuals is the fact that, in most multi-celled life forms, sex is a prerequisite to reproduction. Evolution has arranged it so that sharing genes is mandated. You want to reproduce? You\u2019re going to have to mix your genes with someone else\u2019s.<\/p>\n<p>Seen in this way, obligate sex is a grand communist conspiracy to hold back the strongest individuals and force them to contribute to the community, rather than to \u201cget ahead\u201d on their own.<\/p>\n<p>Most people I know don\u2019t need a lot of motivators in order to have sex. Biology has supplied the motivation. At one level, evolution has given us a powerful sex drive. At a deeper level, evolution has made sure that gene sharing is a prerequisite to reproduction, so that the drive to reproduce is transformed into a drive to copulate.<\/p>\n<h2 class=\"header-anchor-post\">Ciliates and other protists<\/h2>\n<div class=\"pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent\">\n<div class=\"pencraft pc-display-contents pc-reset _pubTheme_ztq6h_1\">\n<div id=\"\u00a7ciliates-and-other-protists\" class=\"pencraft pc-reset header-anchor offset-top\"><\/div>\n<\/div>\n<\/div>\n<p>Single-celled organisms are of three types: Bacteria, archaea, and protists. Small, small, and large. Simple, simple, and complex.<\/p>\n<p>Protists are the large, complicated protozoa from which multi-celled life emerged. The single cell is a million times larger than a bacterial cell and far more complex and structured; but still it\u2019s a single cell.<\/p>\n<p>Paramecia are ciliates. They are an advanced form of protists, capable of changing shape, moving about, and sophisticated goal-directed behaviors.\u00a0<a href=\"https:\/\/www.proquest.com\/openview\/26465152895458ac783d6e1ed36946a3\/1\" rel=\"\">Paramecia can learn and remember<\/a>, though they obviously have no brains or even nervous systems. [<a href=\"https:\/\/link.springer.com\/article\/10.3758\/BF03209695\" rel=\"\">another ref<\/a>]\u00a0<a href=\"http:\/\/jamsat.sums.ac.ir\/article_42486.html\" rel=\"\">No one knows how<\/a> they acquire information or store it. A fascinating topic, but not for today.<\/p>\n<span class=\"embed-youtube\" style=\"text-align:center; display: block;\"><iframe loading=\"lazy\" class=\"youtube-player\" width=\"1200\" height=\"675\" src=\"https:\/\/www.youtube.com\/embed\/sq6Y54mxjOg?version=3&#038;rel=1&#038;showsearch=0&#038;showinfo=1&#038;iv_load_policy=1&#038;fs=1&#038;hl=en-US&#038;autohide=2&#038;wmode=transparent\" allowfullscreen=\"true\" style=\"border:0;\" sandbox=\"allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox\"><\/iframe><\/span>\n<h5>This is lacrymaria, an advanced ciliate. It is a single cell that shapes itself to form a head and neck that can hunt for food.<\/h5>\n<p>In paramecia, reproduction and sex are entirely separate functions. Reproduction is clonal. The paramecium splits in half, its DNA is copied with half going to each daughter cell. Sex is via a process known as conjugation. Two paramecia sidle up to each other, and their cells merge, their chromosomes pair up and some genes cross over (as in sex for higher organisms). Two paramecia separate, but they are not the same two that came together a few hours earlier. Each individual is \u201chalf me and half you\u201d. Conjugation can require\u00a0<a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC6380434\/\" rel=\"\">many hours<\/a>\u00a0in some species. Sharing genes is much slower and, presumably, more costly than reproduction.<\/p>\n<div class=\"captioned-image-container\">\n<figure>\n<div class=\"image2-inset\"><img loading=\"lazy\" decoding=\"async\" class=\"sizing-normal\" src=\"https:\/\/substackcdn.com\/image\/fetch\/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep\/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F882c8ed5-2e62-429c-a79e-cb82436d92b5_800x535.jpeg\" alt=\"\" width=\"800\" height=\"535\" data-attrs=\"{&quot;src&quot;:&quot;https:\/\/substack-post-media.s3.amazonaws.com\/public\/images\/882c8ed5-2e62-429c-a79e-cb82436d92b5_800x535.jpeg&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:535,&quot;width&quot;:800,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:184611,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image\/jpeg&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false}\" \/><\/div>\n<\/figure>\n<\/div>\n<h5>Conjugating paramecia, with merged micronuclei and separate macronuclei visible.