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Rotifers and 60 million years without sex

The fourth story about fantastic creatures and what they can teach us

They live where crawls every licensed biologist. They put up our ideas about how to operate a living organism. They know what we can only dream of. And we? Jealous. Going with them in the valleys, and the depths of the quagmire. Spend the best years of his life and family heritage in search of their secrets. And suddenly find … rotifers tiny worms with incredible chastity. Like the Amazon for the millions of years they do without the males. How do they manage to overcome this temptation? And most importantly, should we follow their example?

Meet the rotifer

Meeting with rotifers often goes unnoticed for a person. Unless he is armed with a microscope: the average size of rotifers near polukilometre. Familiarity is complicated by the fact that rotifers are transparent and lives in the water – almost any fresh (especially rotten) and around the world. Rotifer easily changes its shape, pulling and sticking his head or leg, writing himself to the surface and breaking away from her, crawling and swimming. Therefore, a drop of water can see the microscopic Stingray, a mouse or glass. But that’s all it is, a rotifer.

The closest of the inhabitants of the school are worms, but they do not look like her. A unique feature of rotifers coloradodaily apparatus, which under the microscope resembles a large hairy ears. But it is not the ears and lips-lined with cilia. Cilia beating (from which the lips become similar to the chainsaw), creating a vortex that sucks in all the surrounding detail – bacteria, algae and single-celled animals. For the lips should actually torso, ending with a foot – muscular appendage, which can be attached and crawl. That’s all animal.

Rotifer pulls the lips to food. Source: Don Williams / youtube

Apart from the fact that rotifers are surprisingly voracious and sometimes absorb large ciliates, they are also incredibly tenacious. Fresh water – the environment is unstable and periodically drying up. But rotifers endure drying, falling into suspended animation, and after the moisture back to life. But their superpowers are not over. Among them is the group bdelloidea rotifers (from the ancient Greek. Βδελλα – “leech”), which consists entirely of females. And males and, therefore, the sexual process of them, apparently, was not long ago. At least 60 million years.

To know whether to envy or sympathize with bdelloid Rotatoria, let’s try to answer one of the key questions of biology: why do we need sexual reproduction? C point of view (as, for example, writes in detail, Richard Dawkins in “the ancestor’s Tale”), is the most disadvantageous method of reproduction possible. Imagine: you are going to pass on their genes to the child. You divide your set of chromosomes in half, and then half thrown away. Why such extravagance? (Not to mention the other temptations and difficulties associated with sexual reproduction.)

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It’s all in the diversity. Imagine life as an endless examination in mathematics. Passed survive. In this case, the genome is like a cheat sheet for the exam, and each gene is the answer to one of the tickets. Convenient to have two Cribs from the father and the mother, in the case of one of the parents Learn more earnestly the other. However, it is known that it is finally the correct answer to all questions does not exist. The examiner is resourceful and cunning: you never know what will come to him. So use from time to time, the same, crib, unprofitable. For this, there is chromosomal crossover (or homologous recombination): take the cheat sheet from my mother, put it next to her crib father and shuffled them, that is, change the answers to the same question the mom’s text to my dad’s. We have again two sets of cheat sheets with answers to the same questions, but all mixed up. And the children inherit a hodgepodge of the genes of their parents. On the texts of the tickets (chromosomes) are randomly distributed to the germ cells, which will meet with the same “mixed” chromosomes of the partner. As a result of variability of species and the diversity of genetic material. It increases the chance that the offspring will take the best from their grandparents and be able to withstand new environmental conditions.

Picture: Anatoly Lapushka / Chrdk.

Now imagine animals that have sex process. They are the students who take examinations in the abstracts of the last century. Yes, they are stable and time tested. But what would happen if a student of the day was asks about the theory of relativity, while in his picture of the world does not exist even the law of gravity? Epic fail, expulsion, death. Therefore, the species without sexual reproduction, very little. In addition to the rotifers, this was only isolated armored pincers and crayfish ostracod. Where do they take genetic diversity? In other words, one write off?

Is there life without sex

You can live without a full-fledged sexual reproduction if you’re “only a Very Little Creature,” for example, one-celled. Then you are the boss, and it is possible to share information with others. For example, do bacteria: throw out copies of their DNA molecules and absorb other people from outside. Call it horizontal gene transfer. Single-celled animals can share with each other whole sets of chromosomes (as does a Paramecium). But if you’re multicellular, then to do this scheme just will not work. By inheritance you will pass on only the genes of reproductive cells and, therefore, need not only to eat someone’s DNA, but somehow they implement. The problem lies in the fact that the nuclei of multicellular animals are protected from obtaining an alien DNA (and then suddenly it’s a virus) – digestive system, cell membranes and intracellular mechanisms that breaks down excess DNA. That is why horizontal gene transfer in multicellular much less likely, and for a long time it was believed that and it does not. But then came the bioinformatics methods, and found that everything is more complex.

