How to repress your past
Embryos of the moss-like plant Marchantia polymorpha repress all the genes originating from the sperm while they develop.
The reproductive cells of organisms that reproduce sexually — the egg and the sperm — each contain one copy of the organism’s genome. An embryo forms upon fertilization of an egg by a sperm cell. This embryo contains two copies of the genome, one from each parent. Under most circumstances, it does not matter which parent a gene copy came from: both gene copies are expressed. However, in some species genes coming from only one of the parents are switched on. This unusual mode of gene expression is called genomic imprinting. The best-known example of this occurs in female mammals, which repress the genes on the paternal X chromosome. Genomic imprinting also exists in flowering plants.
Both mammals and flowering plants evolved tissues that channel nutrients from the mother to the embryo during development; the placenta and the endosperm, respectively. Genomic imprinting had, until now, only been described in these two types of organisms. It was unknown whether imprinting also happens in other organisms, and specifically those in which embryos develop inside the mother but without the help of a placenta or endosperm.
Here Montgomery et al. addressed this question by studying the liverwort, Marchantia polymorpha, a moss-like plant. Initial experiments showed that cells in the liverwort embryo mostly expressed the genes coming from the egg, and not the sperm. All the genetic material coming from the sperm had a molecular marker or tag called H3K27me3. This mark, which also appears on the paternal X chromosome in female mammals, switches off the genes it tags. M. polymorpha embryos thus suppress gene expression from all of the genetic material from the father, relying only on maternal genetic material for development. When Montgomery et al. deleted the maternal genes necessary for making the H3K27me3 mark, the paternal genes switched on, and this led to the death of the embryos. The survival of M. polymorpha embryos therefore depended on keeping only one set of genes active.
Taken together these experiments indicate that genomic imprinting evolved about 480 million years ago, about 320 million years earlier than previously thought, in organisms for which embryo development depended only on one parent. This means there are likely many more organisms that control gene expression in this way, opening up opportunities for further research. Understanding imprinting in more detail will also shed light on how sexual reproduction evolved.
Originally published at https://elifesciences.org/digests/79258.