<\/h5>\n<p>&nbsp;<\/p>\n<p>There are boy paramecia and girl paramecia. Two sexes. Other ciliates have multiple sexes with complicated rules about who can mate with whom. (The\u00a0<a href=\"https:\/\/brainly.com\/question\/28710745\" rel=\"\">Tinder app for ciliates<\/a>\u00a0is way over budget and past its development deadline.)<\/p>\n<h2 class=\"header-anchor-post\">Protist genetics<\/h2>\n<div class=\"pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent\">\n<div class=\"pencraft pc-display-contents pc-reset _pubTheme_ztq6h_1\">\n<div id=\"\u00a7protist-genetics\" class=\"pencraft pc-reset header-anchor offset-top\"><\/div>\n<\/div>\n<\/div>\n<p>You and I each have one single genome, a set of 23 pairs of chromosome that sits in the nucleus of every cell in our bodies. Complicated chemical tags are attached to the chromosomes that determine which genes are active when and where. The DNA in each cell is actively being transcribed all the time, and this transcription controls the cell\u2019s metabolism.<\/p>\n<p>Ciliates have a different system. Each cell has a micronucleus containing a pristine copy of the DNA, and a macronucleus, with multiple copies of each gene in a DNA loop called a plasmid. The macronucleus is the workhorse, from which all gene products are transcribed, while the micronucleus retains a single archival copy of each chromosome.<\/p>\n<p>When a paramecium splits in two, the micronucleus makes a copy of every chromosome, so there is a fresh new micronucleus for each daughter cell. The macronucleus, however, just splits in two, in a process called\u00a0<a href=\"https:\/\/www.sciencedirect.com\/science\/article\/pii\/S0040580922000272#:~:text=Unlike%20most%20eukaryotes%2C%20the%20macronuclei,in%20two%20identical%20daughter%20nuclei.\" rel=\"\">amitosis<\/a>. Each daughter cell gets half of the macronucleus, containing an arbitrary selection of half the plasmids.<\/p>\n<p>Remember that the macronucleus contains many copies of each gene, so usually the arbitrary split results in at least one copy of each gene in each half. The macronucleus regrows itself as the daughter cell grows and matures. Plasmids in the macronucleus are always copying themselves, creating copies of copies, but the archival copy remains in the micronucleus, where it is protected from chemical disturbances.<\/p>\n<p>Over hundreds of cell divisions, the macronucleus becomes dysfunctional. The cell languishes and cannot continue growing and reproducing. This is\u00a0<a href=\"https:\/\/www.cancer.gov\/publications\/dictionaries\/cancer-terms\/def\/senescence\" rel=\"\">cell senescence<\/a>, ciliate style. (It does not have to do with\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41556-022-00842-x\" rel=\"\">telomere length, which is a trigger of cell senescence<\/a>\u00a0in multicelled organisms.)<\/p>\n<p>The only cure for this \u201cdisease of old age\u201d is to create a new macronucleus, using the archival chromosomes in the micronucleus.<\/p>\n<p>Any self-respecting, rugged individual paramecium would just spit out its old macronucleus and create a new one. But they have been tamed. There are no rugged individuals left, and they all have to live by the rule. The rule says that you don\u2019t get to make a new macronucleus \u2014 even if your life depends on it \u2014 unless you share your genes.<\/p>\n<p>Renewal is only available to the cell pursuant to conjugation. A new macronucleus is only created after gene exchange.<\/p>\n<h2>How did senescence in Ciliates evolve?<\/h2>\n<p>The process described above is a kind of senescence for ciliates. After one or two hundred cell divisions, the cell line loses its ability to reproduce. The macronucleus poops out, for reasons that are not fully understood. Perhaps it is mutation or DNA damage. Perhaps the different copies of copies of copies of plasmids in the macronucleus become unbalanced, with too many copies of some plasmids and a shortage of others. In any case, the cell needs a new macronucleus.<\/p>\n<p><strong>The interesting thing is that the cell is perfectly capable of creating a new macronucleus from the archival copy of DNA stored in its micronucleus, but it does not do this.<\/strong><\/p>\n<p>Evolution has decreed that the creation of a new macronucleus from the micronucleus can only occur subsequent to conjugation. In other words, aging and death are built into the life plan for paramecia, and sharing genes is the only way to get a new start.