快猫短视频

SOMETHING strange is going on. Viruses all over the world are preserving a
stretch of genetic material that until now was considered redundant. Given that
DNA with no function tends to change rapidly because of random mutation, this
suggests that this particular piece of 鈥渏unk鈥� DNA must be of some use to the
viruses.

Jim Van Etten and his colleagues at the University of Nebraska found the
mysterious sequence within a gene carried by chlorella viruses. These viruses
live in algae in lakes around the world, and the gene the Nebraska team was
studying repairs damage to viral DNA caused by ultraviolet light.

Non-coding DNA sequences inside genes are called introns. These are generally
considered junk DNA, containing no useful information. They tend to change more
rapidly than protein-coding DNA because they are not under any selective
pressure to hang on to a useful sequence. Even the few introns that do have
functions鈥攕uch as jumping into other genes鈥攐nly conserve a limited
amount of genetic material.

The Nebraska scientists determined the DNA sequence of the repair gene from
42 chlorella viruses isolated over a period of 12 years from the US, China,
Australia and Argentina. They found that 15 of the viruses shared an identical
copy of the intron. Another four viruses contained almost exact copies of a
related, but slightly smaller intron. 鈥淚 don鈥檛 know of any other examples of
this type of intron sequence conservation. There has to be selective pressure to
maintain the sequence with that kind of fidelity,鈥� says Van Etten.

鈥淭here鈥檚 something unusual going on here,鈥� agrees Arlin Stoltzfus of the
Center for Advanced Research in Biotechnology in Maryland. He thinks that the
intron sequence could be identical in the viruses simply because it has spread
though the virus population recently and has not yet had time to mutate. But
such a rapid spread implies that the intron must be conferring some selective
advantage.

This doesn鈥檛 have to be the case, however. David Penny, a theoretical
biologist at Massey University in New Zealand, thinks that the intron could be
compared to a 鈥渟elfish gene鈥�, good at ensuring its own survival without
necessarily benefiting the virus. 鈥淲hat is good for a particular gene need not
be best for the individual that houses the gene,鈥� he says.