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Search results of the horizontal transfer of genes in modern genomes is the same to the problem that regularly confronts every teacher: how to determine who copied who, and catch bouncer? For this bioinformatics to compare genomes of closely related species and looking for differences. For example, if we compare the genomes of mouse and rat and see that the latter has a completely new gene that has no analogues on the mouse, we can assume that it appeared from the outside. Then we are looking for similar genes in other organisms, this time far-bacteria, fungi, plants, etc. And if you can find similarities, our “anti-plagiarism” indicates that specialist caught.

Judging by the comparison of the gene pathways, about 60 million years ago. This is evidenced not only analysis of their genomes, but also the unusual structure of chromosomes. Most of the animals carry in the cell a double set of chromosomes, with each pair (homologous chromosomes) contain the same genes. In the process of formation of gametes homologous chromosomes, each other (conjugium), connect their “identical” sites and exchange parts, as we have mentioned above. But the chromosomes of rotifers over millions of years have lost homology. Now their content is so different from each other that they can not change the information. Therefore, to form sex cells are unable to rot, and sexual reproduction is impossible for them, even if they decided to resort to it.

So, in the world of rotifers, no sex. But there are alternatives. Have someone else’s genes. According to some reports, up to 14% of the genome rotifers are arrogantly written off by other organisms (for comparison: in most animals is less than 1%). There is a suspicion that the rotifers helps the regular drying. Since DNA consists of two chains, stuck together, integrate into a foreign matter. When the mismatch repair system is trying to repair such a break, they are looking for any and all DNA that is comfortable to grip. Similar to what happens with broken “spur” on which is recorded a response: student is frantically looking for anyone. Dehydration in DNA can arise such double breaks. And indeed, those species that are often experiencing drought, “written off” of genes found more. Probably, an important role is played here by the dangerous proximity of the ovary to the intestinal path from the food to the offspring is short.

However, the problem is that you can not embed them in your abstract. If it is differently numbered or it is written in a different hand (or worse in another language), it will be uncomfortable. This prevents the active retired in unrelated organisms – their genes are surrounded by regulatory sequences of unusual, and sometimes even different features of the genetic code (e.g., the frequency of occurrence of certain nucleotides). They are more like notes, borrowed from the students of another faculty. To they began to work, leaving a generation of the rewriting. Therefore, we can distinguish genes, “written off” a long time ago, from the recent acquisitions. The longer gene lives in the genome of the host, the closer it is to the active regions of the nucleus (farther from the ends of chromosomes) and the more structure of the host.

Foreign genes are often very useful. For example, bacteria, fungi and plants has been taught rotifers to better digest food, neutralize toxins and free radicals. But to tame foreign genes is difficult and not always possible, so at least some of them survived. And it is quite difficult to replace the full sexual process. But a couple of years ago, rotifers caught cheating each other: in their genome showed signs of horizontal transfer from individuals of their own and other species. So far, however, it is unclear how exactly genes of rotifers from one fall to another. But now we can not with the former categorically to assert that the rotifers, no sex at all. It is, but alternative.

Picture: Anatoly Lapushka / Chrdk.

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And we can do that?

Whatever was said in the myths, people have little chance of becoming Amazons. Like other mammals, we can not reproduce without a partner. On those rare occasions when it is in one way or another, and the egg begins to divide itself, in the ovary develop teratomas – tumors large and intimidating. As for horizontal gene transfer, it just does not pass us by. Some parts of our DNA, as it is believed, also came from bacteria and single-celled animals, for example the genes that determine blood group. However it happened, most likely, in the early stages of vertebrate evolution, when man as a separate species was far away. Horizontal transfer may occur in individual cells and now – for example, when stomach cancer or leukemia occurs in the cells. Maybe that’s somehow contributes to their survival in the harsh pressure of the immune system. But by inheritance to pass this alien DNA, most likely, will not work. Our sex cells are carefully protected from external influences, and the likelihood of the incorporation of DNA from outside. But it is likely that we will learn from the genome editing of embryos and learn how to manipulate human genes consciously. And then rotifers will be jealous of us.

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