Photo courtesy of Jeramiah J. Smith and Chris T. Amemiya

Overboard! Most cells in adult lampreys have 20% less DNA than their newly fertilized eggs do.

Fish Throws Away Its Genes as It Grows

Liz is a staff writer for Science.

Whether it's its extraterrestrial looks or status as a "living fossil," there's always been something fishy about the sea lamprey. Now scientists have added another oddity to the creature's repertoire: The lamprey jettisons 20% of its genome during development.

Jeramiah Smith of the University of Washington, Seattle, first suspected something strange while piecing together the sea lamprey's genetic sequence. The postdoctoral fellow and his colleagues tried labeling live lamprey cells using a technique that detects broken DNA. "Every cell in the embryo was [labeled] as dying," he recalls. So he took a closer look to see what was going on and got a big surprise.

Working with Chris Amemiya of the Benaroya Research Institute at Virginia Mason in Seattle, the group found that lamprey sperm DNA had sequences not found in lamprey liver and that overall the sperm genome was millions of bases longer. The sperm DNA included a highly repetitive sequence called Germ1, and by monitoring the loss of Germ1 during development, Smith was able to track the genome's reorganization.

His group found that the egg and sperm have the full genome, but shortly after fertilization--about the time the new genome turns on--the genome starts to be pared down. By day two, the amount of Germ1 has decreased substantially. And by the time the larvae hatch, it's almost entirely disappeared, Smith's team reports online this week in the Proceedings of the National Academy of Sciences.

The group has also identified specific genes in sperm DNA that were lacking in liver. One of them, called SPOPL, may help stabilize the DNA-protein complex called chromatin, but Smith says he suspects "probably hundreds" more are missing from various places in the genome.

The discovery "opens up a whole new area of thought, as it's contrary to what you generally think happens in vertebrates," says Marianne Bronner-Fraser, a developmental biologist at the California Institute of Technology in Pasadena. Typically, vertebrates silence genes by modifying them or their surroundings chemically, not by getting rid of them.

Paring down might be economical, as each dividing cell has less DNA to replicate. But Smith thinks that 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.

The work "is a very valuable contribution to our understanding of the dynamic nature of genome evolution," says embryologist Mark Martindale of the University of Hawaii, Honolulu.

Other organisms undergo genomic reduction, notes molecular biologist Guy Drouin of the University of Ottawa, Canada, who studies this phenomenon in tiny invertebrates called copepods. "The real breakthrough will come when someone is able to figure out the mechanism by which [genome reduction] occurs." One nice thing about lampreys, he says, is that they are easy to study--so perhaps these strange creatures will help solve the mystery.

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Posted in Plants & Animals, Biology