Humans are more complicated than threadworms

How complex an organism is can hardly be determined by the number of its genes – researchers propose the interactome as a new yardstick for this

What really makes an organism? "It’s in the genes", it was said for a long time – not least the Human Genome Project in the 90s of the last century gave rise to the hope of deciphering all the little aches and pains in the human genome, the functioning or non-functioning of the human body, not least cancer, and finally achieving eternal life.

In the meantime, science has become disillusioned. The genome is obviously not the map to the fountain of youth that had been hoped for. The sequence of base pairs in the DNA also says little about the complexity of an organism: Otherwise, the common cabbage, with about 100.000 genes, humans are among the most complex creatures on earth, with 25,000 genes.000 genes in the midfield, somewhere between threadworm (19.000) and thale cress (25.500). Of course, we can’t take this lying down, so researchers have been looking for other criteria for a long time, at least partly in the hope of bringing the human species closer to the crown of evolution.

Humans are more complicated than threadworms

To do this, they first looked at how many of these genes are actually transcribed into RNA at any given time in a cell – the so-called transcriptome. The transcriptome is far more dynamic than the genome; it is subject to environmental influences and hormonal regulation and thus also reflects the state of a cell. Humans and chimpanzees, for example, share 98.7 percent of their genetic material – and yet differ greatly in terms of which gene is copied from the total supply at which point in time. How this process takes place has apparently changed more rapidly in the course of evolution than the gene pool itself.

However, the transcriptome does not yet reflect the complexity of life. It is possible that a single coding gene sequence can give rise to completely different proteins on its way through RNA and mRNA – the effect is known as splicing. The next step after the transcriptome is therefore the proteome – the totality of all proteins in an organism. If the transcriptome is already dependent on environmental influences, this is even more true for the proteome. But the total number of available proteins (in humans about half a million to a million) says something about how complicated an organism is structured?

The legitimate argument against this is that it is more a matter of how these two interact. Science calls the totality of possible protein interactions the interactome – and has thus found a new buzzword and a new beacon of hope. How proteins work together and against each other is something biologists are currently eagerly exploring, in the hope that understanding these processes will also open the way to understanding many diseases. However, here the research is still relatively at the beginning, because it is complicated to bring possible and actual combinations in relation to each other. In an article in the publications of the U.S. Academy of Sciences, a team of European researchers now demonstrates a method for estimating the coarseness of the interactome.

For this purpose, the scientists use methods of network statistics. This enables them to draw conclusions about the overall size of the networks under consideration (interactomes) from existing but not completely recorded interaction networks. These conclusions are of course dependent on the previous "measurement" results of interactome research. However, they already yield interesting figures: The human interactome is about three times coarser than the nematode (although the coarseness of the proteome is comparable) and a whole order of coarseness more powerful than the fruit fly, which in turn should be characterized by twice as many protein interactions as baker’s yeast. The human hour with it with 650.000 interactions at the top. However, it still has only a very sparsely populated network, because this number corresponds to just 0.2 percent of the theoretically possible interactions. The authors of the study provide a small consolation: in other species, the network looks even darker.

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