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This Week

24 June 1989

Sperm provides a bridge between species

ITALIAN biologists claim that mouse sperm washed with fluids containing
fragments of foreign DNA can pick up the stray genes and carry them into
the eggs they fertilise, producing transgenic animals. The amazing experiment,
described as the biological equivalent of cold fusion, is published in this
week’s issue of Cell.

If confirmed by other investigators, this discovery will revolutionise
animal breeding. At the moment, biologists produce transgenic animals by
two laborious and often unsuccessful procedures. They either inject fertilised
eggs with foreign DNA, by hand under a microscope, or infect unspecialised
embryonic cells with viruses engineered to carry a foreign gene.

The Italian finding also promises to alter our understanding of the
processes underlying evolution. It suggests that sperm could transmit DNA
across species boundaries, transforming the course of evolution.

Marialuisa Lavitrano and her colleagues at the University of Rome incubated
sperm with cloned DNA known as pSV2-CAT. Mouse eggs fertilised with such
sperm were then placed in the oviducts of foster mothers. In an analysis
of 250 offspring, the researchers detected the foreign gene, stably integrated
into the genetic material of the offspring, in about one-third of cases.


The researchers ruled out simple contamination of the eggs by the foreign
DNA by showing that the alien genes in the transgenic mice bore methyl groups
in positions unique to mammals, and so must have been replicated in the
mouse cells.

No one has any idea how the sperm are able to take up DNA. It happened
rapidly, in less than 15 minutes, but only in live sperm. So the researchers
suspect that the foreign DNA enters the head of the sperm, rather than just
being attached to its surface. Surprisingly, a single sperm can apparently
capture as many as 4000 molecules of the pSV2-CAT DNA. The research drops
a bombshell into the long-standing controversy about whether the exchange
of genetic material between species has been important in evolution.

Most biologists have concluded that only certain retroviruses could
shuttle genes between species. If, as the new findings from Rome suggest,
sperm can carry stray DNA into eggs, the process may be much more widespread.
However, such a random process seems unlikely to introduce beneficial genes
into a species. So semen may contain substances that suppress a sperm’s
ability to take up foreign DNA, say Max Birnstiel and Meinrad Busslinger
of the Research Institute of Molecular Pathology in Vienna, writing in Cell.

Researchers in the field are fascinated but wary. ‘It is quite a breakthrough,’
says Harry Moore, who studies sperm at the Institute of Zoology in London.
‘But it has got to be repeated. It’s surprising that sperm would do that,
and that it hasn’t been picked up before.’ John Clark, part of a team in
the Agricultural and Food Research Council that produced transgenic sheep,
says: ‘It is an experiment we will all be repeating. If it is independently
confirmed, it will be terrific. It will make the production of transgenic
farm animals much easier.’

Birnstiel and Busslinger also urge caution. The experiments will form
‘a cornerstone in biology . . . if they can be repeated by others’, they
say. But, according to the researchers in Vienna, the findings may ‘also
arouse those who see this as another turn of the screw in our estrangement
from nature, facilitating a comprehensive genetical management in animal