Male Y chromosomes evolving faster, reveals DNA analysis of humans and apes

A groundbreaking study has shed light on the dramatic evolution of the Y chromosome, also known as the male sex chromosome. Scientists analysed ape sex chromosomes and discovered that only a small fraction of the DNA sequences in the primates’ Y chromosome — 14-27 per cent — are relatively the same as their human counterparts, suggesting a much faster rate of its evolution.

Remarkably, the X chromosome, which is present in both males and females, has remained more stable. Over 90 per cent of the ape X chromosome sequences aligned [staying relatively the same over the course of evolution] to the human X chromosome, the study published in journal Nature noted. 

X and Y chromosomes play roles in sexual development and fertility, along with many other biological functions.  

“Some of these [ape] species diverged from the human lineage only seven million years ago, which is not a lot of time in terms of evolution. This shows that the Y chromosomes are evolving very fast,” Kateryna Makova, professor at Pennsylvania State University and one of the study’s authors, said in a statement.

Prior to this study, sex chromosomes in our closest relatives, non-human apes, were only partially characterised, despite the fact that human sex chromosomes had been completely sequenced.

So the team set out to sequence the sex chromosomes of six ape species and investigate their structure and evolution.

They included great apes — bonobo, chimpanzee, western lowland gorilla, Bornean orangutan and Sumatran orangutan and siamang, representing gibbons (lesser apes).

Bornean and Sumatran orangutans diverged from each other approximately 1 million years ago, while chimpanzee and bonobo diverged around 2.5 million years ago. The human lineage diverged from the different ape lineages roughly 7-20 million years ago.

The scientists also used a computational tool known as alignment to identify chromosomal regions that have remained relatively unchanged throughout evolution. This information could shed light on how different evolutionary pressures affect different parts of the genome.

The faster evolution of Y chromosomes could be explained by the fact that sperm production requires more DNA replication, which increases the likelihood of mutations occurring, as compared to egg production. 

The study also discovered that Y chromosomes between ape species varied more than X chromosomes. For example, the male chromosome of the Sumatran orangutan is twice as long as that of gibbons.

Further, about 71–85 per cent and 62–66 per cent of Y and X chromosome lengths, respectively, are made up of repetitive elements (sequences). 

One type of repeat is the palindrome, similar to language palindromes such as “racecar” or “kayak,” in which the letters in the first half of the word repeat in reverse in the second half of the word. This means the sequence of the letters is the same forwards and backwards. 

DNA is made up of four letters, representing four chemicals: Adenine, guanine, cytosine and thymine. The DNA palindromes can be over one hundred thousand letters long. The DNA palindromes on the primate X and Y chromosomes almost always contain genes, a unit of heredity passed from parent to offspring. 

The genes on the palindrome repeat in many copies along the length of the chromosome. This effectively acts as a backup copy because each cell hosts only one Y chromosome. If a gene on the Y chromosome is damaged, the copies could help repair the damage.

Studying sex chromosomes does not just help understand human evolution, it can also explain ape sex chromosome evolution, which could inform conservation efforts in non-human ape species, all of which are endangered.