A new model developed at Tel Aviv University offers a possible solution to the scientific question of why neutral sequences, sometimes referred to as “junk DNA”, are not eliminated from the genome of living creatures in nature and continue to exist within it even millions of years later.

According to the researchers, the explanation is that junk DNA is often located in the vicinity of functional DNA. Deletion events around the borders between junk and functional DNA are likely to damage the functional regions and so evolution rejects them. The model contributes to the understanding of the huge variety of genome sizes observed in nature.

Border Induced Selection

The model describes a phenomenon which the team of researchers refer to as “border induced selection,” and was developed under the leadership of PhD student Gil Loewenthal in the laboratory of Prof. Tal Pupko from the Shmunis School of Biomedicine and Cancer Research at the The George S. Wise Faculty of Life Sciences and in collaboration with Prof. Itay Mayrose, also from TAU’s Faculty of Life Sciences. The study was published in the journal Open Biology.

Throughout evolution, the size of the genome in living creatures in nature changes. For example, some salamander species have a genome ten times larger than the human genome. “The rate of deletions and short insertions, dubbed ‘indels’, is usually measured by examining pseudogenes,” explains Prof. Pupko. “Pseudogenes are genes that have lost their function, and in which there are frequent mutations, including deletions and insertions of DNA segments.”

In previous studies that characterized the indels, it was found that the rate of deletions is greater than the rate of additions in a variety of creatures including bacteria, insects, and even mammals such as humans.

Prof. Tal Pupko

Reverse Bias for Short Segments

The question the researchers sought to answer is how the genomes are not deleted when the probability of DNA deletion events is significantly greater than DNA addition events: “We have provided a different view to the dynamics of evolution at the DNA level,” says Loewenthal. “When measuring the rate of indels there will be more deletions, but the measurements are carried out in pseudogenes which are quite long sequences. We assert that in shorter neutral segments, deletions would likely remove adjacent functional segments which are essential for the functioning of the organism, and they will therefore be rejected [through ‘border-induced selection’]. Accordingly, we assert that when the segment is short, there will be a reverse bias – there will be more insertions than deletions – and therefore short neutral segments are usually retained.”

“In our study, we simulated the dynamics of indels, while taking into account the effect of ‘border-induced selection,’ and compared the simulation results to the distribution of human intron lengths (introns are DNA segments in the middle of a protein-coding gene, which themselves do not code for a protein). A good match was obtained between the results of the simulations and the distribution of lengths observed in nature, and we were able to explain peculiar phenomena in the length distribution of introns, such as the large variation in intron lengths, as well as the complex shape of the distribution which does not look like a standard bell curve.”