<\/p>\n<p>Contrast this to the situation for mammals like us. In both ciliates and mammals, sharing of genes is compulsory. But in mammals, sharing of genes is enforced by making sex a prerequisite for reproduction; while in ciliates, it is aging that enforces gene-sharing. The paramecia offspring will eventually get old and die if they don\u2019t conjugate, and conjugation resets the clock for a new lease on life.<\/p>\n<p>Notice that the cell that gets a new lease on life is\u00a0<em><strong>not<\/strong>\u00a0<\/em>the fabulously successful cell that had created hundreds of generations of copies of itself. That genome has reached a dead end. The cell that gets to start over with a new micronucleus is a hybrid, with half its genes from each of two cell lines.<\/p>\n<p>\u201cAha,\u201d you say, \u201cI\u2019m going to beat the system by cloning, then conjugating with my twin. That way I can hold on to this very special combinations that is me\u00a0<em><strong>me ME<\/strong><\/em>.\u201d<\/p>\n<p>Not so fast. There are at least two different sex types, and your twin is the same as your type. You can\u2019t inbreed. It\u2019s against the rules.<\/p>\n<p>You can clone yourself for awhile, but your clonal lineage is a dead end. Only the community survives long-term, not the individual.<\/p>\n<h2 class=\"header-anchor-post\">How did sex first become mandatory?<\/h2>\n<p>What was the evolutionary mechanism by which aging was imposed on paramecia, and gene-sharing was the only means by which they could escape the grim reaper? AFAIK, I have been the only one to ask this question.<\/p>\n<p>(There has been lots of academic attention to the\u00a0<a href=\"https:\/\/www.routledge.com\/The-Masterpiece-of-Nature-The-Evolution-and-Genetics-of-Sexuality\/Bell\/p\/book\/9780367339272?srsltid=AfmBOoqpJllLmQJVPc00air2iG0R_GvY7cRc6C4qIkhOr6k1TW0e40V1\" rel=\"\">evolution of sex<\/a>, which is long recognized as an evolutionary mystery. But little or none to this closely-related mechanism from hundreds of millions of years earlier.)<\/p>\n<p>I created a computer model a few years ago, but was unable to publish it at the time. The peer review system worked as it should, and reviewers pointed out to me that I had misunderstood the dynamics of senescence and reproduction in ciliates. I have recently rewritten the ms to reflect the understanding that I describe above, and yesterday resubmitted the ms for publication. In the meantime, the preprint is <a href=\"https:\/\/www.researchgate.net\/publication\/384039690_Origin_of_Cellular_Senescence_in_Ciliates\" rel=\"\">on ResearchGate <\/a>\u00a0and on <a href=\"https:\/\/biorxiv.org\/cgi\/content\/short\/2024.09.17.613400v1\">BioRxiv<\/a>.<\/p>\n<p>My ms actually includes two computer models, reflecting the mysterious nature of the phenomenon I\u2019m modeling. I don\u2019t claim to explain how senescence evolved, only to offer scenarios as a start to exploring this important transition in evolutionary history.<\/p>\n<p>Population diversity is important for two principal reasons.<\/p>\n<ol>\n<li>Diversity is the fuel for evolutionary change. A homogeneous population can only evolve one tiny mutation at a time, and so the population can\u2019t adapt when circumstances change. A diverse population offers the opportunity to try new combinations in response to a changing environment.<\/li>\n<li>Diversity offers resistance to epidemics. A parasite is a simpler organism that is usually optimized to prey upon one particular genotype. In a homogeneous community, the parasite spreads seamlessly from one individual to the next; in a diverse community, the parasite has a harder time.<\/li>\n<\/ol>\n<p>I constructed two separate computer models, based on these principles and also structured in ways that made the two models as different as I could conceive them.<\/p>\n<p>The results showed that, by adjusting parameters, I could make each of these models produce senescence as an adaptation, with rejuvenation tied to gene sharing.<\/p>\n<p>If you\u2019re skeptical about computer models, as I am, you\u2019ll want to know whether the particular parameters that are friendly to selection for senescence are typical of what we find in nature. But the models are abstract and idealized, difficult to relate to complexities in the real world \u2014 let alone the real world two billion years ago, when sex first appeared. So these models are just a start, just a suggestion.<\/p>\n<h2 class=\"header-anchor-post\">The big picture<\/h2>\n<p>If you\u2019re a paramecium, you can clone yourself for awhile, but your clonal lineage is a dead end. Only the community survives long-term, and not the individual.