Scientists at the Francis Crick Institute have recently made groundbreaking discoveries that revealed incredible insights into the embryonic development of opossums. These results stand in stark opposition to the embryonic development observed in eutherian mammals. A recent study published in the journal Nature just uncovered something surprising. Unlike their eutherian counterparts, opossum embryos do not experience a full-scale DNA methylation reprogramming wipe event. This research is the first to explore these epigenetic changes in opossum embryos. It helps us understand some profound concepts behind their special evolutionary processes.
Opossums divergenced from the other eutherian mammals about 160 million years ago, resulting in some very special and significant evolutionary differences. The patterns of DNA methylation across opossum eggs, sperm, and embryos were mapped by the researchers. What they saw in opossum eggs and sperm was a much greater degree of methylation similarity. This is in sharp contrast to the levels seen in eutherians. This similarity is especially interesting given what we know to be the evolutionary mechanisms that separate these groups.
Wazeer Varsally, the study’s co-first author and a Principal Laboratory Research Scientist at the Crick, explained this important finding. He went on to demonstrate that deleting methylation in the placenta allows transposons to become expressed in opossum embryos. Transposons, often called “jumping genes,” are key players in genetics. They assist in changing both the timing and location of gene expression within the host organism. Further, this finding highlights the importance of these genetic elements in the developmental biology of marsupials.
James Turner, left, Principal Group Leader of the Sex Chromosome Biology Laboratory and senior author of the study. He highlighted that opossums develop at a slower rate and undergo a series of developmental stages in comparison to eutherians. This decelerated development could be due to their unusual reproductive strategy which is different from placental mammals. Together, these findings propose that early development represents a wave of active DNA methylation erasure. That’s possible, but this evolutionary process could have developed to improve this embryo’s placenta-making abilities — in itself a major evolutionary innovation.
This research has deep implications. It unveils exciting new clues to the striking differences in mammalian embryo development between species. This research further helps to elucidate the biology of this unique group of mammals, the marsupials. It feeds into bigger questions on the evolution of mammals and reproductive strategies.