The plague epidemics that swept Europe repeatedly took such a toll in human lives that variants of key immune system genes, which gave a survival advantage, spread in people living there.
Research just published in the Proceedings of the National Academy of Sciences uncovered evidence for such evolutionary selection by examining two ethnic groups in present-day Romania — those of European ancestry and the Rroma, also known as gypsies. The latter are the descendants of people who migrated from north-western India and settled in Europe about 1,000 years back.
A team of European scientists, along with colleagues in India, examined genetic variations, known as ‘single nucleotide polymorphisms’ (SNPs), in 100 individuals each of European ancestry and from the Rroma community.
These SNPs were then compared with the ones found in 500 Punjabis, who were taken as representing people from the geographic origin of the Rroma.
The scientists looked for genetic variations that were similar in those of European lineage and the Rroma and where both differed from the Punjabis.
This allowed them to identify places in the genome where the two Romanian ethnic groups had been subjected to common evolutionary pressures.
“This paper shows that two human populations with a different ethnic and genetic background show similar evolutionary pressures due to the environment in which they live,” observed Mihai G. Netea of the Radboud University Nijmegen Medical Center in The Netherlands, one of paper’s senior authors. Both “European and Rroma populations show positive selection in similar genes of the immune system,” he said in an email.
As the large plague epidemics in Europe had death rates of up to 30 per cent to 50 per cent, “it is rational to hypothesise that plague had major evolutionary effects on the immune system of European populations,” the paper noted.
The worst of these epidemics was the one known as the Black Death that devastated the region in the 14th century and which occurred after the Rroma had settled there.
The Indian subcontinent, on the other hand, had escaped the Black Death and the plague epidemics it suffered had far less impact.
The Europeans and Rroma showed positive evolutionary selection in SNPs associated with three genes relating to the immune system that cluster on the same chromosome. Those genes produce proteins — Toll-like receptors 1, 6 and 10 — that were involved in the immune defence against bacteria, including Yersinia pestis , the one that causes plague.
The scientists were also able to show that white blood cells with the three gene variants found in Europeans and Rroma gave a stronger immune response when exposed to Y. pestis.
The hypothesis was that these gene variants increased the survival of individuals who faced the plague, according to Prof. Netea.
It was not possible to study this as the research did not investigate patients with plague. “It is a plausible hypothesis, but no more than that,” he said in his email.
The analysis also showed that a cluster of four genes, including SLC45A2 that produced light skin pigmentation, was among those that had been evolutionarily selected in Europeans and Rroma.
Not everything was related to the plague and many other pressure factors may have been involved, remarked Prof. Netea.
“Studies of this nature help us identify genes, particularly in the immune pathways, that are under evolutionary pressure,” said B.K. Thelma of the University of Delhi’s Department of Genetics, who is one of the paper’s authors.
