The beak – a living fossil

All freshwater bodies combined represent a small area compared to that of the sea or land. As a result, competition among freshwater inhabitants will be less severe than elsewhere. New forms will form more slowly, while old forms will die off more slowly. That’s why it’s in fresh water that we find seven types of ganoid fish, remnants of a once dominant order. In fresh water in general, some of the most anomalous forms known in the world today can be found, such as [dziobak] Ornithorhynchus and [prapłaziec] Lepidosiren, which, like the fossils, unite to some extent the orders now widely separated on the natural scale [on the ladder of being, in systematics].. Te anomalne formy można niemal nazwać żywymi skamieniałościami – przetrwały do dnia dzisiejszego, zamieszkując ograniczony obszar i będąc w ten sposób wystawione na mniej dotkliwą konkurencję . (Karol Darwin, O pochodzeniu gatunków).

Already Charles Darwin called the beak a living fossil, pointing to its nature as an intermediate link between two clusters: reptiles and mammals. Nowadays, thanks to advances in paleontology and phylogeography, we have come to know many more such peculiarities. We admit the nature of living fossils to such common fossils as platypuses, cockroaches and horsetails. On the other hand, some scientists reject the very concept of living f ossils and relict species in general. Regardless of which of these approaches triumphs in research and didactics, steak biology in the broadest sense will remain an inexhaustible source of examples and puzzles for future generations of researchers. Although we don’t know how to breed them in a zoo, we have had cell lines of these marvels for more than 40 years [3, 7].

Once slowly

Platypus genes have been studied for decades. Already the initial version of the genome sequencing was a sensation worthy of publication in Nature (May 8, 2008). The attention of scientists and readers was captured by typically reptile and typically mammalian elements. Two genes previously characteristic only of fish, amphibians and birds as typically oviparous animals were also identified. The genome as a whole turned out to be small. It counts a mere 2.3 billion base pairs, forming just over 18,500 protein-coding genes. The number of genes thus turned out to be fairly typical for mammals. Little! More than _4/5 of our protagonist’s genome was consistent with the preliminarily sequenced genomes of typical placentals and marsupials.

For 15,312 of the 18,527 functional genes mentioned earlier, orthologs (in the big picture: 1:1 equivalents) were immediately found for 5 other mammals. All of these pecking orthologs encode metabolism, DNA replication or splicing. These activities take place the same way in all oviparous mammals, so their genes evolve rather slowly [1, 3, 5, 7].

Once quickly

The remaining genes (without orthologs) change more rapidly. Nearly 2,000 of them encode olfactory receptor proteins. Just as the beak phenotype lacks a uterus and true nipples, so the genotype lacks many genes typical of all viviparous mammals. Among them, as indispensable as they may seem, such as those encoding vitellogenin II, cytochrome P450, glutamine synthetase and melatonin receptor 1C. Scientists have been intrigued for decades by the mechanisms of sex determination in stevedores. As is well known, sex in birds is determined at the genetic level inversely than in mammals. Females have ZW genes, males ZZ, while female placentals have XX and males have XY.

In today’s reptiles, a considerable variety of genetic mechanisms responsible for sex have been preserved, since the sex of certain crocodiles and turtles depends on the heat of the environment. In the case of the beak, despite its oviparity, beak and ravenous bones, a typically mammalian determination system with XY males and XX females was expected. Meanwhile, the platypus shocked again. In the field of genetic foundations of sex, it turned out to be similar to reptiles, yet unlike the rest of the insectivores. Not even to the as closely related spiny dogfish with X0 sex determination system [1, 2, 3, 6].

The beak, however, more reptilian than thought

After decades of searching, researchers have finally accepted that male beaks do not have a Y chromosome or the SRY gene, crucial for male sexual characteristics (even a human with two XX, but active SRY phenotypically and psychologically is considered male). Instead, some homologs of genes typical of birds and reptiles have been discovered. It’s been 20 years since scientists from the Australian National University observed a whole 10 sex chromosomes in the beak instead of the 2 characteristic of placentals. Mr. Beak thus has a whole chain of XYXYXYXYXY heterosomes, instead of the modest XY like humans.

The use of the X and Y symbols to describe the beak genome is controversial, since the X chromosome of platypus bears a confusing resemblance to the typically avian Z, rather than the X heterosomes typical of the bagworms or placentals. Could it be homologous to the chromosomes of reptiles and birds? Despite strenuous research, no human X orthologs have been found. The genetic underpinnings of sex determination in the steak beaks turned out to be much more archaic and similar to zauropsids than had been supposed. The sex determination of the beak is as bird-like as its eggs or beak. On the other hand, the sex determination system in viviparous animals (bagworms and placentals), based on X and Y heterosomes, turned out to be a much more recent evolutionary invention than previously thought.

