Rolf Ohlsson

Rolf Ohlsson

Rolf Ohlsson obtained his PhD from the University of Lund, Sweden. As a post-doctoral researcher he studied gene expression, RNA processing and post-translational processing. He pioneered epigenetic research as a professor at the Karolinska Institute, Stockholm, Sweden, more than 20 years ago. An exploration of the epigenetic dimension of evolution was made possible by a move to the Evolution Biology Centre, Uppsala University. During this time he discovered genomic imprinting to demonstrate the evolutionary conservation of an epigenetic phenomenon. He also identified a new gene that acquired expression patterns during early mammalian radiation overlapping with the evolution of the imprinting phenomenon. The physical interactions between chromosomes to influence their epigenetic states have more recently been documented to be evolutionarily conserved providing a potentially new perspective of mechanisms underlying evolution of chromosomes and their functions. Rolf Ohlsson has recently moved back to the Karolinska Institute as a strategic professor to investigate links between epigenetic states, chromatin conformation and nuclear structure in association with DNA replication and transcription in development, evolution and disease.

The epigenetic perspective of evolution:

The imprinted connection

For example, one of the variants of Linnea vulgaris discovered by Linné reflects a so-called epimutation at the Lcyc locus that has been stable for several hundreds of years, at least. The impact of epigenetics on evolution is, however, best visualised by the phenomenon of genomic imprinting, which states that one of the parental alleles is inactivated in a parent of origin-specific manner.

This state of hemizygosity must provide advantages that trump the danger of having only one allele in an active state. Indeed, imprinting has been strongly associated with reproductive strategies and behaviour. Thus, imprinted genes fall into the categories of promoting growth only when they are expressed monoallelically from the paternal allele, and antagonizing growth when expressed only from the maternal allele.

Moreover, genes controlling neocortex development requires maternal inheritance, whereas genes promoting cognitive traits are expressed only from the paternal alleles. Using one imprinted domain as bait, we have been able to identify a network of imprinted domains that physically interact with each other irrespective of whether they are from the same or different chromosomes. This network of physically interacting chromosomes is conserved from mouse to man and depends on one single imprinted domain (Igf2/H19) that is the only known cluster of imprinted genes that can be found in marsupials.

We propose that the imprinted phenomenon reflects early evolutionary events that were initiated from the Igf2/H19 domain to spread throughout the mammalian genome in an ancestor common to marsupials and eutherian mammals.