## The small but mighty nematode is an excellent model

At the end of the 1960s, the South African biologist Sydney Brenner (1927 – 2019) decided to leave the field of molecular biology, with an explanation that everything essential in it had already been discovered. He decided to find an animal that would help him answer important questions in developmental biology: what determines cell differentiation during ontogenesis.

From all the candidates, a small (approx. 1 mm) self-fertilising worm Caenorhabditis elegans, met all the requirements for a model organism (Table 20.1), including simple breeding (in Petri dishes with agar and bacteria that are food for the worms). C. elegans has a short life cycle (~21 days at 20oC), starting with embryogenesis inside the egg. The nematode hatches as a 1st stage larvae (L1) and proceeds through 4 periods of growth (L1-4), each separated by a molt – where the cuticle (skin) of the worm is shed, much like a snake (Figure 20.6). Upon reaching adulthood, each worm will produce around 300 genetically identical progeny over a 4 day period. After reproduction, the worms start to age, in a similar manner as humans. An interesting phenomenon of the life cycle of C. elegans is that, in the case of starvation during the 2nd larval stage (L2), the worm can exit the trajectory leading to an adult individual and reach the stage of so-called dauer larva, where the worm will undergo a significant slowdown in metabolism and various other physiological and anatomical changes (e.g., closing of the mouth opening). This allows its survival for several months instead of 2 weeks. Genetic analyses of this developmental "bypass" identified several genes that control entry (and exit) from the dauer larva stage. It has been shown that changes in these genes in adults can lead to a 3-fold extension of their life. Interestingly, these genes code for proteins that participate in the regulation of metabolism in human cells and belong to the pathways that are regulated by the hormone insulin. The study of such an exotic developmental stage as the dauer larva in C. elegans has significantly contributed to our understanding of the molecular mechanisms of aging, even in mammals like us.

In addition to the advantages mentioned above, C. elegans provides a few more important benefits: First, we know the exact pedigree of every cell that makes the worm. Together with colleagues, Robert Horvitz and John Sulston, mapped the fate of each cell and created a cellular family tree, which is commonly called a lineage. The fact that each individual worm has the same cell pedigree (we call this eutely) makes it possible to identify genes involved in cell fate determination in C. elegans bodies. The importance of the study of C. elegans for human medicine was shown, for example, in the study of genetic control of cell death. Of the 1090 cells that are created during ontogenesis, exactly 131 die in a process known as apoptosis, so that the adult hermaphrodite worm has 959 cells. By identifying mutants in which this type of programmed cell death does not occur, it was possible to reveal the genes whose products are involved in the implementation of apoptosis. Some genes activate apoptosis (inactivation of such a gene led to an excess number of cells in an adult), while others inhibit it (such a mutant would have fewer cells). One of the genes discovered in this way was ced-9(the abbreviation ced is derived from cell death), which encodes an inhibitor of apoptosis. Human cells have been shown to contain a gene encoding a similar protein called Bcl-2. In the case of some oncological diseases (e.g., B-cell lymphomas), this protein is produced in high concentration, which leads to the suppression of apoptosis even under conditions that normal cells evaluate as dangerous. Due to the overproduction of the apoptosis inhibitor Bcl-2, tumour cells lose this ability and continue to divide instead of dying. Similar to the case of ced mutants, genes were also found in C. elegans that regulate the formation of specialised cell types, including 302 neurons. The defined (and low) number of neurons makes C. elegans an excellent object for the study of neurogenesis (formation of the nervous system), because all connections are mapped in detail and it is thus possible to study their role in the relevant biological process (e.g., movement, excitability, responses to odour signals).

The second benefit of C. elegans is that it has had its genome completely sequenced so that we now know what every gene is and where it is in the genome. In fact, this humble 1mm long worm was the first multicellular eukaryotic organism to have its genome sequenced. Containing 100 million bases of DNA, the genome has around 20,500 protein-coding genes arranged across 6 chromosomes (5 autosomes and a sex chromosome).

Taken together, C. elegans is an extremely valuable model organism with many thousands of researchers working with it on various topics. Research with C. elegans continues to be an important source for new discoveries, as shown by the many Nobel Prizes awarded to people working with worms!