It must have emerged only after the evolutionary lineage of steatids and viviparous mammals separated. Platypus spermatozoa contain either the entire 5Y set or the 5X set. During meiosis, the heterosomes of the steakpods form something like a chain. In the absence of the SRY gene, it is not very clear how the 5Y set determines technically (or not?) the male sex in the beak [4, 6].

But balls!

The genes, and consequently the proteins they encode, responsible for the fertilization of the egg and the earliest divisions of the embryo, turned out – as expected this time – to be something between a typically reptilian and typically mammalian set. KIR (killer cell immunoglobulin-like receptor) paralogs alone were counted in the beak 214 (against less than 20 in humans). The number of 15 given in the Polish Wikipedia for humans needs to be increased by at least 1 (14 active plus 2 pseudogenes).

In Homo sapiens, it is the KIRs that are responsible for much of the fertility problems, including so-called immune infertility, habitual miscarriages or implantation problems after IVF. Such a large number of perhaps functional KIRs in platypus is due to the complex reproductive biology combining the typically reptilian stage of development in the egg with the almost mammalian (though still similar to mechanisms in oviparous reptiles) stage of feeding the embryo through the egg walls in the female’s reproductive tract [2, 3, 6, 7].

Small is beautiful

The beak also proved peculiar in the area of non-coding RNA proteins. For these were observed in it less than in the rest of mammals, 1220 except for small nuclear RNAs. The latter, conversely, platypus has some 2,000 copies, so ten times more than typical mammals. The general differences in the size of the 52 chromosomes of platypus bring to mind associations with the micro- and macro-chromosomes of typical reptiles. The genome of our hero is dominated by finer microchromosomes. Here, however, the similarities with birds end, for the genes located on them do not overlap with homologous chicken genes [3, 4, 5, 7].

A little one can do more (spoil?)

There is a veritable rash of short, scattered nuclear retrotransposons lacking long, terminal (SINE)-like repeats, numbering about 40,000. SINE retrotransposons travel through genomes by retrotransposition, forming short repeats of the genetic code at the integration site. Scientists are still discussing the possible functions of these stretches. However, they agree on their significant impact on the functioning of entire genomes. In humans, links have been seen between their presence and certain cancers.

SINEs are not bad for markers in phylogenetic analyses because of the marked differences between species. On the other hand, the beak’s piRNA, an RNA associated with the Piwi protein, responsible for methylation and DNA silencing, turned out to be remarkably similar to the RNAs of the placentophagous and the placental mammals. In platypus, it is characterized by a higher number of repeats and transposon-protective structures than in viviparous mammals [4, 5, 7].

Art for art’s sake? Or art for the sake of art?

Because of the early divergence of stevedores and viviparous mammals and the small number of surviving species of the former group, the beak has become a frequent object of evolutionary biology research. But isn’t chiseling away at its genome and proteome a kind of art for art‘s sake? Finding answers to questions important only to a group of harmless weirdos? Wasting time and resources that should rather be spent on fighting cancer and aging? Or protecting against biological weapons?

The scientists, appealing for further research funding, also cite the potential for practical applications of the results of their work on the beak for all of humanity. Among other things, they promise a better understanding and, as a result, more effective treatment of autoimmune diseases and more efficient production of vaccines for humans, livestock and game [3, 4].

In the article, I used, among other things. z:

1. Baldwin, J., Temple-Smith, P.D. 1973 Distribution of LDHX in mammals: presence in marsupials and absence in the monotremes platypus and echidna. Comp. Biochem. Physiol. 46B: 805-811.
2. Bick, Y., Jackson, W. 1967 A mammalian X-O sex chromosome system in the monotreme Tachyglossus aculeatus determined from leucocyte cultures and testicular preparations. Am. Nat. 101, 79-86.
3. O’Brien, S. 2004 The Platypus Genome Unraveled. Cell 133 (6), 953-955.
4. Grützner, F., Deakin J., Rens, W., El-Mogharbel, N., Graves, J.A.M. The monotreme genome: a patchwork of reptile, mammal and unique features? Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 136(4), 867-881.
5. Kirsch, J.A., Mayer, G.C., 1998 The platypus is not a rodent: DNA hybridization, amniote phylogeny and the palimpsest theory. Phil. Trans. R. Soc. B 353, 1221-1237.
6. Watson, J.M., Frost, C., Spencer, J.A., Graves, J.A.M. 1993. Sequences homologous to the human X-and Y-borne zinc finger protein genes (ZFX/Y) are autosomal in monotreme mammals. Genomics 15: 317-322.
7. Wesley, C., Warren, LaDeana W.,, Graves, J., Ewan Birney, E., Ponting, C., Grützner, F, Belov, K., Miller W., Clarke L.. 2008. genome analysis of the platypus reveals unique signatures of evolution. Nature 453 (7192), pp. 175-183.