<\/p>\n<p>For multicelled creatures, we can\u2019t clone ourselves at all. Each of us is destined to be a one-off experiment in combinatorics, never to be repeated. It is the gene pool that has Darwinian staying power, not the particular combination that is you or me.<\/p>\n<p>This principle has deep consequences for the whole process of evolution. It means that, in the long run, it is the fitness of the community that matters. Individual fitness is fated to be a transitory phenomenon. It is community fitness that can persist, can compete, can flourish in Darwin\u2019s competition.<\/p>\n<p>This is the opposite of the \u201c<a href=\"https:\/\/www.thriftbooks.com\/w\/the-selfish-gene_richard-dawkins\/247478\/item\/4026410\/\" rel=\"\">selfish gene<\/a>\u201d. Richard Dawkins (and\u00a0<a href=\"https:\/\/www.google.com\/books\/edition\/The_Genetical_Theory_of_Natural_Selectio\/sT4lIDk5no4C\" rel=\"\">R. A. Fisher<\/a>\u00a046 years before him) had it exactly backwards. According to the selfish gene dogma, which dominated evolutionary theory from 1970 onward, Darwinian competition takes place primarily at the individual level. That was the old theory. But sex changes this. The meaning of sex is precisely to lift the level of evolutionary selection from the individual to the community.<\/p>\n<p>Life is evolved for cooperation, just as much as for competition.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>In Darwinian terms, sex is good for the community, bad for the individual. If this statement doesn\u2019t make sense to you, perhaps you are thinking that sex is a part of reproduction, and reproduction is the very definition of Darwinian fitness. But sex is the sharing of genes. Reproduction is the process of creating new &#8230; <a title=\"Sex and the Single Paramecium\" class=\"read-more\" href=\"https:\/\/scienceblog.com\/joshmitteldorf\/2024\/09\/15\/sex-and-the-single-paramecium\/\" aria-label=\"Read more about Sex and the Single Paramecium\">Read more<\/a><\/p>\n","protected":false},"author":65,"featured_media":1249,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1247","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized"],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.4 (Yoast SEO v27.4) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Sex and the Single Paramecium - Josh Mitteldorf<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/scienceblog.com\/joshmitteldorf\/2024\/09\/15\/sex-and-the-single-paramecium\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Sex and the Single Paramecium\" \/>\n<meta property=\"og:description\" content=\"In Darwinian terms, sex is good for the community, bad for the individual. If this statement doesn\u2019t make sense to you, perhaps you are thinking that sex is a part of reproduction, and reproduction is the very definition of Darwinian fitness. But sex is the sharing of genes. Reproduction is the process of creating new ... 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The surprising fact that our bodies are genetically programmed to age and to die offers an enormous opportunity for medical intervention. It may be that therapies to slow the progress of aging need not repair or regenerate anything, but only need to interfere with an existing program of self-destruction. Mitteldorf has taught a weekly yoga class for thirty years. He is an advocate for vigorous self care, including exercise, meditation and caloric restriction. After earning a PhD in astrophysicist, Mitteldorf moved to evolutionary biology as a primary field in 1996. He has taught at Harvard, Berkeley, Bryn Mawr, LaSalle and Temple University. He is presently affiliated with MIT as a visiting scholar. In private life, Mitteldorf is an advocate for election integrity as well as public health. He is an avid amateur musician, playing piano in chamber groups, French horn in community orchestras. His two daughters are among the first children adopted from China in the mid-1980s. Much to the surprise of evolutionary biologists, genetic experiments indicate that aging has been selected as an adaptation for its own sake. This poses a conundrum: the impact of aging on individual fitness is wholly negative, so aging must be regarded as a kind of evolutionary altruism. Unlike other forms of evolutionary altruism, aging offers benefits to the community that are weak, and not well focussed on near kin of the altruist. 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