What We Talk About When We Talk About Junk DNA

Junk DNA is a popular yet controversial concept that states that organisms carry in their genomes DNA that has no positive impact on their fitness. Nonetheless, biochemical functions have been identified for an increasing fraction of DNA elements traditionally seen as Junk DNA. These findings have been interpreted as fundamentally undermining the Junk DNA concept. Here, we reinforce previous arguments that this interpretation relies on an inadequate concept of biological function that does not consider the selected effect of a given genomic structure, which is central to the Junk DNA concept. Next, we suggest that another (though ignored) confounding factor is that the discussion about biological functions includes two different dimensions: a horizontal, ecological dimension that reflects how a given genomic element affects fitness in a specific time, and a vertical, temporal dimension that reflects how a given genomic element persisted along time. We suggest that Junk DNA should be used exclusively relative to the horizontal dimension, while for the vertical dimension, we propose a new term, Spam DNA, that reflects the fact that a given genomic element may persist in the genome even if not selected for on their origin. Importantly, these concepts are complementary. An element can be both Spam DNA and Junk DNA, and Spam DNA can also be recruited to perform evolved biological functions, as illustrated in processes of exaptation or constructive neutral evolution.

Automated summary

The concept of junk DNA, although undermined by the discovery of biochemical functions in traditionally considered junk DNA elements, is still valid. The authors highlight the inadequate concept of biological function and the need to consider two dimensions of discussion - horizontal and vertical - when discussing biological functions.