Sea lampreys are parasites native to the northern and western Atlantic Oceans that suck blood and other vital fluids from their fellow fish. They have the distinction of being possibly the first destructive invasive species in North America; they entered the Great Lakes through the Welland Canal in the 1830s and have been killing trout there ever since.
They also have the distinction of splitting off from the rest of the vertebrate lineage very early, about 550 million years ago, before the evolution of the jaws. This makes lampreys useful as a model organism to shed light on the evolution of various vertebrate traits. But our studies have also revealed some strange features of lampreys, including the fact that they lose hundreds of genes early in development.
Sea lampreys have several traits that other (jawed) vertebrates do not, suggesting that these traits were either (a) present in our last shared ancestor and lost by us, or (b) arose since lampreys diverged from the lineage of vertebrate jaws. One of these features is a real quirk: programmed genome rearrangement. During this process, sea lampreys shed about 20 percent of their genome during embryonic development. A few cells do not undergo this process and they pass on the otherwise missing DNA to another generation.
When this genome rearrangement was discovered in 2009, scientists speculated that the deleted genetic sequences must be important to germ cells and/or early in embryonic development, but harmful to more mature cells. But sea lampreys have a highly repetitive genome, consisting of 99 chromosome pairs in the germline, so sequence data confirming this speculation has been difficult to generate. The post-deletion genome was completed in 2013, but the full germline genome took until this week to complete.
Comparing the two genomes revealed that hundreds of genes are eliminated in programmed genome rearrangement. The human versions of these genes are involved in processes such as cell division, migration and adhesion, as well as dedication to a specialized state. These are biological functions that are in fact vital during embryogenesis, but are very problematic for somatic cells, where they can promote aggressive cancers.
In addition, the mouse versions of these genes are turned off in embryonic stem cells and are presumably later released from this silencing when these cells eventually differentiate into germline cells. So all vertebrates must somehow deactivate these growth-promoting genes when they mature from an early embryonic state.
Sea lampreys do this by physically eliminating the genes, while mice (and probably humans) do this by silencing them. There can be pros and cons to both approaches, so the topic is worth looking into further. And it would be interesting to find out how jabs target specific genes for deletion, as that could be a useful tool to have in our genetic arsenal.
Natural Genetics2018. DOI: 10.1038/s41588-017-0036-1 (About DOIs).
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