Sex is not reproduction.<\/b><\/p>\n
Sex originally had nothing to do with reproduction, and how the two became bound so tightly together is a subject of ongoing debate in the evolutionary community for a hundred years. \u00a0Sex is the sharing of genes. \u00a0Your children get half your chromosomes and half from your spouse. \u00a0What is more, individual genes within a chromosome can cross over<\/a> between a mother\u2019s chromosome and \u00a0a father\u2019s in an exchange that greatly enhances the possible combinations. \u00a0Sex serves a profound evolutionary purpose, boosting evolvability by broadening the competition, making possible trials of many diverse combinations. \u00a0Sex is a democratizing force in opposition to the selfish gene, tying together the fate of an entire community. \u00a0Any selfish gene that wants to get very far in evolutionary competition has to learn to work well with a variety of other genes in the community. \u00a0This puts a damper on the selfish advantage of any gene that provides only a temporary advantage, or whose advantage depends on stealing from others.<\/p>\n In \u201chigher organisms\u201d–that\u2019s you, me, and the cockroach–sex and reproduction have been so tightly integrated that it\u2019s not possible to reproduce without sharing genes. Ultimately, it’s the prospect of reproduction that provides the “carrot” (not what you were thinking?) \u00a0In some animals, including humans, sexual activity modestly enhances life expectancy. \u00a0That might be a carrot if you think of it as a motivation to have sex, or a stick if you think of early death as punishment for abstience.<\/p>\n In many protozoans, the functions of sex and reproduction are completely separate. \u00a0Sex occurs between two protozoans of the same species via conjugation<\/i>, and it may occur only once in a few\u00a0hundred lifetimes. \u00a0But reproduction is something every protozoan does individually, by dividing in half.<\/p>\n In conjugation, two cells sidle up to each other and the membranes between them dissolve. \u00a0Two cells actually fuse, and then the nuclei of those cells (containing the genetic material) fuse as well. \u00a0The identities of the two are thoroughly scrambled to create two new individuals. \u00a0Then two cells emerge from conjugation, but the cells that emerge cannot be identified individually with the original cells. \u00a0You and I have both become \u2018half me and half you\u2019.<\/p>\n \u00a0<\/strong><\/strong><\/p>\n Sex and the Single Protozoan: What\u2019s in it for Me? <\/b><\/p>\n In animals and plants, sex and reproduction have been tied tightly together so that organisms can\u2019t revert to reproduction on their own. \u00a0But in protozoans, where is the motivation to participate in conjugation? \u00a0What carrot or stick assures that cells take time out from the serious business of reproduction to share their genes?<\/p>\n The carrot is entirely subjective. \u00a0Disclaimer: I, myself, have never had personal experience as a protozoan. \u00a0But several protozoans whom I know and trust have told me that it is an experience like no other. \u00a0I mean, like cosmic, man, really far out. \u00a0The feeling of that cell wall dissolving and new cytoplasm pouring in is an experience of a lifetime–once in a hundred lifetimes, actually. \u00a0Even though they are merging with just one other individual, what they describe is an experience of sublime oneness with all of creation. \u00a0Groovy.<\/p>\n Still, in case the experience itself is not sufficient reward for sharing, nature has provided a stick as well as a carrot. \u00a0And that is that if you go for too many generations just reproducing clonally, never sharing genes, then you die. \u00a0This is the origin of replicative senescence, another name for telomere shortening.<\/p>\n Every time the cell reproduces, the telomere gets a little shorter (in both daughter cells). \u00a0If the daughter cells go on like this for more than a few hundred generations, they run out of telomere. \u00a0The remedy: sex=conjugation. \u00a0When two cells conjugate, the two cells that emerge have long telomeres again, and a fresh lease on life. \u00a0Telomerase is brought out of deep storage to rescue the chromosomes and restore their telomeres to full length. \u00a0The cells that emerge begin life anew and can replicate hundreds of times more before their descendants have to worry again about a shortage in the telomere department.<\/p>\n \u00a0<\/strong><\/strong><\/p>\n Telomerase Rationing Enforces Gene Sharing in Protozoans<\/b><\/p>\n The interesting thing is that telomerase is right there in the genome, always available to be expressed. \u00a0The cell could, in principle, get out a little telomerase every time it replicated, so it would not lose telomere length at all, ever. \u00a0In this sense, it is an artificial shortage. \u00a0Telomerase is withheld by evolutionary programming, and the cell is coerced into sharing genes every so often in order to get its hands on the telomerase it needs to continue living and reproducing. \u00a0The temptation to cheat must be enormous. \u00a0The telomerase gene must be hidden away (by epigenetic programming) so well that a chance mutation can\u2019t create a rogue daughter cell that is \u201cimmortal\u201d in the sense that it can go on dividing and dividing, never sharing its genes.