For most organisms, genetic inheritance explains why offsprings are similar to their parents. However, there remains an exception to the rule — and two teams have investigated a case where a biological trait was transferred not through genes but a memory mechanism.
Two research groups from the University of Geneva (UNIGE) in Switzerland are investigating locations of centromeres — specialized DNA sequences in a chromosome that plays an important part in cell division. They discovered that in the worm species Caenorhabiditis elegans — a free-living, transparent nematode only 1mm long on average — the location of the centromere is transmitted not by genetic inheritance but by something researchers described as an "epigenetic memory mechanism."
Their findings to this exception appear in the latest PLOS Biology, in a report titled "Transgenerational inheritance of centromere identity requires the CENP-A N-terminal tail in the C. elegans maternal germ line."
Understanding the Mechanisms of Biological Trait Transmission
The passing of a biological trait occurs in living organisms — from the largest mammals to the smallest microbes — and includes physical and even behavioral traits passed down from parents to offspring. This process is known as genetic inheritance, and The Cell: A Molecular Approach (2nd Edition) explains it as the passing of genetic information that specifies structure and function from a parent organism. This is usually mediated by the DNA, more specifically the replication at each of the cell division processes that copies the gene containing a specific makeup or predisposition.
However, there are instances where a biological trait is transmitted to an offspring without going through the processes of genetic inheritance, and these are called epigenetic phenomena. This particular case was observed by the UNIGE researchers examining the physical separation of chromosomes into two daughter cells as cell division occurs. One team, led by professor Florian Steiner from the Department of Molecular Biology, who also serves as the last author of the study, looks into centromeres in the nematode C. elegans. They noted how certain chromosomal structures act as "anchors" for the memory mechanism that leads the correct distribution of chromosomes to the daughter cells. More importantly, a failure in this distribution could lead to the death of the cell and has been observed in the development of cancerous cells.
"Studying these processes is greatly facilitated in C. elegans, since this small worm is transparent and allows live observation of cell divisions and the fate of chromosomes from one generation to the next," explains Reinier Prosée, first author of the study and a researcher from the Department of Molecular Biology, in a news article from the University of Geneva.
Memories Help Determine Physiological Development
The other group, led by UNIGE Faculty of Medicine professor Monica Gotta, is also analyzing the protein that defines the placement of the centromeres on the C. elegans chromosome together with Steiner's team. Their collaborative effort led to the discovery of how the protein navigates to the right location — thanks to a specific region that "guides" the protein.
Researchers then created a mutation in the nematode in which the DNA does not have the necessary codes to "guide" the protein. Steiner explains that they initially predicted the mutant would not be viable since the position of the centromeres could not be pinpointed since the DNA region was removed. However, the protein still managed to reposition itself and remain functional. It led the researchers to discover that once the centromeres are correctly defined from the parent, it is passed down to the offspring thanks to the cells "remembering" this particular configuration, even without the DNA codes that supposedly guide the chromosomal structures.
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