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Almost all stories about the lamprey are strange. It has been called the alien fish, the living fossil, and has some of the scariest teeth in the sea. But the newest characteristic of the fish might be strangest yet: it throws away parts of its genome as it grows.
A considerable portion of the human genome is not active. Some of it is so-called ‘junk-DNA’ that has no known purpose. Some of it is composed of DNA that is only used during development and then never again. Some of it is harmful, mutated genes that have yet to be activated, that lay silent and dangerous undetected. But it is all there, every last bit that a human was born with.
Until recently, it was thought that virtually all vertebrates followed the same pattern. But the ever-unusual lamprey fish has proven the exception to the rule. Over the course of its development, the lamprey “jettisons 20% of its genome.” [Science]
This phenomenon was discovered by the wonder of scientific accident. A graduate student at University of Washington in Seattle was trying to piece together the genome. By using a fluorescent dye on live lamprey embryos that lights up broken DNA, Jeramiah Smith observed something strange. "Every cell in the embryo was [labeled] as dying," Smith said. [Science] Broken DNA does usually mean a cell is dying. However, as lamprey embryos were clearly growing into healthy fish, it was clear the cells were neither dead nor dying. But what of all that broken DNA?
Researchers went to the beginning of the development story: sperm. They found that “lamprey sperm DNA had sequences not found in lamprey liver and that overall the sperm genome was millions of bases longer.” [Science] At some point directly after fertilization, both egg and sperm cells begin to shed genomic data. That missing liver gene, SPOPL (which actually does the important job of stabilizing DNA-protein chromatin, and thus cannot be categorized as throw-away DNA), is thought to be one of many missing genes all across the lamprey’s system.
Genomic reduction is not completely unique to vertebrates. Copepods do it, too. But having more than one model is key to understanding this quirk of genomic evolution. The technique is somewhat economical; it takes less energy and space to copy over a shorter DNA sequence. But Smith thinks it extends farther than that. He believes “the cell might be getting rid of genes that are good for the tissue destined to become egg and sperm--such as ones that stimulate rapid growth and proliferation--but which could make a specialized cell cancerous.” [Science]
So it is a matter of risk vs. benefit. Losing important genes in order to stop cancer. Such selective genome reduction may hold important insight into yet another mystery of evolution.
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