<\/p>\n We are the descendants of these cells, a billion years on, and we have inherited the same system of telomerase rationing that the protozoans have to live with. \u00a0In protozoans, the artificial shortage of telomerase is the \u201cstick\u201d that enforces the imperative to Share Genes! \u00a0In humans (and most other mammals) our stem cells divide during an individual lifetime, and very little telomerase is available, so the telomeres of our stem cells gradually shorten with age, and this is one of the primary aging clocks<\/a>, one of a handful of deep roots of human aging.<\/p>\n Last week, ScienceBlog featured an article<\/a> about one such protozoan that reproduces by simple mitosis without telomerase, and conjugates occasionally to exchange genes and renew its telomeres. \u00a0This is the pond-dwelling cilliate Oxytricha trifallax<\/i>.<\/p>\n \u00a0<\/strong><\/strong><\/p>\n Two Cell Nuclei<\/b><\/p>\n Oxytricha<\/i> and other protozoans (but not higher life forms) actually have two cell nuclei. \u00a0There is a micronucleus that contains the original copy of all the chromosomes. \u00a0Then there is the macronucleus that contains many working copies. \u00a0In the course of the cell\u2019s metabolism, it is the macronucleus that directs all the cell\u2019s activities. \u00a0The original copies of the genes are preserved in the micronucleus, which is only active during reproduction and conjugation. \u00a0Genes in the micronucleus are organized strictly onto chromosomes. \u00a0Genes in the macronucleus are cut apart for easy access. \u00a0The number of copies of each gene is proportional to the amount of activity that that gene needs to contribute to the cell\u2019s metabolism.<\/p>\n During clonal reproduction, chromosomes in the micronucleus are copied to make two identical new micronuclei. \u00a0But the macronucleus is simply split in two, half going to each clone. \u00a0After reproduction, the micronucleus fortifies the macronucleus with new gene copies.<\/p>\n But conjugation brings a truly fresh start. \u00a0After conjugation, the two macronuclei are destroyed and digested! \u00a0This is so new instructions can be read from the new library of genetic material, coming from two organisms that are slightly different. \u00a0The old macronuclei are destroyed, and new ones are created separately in each daughter cell, with faithful copies of that daughter\u2019s genes.<\/p>\n In Oxytricha<\/i>,<\/p>\n the level of DNA processing in the formation of a new macronucleus is extraordinary: the original micronucleus chromosomes are fragmented at tens of thousands of positions, 95% of the DNA complexity is lost, and the resulting chromosomes– sometimes referred to as \u201cnanochromosomes\u201d–are amplified to thousands of copies each. Each macronucleus chromosome typically contains a single gene flanked by very short telomeres. The size of these molecules ranges from 0.25 to 35 kb, and each is present at an average of 1000 copies. \u00a0[Genome Inst, Washington Univ St Louis<\/a>]<\/p>\n The ScienceBlog summary referred to a recent article by Laura Landweber\u2019s group<\/a> at Princeton:<\/p>\n Here, we report the Oxytricha<\/i> germline [micronucleus] genome and compare it to the somatic [macronucleus] genome to present a global view of its massive scale of rearrangement. \u00a0The remarkably encrypted genome architecture contains >3,500 scrambled genes, as well as >800 predicted germline-only genes expressed, and some posttranslationally modified, during genome rearrangements. \u00a0Gene segments for different somatic loci often interweave with each other. \u00a0Single gene segments can contribute to multiple, distinct somatic loci. Terminal precursor segments from neighboring somatic close to each other, often overlapping.<\/p>\n The message is that the genome evolves by assembling pieces of genes that had been found useful in different circumstances in the past, and that much of the genome is assembly instructions for piecing together parts from different locations to form a functional whole. \u00a0Many of the pieces are re-used.<\/p>\n This presages the fact for higher organisms that only 5% of our DNA is genes, and the rest is epigenetic instruction, dictating when and where each gene is to be expressed.<\/p>\n <\/p>\n Brief Updates<\/b><\/p>\n I did another 4-day fast this week. \u00a0It seemed easier than my first long fast last spring<\/a>. \u00a0There were no craving or headaches. \u00a0I\u2019m not able to run or to swim or do interval training while fasting, but I can bicycle and do yoga and walk for hours on end. \u00a0I bicycled more than 20 miles (to attend a conference) on my fourth day, and didn\u2019t feel drained. \u00a0I\u2019m not very productive during a fast, and my concentration is pretty diffuse. \u00a0Sleep is often interrupted by hunger. \u00a0But on the positive side, there\u2019s a calm and peace that comes over me, and it\u2019s easy to be content just being where I am.<\/p>\n Science Magazine had 2 feature articles [Ref1<\/a>, Ref2<\/a>] and a summary news article<\/a> on CRISPR dynamics<\/a> in E. coli<\/i> bacteria. \u00a0The articles describe how CRISPR RNA is able to locate stretches of foreign DNA, planted there by an invading virus, and excise it from the genome. \u00a0I still can\u2019t say that I understand how the cell knows what to look for.<\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":"![\"\"](\"http:\/\/faculty.clintoncc.suny.edu\/faculty\/michael.gregory\/files\/bio%20102\/Bio%20102%20Laboratory\/protists\/paramecium_multimicronulleatum_conjugating_X_200.JPG\")
![\"\"](\"http:\/\/genome.wustl.edu\/image\/2\/370\/370\/5\/images\/misc\/oxytricha_trifallax.png\")
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