Parasitic invasion credited with evolution of sex
|09:30 08 May 04|
|NewScientist.com news service|
The first inkling of maleness began when parasitic bacteria jumped between cells, dragging their host's genes with them. And according to the researcher who came up with the controversial idea, the vestiges of this inauspicious beginning persist in the sperm of animals today.
Some time between 2000 million and 700 million years ago, bacteria entered into an uneasy truce with larger cells. These cells were the precursors of complex eukaryotic cells, that eventually evolved into today's multicellular animals and plants.
The bacteria wound up losing around 90 per cent of their genes to the host nucleus and became mitochondria - the energy-generating components of complex cells. But modern mitochondria are so intimately involved in sexual reproduction that one scientist thinks they may even have been responsible for the evolution of sex itself.
Chris Bazinet, at St John's University in New York believes that early mitochondria were mischievous. They could have colonised new hosts by bursting out and jumping to nearby cells.
Paradoxically, this might have benefited the host cell if the mitochondria took genes from the nucleus with them. Sharing genes can be a big plus because it allows a cell to adapt to new environments or threats.
This in turn could have led to the process of gene donation and acceptance becoming formalised and controlled by the host's genome. "Donators" were proto-males and "acceptors" proto-females (Bioessays, vol 26, p 558).
Bazinet accepts his idea is highly speculative, but he says the bizarre behaviour of mitochondria in the fruit fly Drosophila might point to their earlier behaviour. "Mitochondria do some weird and complicated things in the production of sperm and egg cells that they don't do anywhere else in the body," he explains.
When Drosophila sperm form, for example, mitochondria move into position using a bundle of actin fibres resembling a comet's tail. Strangely, while these mitochondria do not make it into the final cells, that shuffle is integral to sperm development - if it is disabled, the fly is sterile.
The movement looks very like the technique a parasitic bacterium called Rickettsia uses to push itself into neighbouring cells. And Rickettsia are thought to be close relatives of the bacteria that became mitochondria. Bazinet believes that this odd behaviour may be a vestige of an ancient process that led to the development of sperm and eggs.
RNA from mitochondria has also been found within the nucleus of human and mouse sperm. No one is sure what it is doing there, but it suggests that mitochondria play an intimate role in sperm development.
Rick Michod, an evolutionary biologist at the University of Arizona in Tucson, agrees that mitochondria had a hand in the evolution of sex, but he departs from Bazinet on the details.
Sexual systems were reorganised when cells took on bacteria as mitochondria, he says, "but systems of genetic exchange were already in place, before the eukaryotic cell evolved". Genetic studies in bacteria have revealed that extensive gene exchange is the norm between different bacteria even though they do not have formal sexual reproduction.
Bazinet hopes to test his theory by working out whether Rickettsia transfer host genes from cell to cell. The bacterium, which causes a serious, tick-borne disease called Rocky Mountain Spotted Fever, spends time in the nucleus of human cells.
If they can pick up and transfer host genes, then perhaps this also happened in the earliest eukaryotes. So did early mitochondria set our distant ancestors on the route to males and females? The jury is out.
Lynn Margulis of the University of Massachusetts in Amherst, who revived the 19th-century idea that mitochondria are descended from bacteria, is reluctant to go that far. "Bazinet has shown that mitochondria behave just like their bacterial ancestors," she says. "But I wouldn't make his grandiose claim about mitochondria being responsible for the whole of the evolution of sex."
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