Using fluorescent tagging technology, the researchers have bought themselves a front row seat to the fight, allowing them to decipher the strategies males use to ensure their sperm (and not another male's) reach the egg, already helping to debunk a recent theory about sperm competition in Drosophila, according to a study published online today (March 18) in Science Express.
"It's one of the most exciting papers in the sperm competition field I've seen for ages," said evolutionary biologist linkurl:David Hosken;http://biosciences.exeter.ac.uk/staff/index.php?web_id=david_hosken of the University of Exeter in the UK, who did not participate in the study.
"The paper is really the methodological breakthrough that enables us to capture a glimpse of sperm competition in action," said evolutionary biologist linkurl:Tommaso Pizzari;http://www.zoo.ox.ac.uk/staff/academics/pizzari_t.htm of the University of Oxford, also not a co-author. "What happens inside the female has remained, up till now, a bit of a black box."
Whenever females mate with multiple males, there is an opportunity for competition. Specifically, the sperm of her mates compete with one another inside her reproductive tract. Females' tendency to mate with more than one male is now recognized as "more rule than exception," in the animal kingdom, and as a result, "sperm competition is widespread and rife and is driving a lot of biodiversity," said evolutionary biologist and study author linkurl:Scott Pitnick;http://biology.syr.edu/faculty/pitnick/pitnick.htm of Syracuse University in NY.
But until now, the mechanisms of sperm competition have been largely speculative, inferred on the basis of how many of the female's offspring each of her mates sires, or other indirect outcomes.
By differentially tagging protamine genes -- which code for the nuclear proteins that compact DNA in sperm heads even more tightly than histones do -- in two different lines of Drosophila melanogaster, Pitnick and his colleagues tracked the journey of two male ejaculates through the female reproductive tract. Still snapshots after the second male copulated with the female, as well as short 10-second videos of the action, revealed the "complex dance" of the green sperm of one male (tagged with GFP) and the red sperm of the other (tagged with RFP). Part of the action: female ejection of sperm from specialized female storage organs, intermixing of sperm, and sperm displacement by other sperm from storage organs. "We were baffled at first by the incredible activity of sperm," Pitnick said.
The researchers did not, however, find evidence to support a recent theory that the second male's ejaculate incapacitates the first male's sperm, proposed in Drosophila research. Interestingly, another study published this week in Science did find evidence of this tactic in ants and bees -- "the champions of sperm storage," said Pizzari. In those species, male seminal fluid appears to have the ability to recognize kin, and can therefore help its own sperm survive while making it harder for the sperm of other males.
Applying the new fluorescent techniques to the hymenoptera (ants and bees) as well as other species will be useful to further detail the different mechanisms of sperm competition and how they vary, said Pizzari. "This gives us a very powerful descriptive tool to understand the mechanisms that underlie variation in paternity, which is really the outcome of sperm competition."
Pitnick and his colleagues have already developed fluorescently-labeled sperm in two other species of Drosophila as well as a species of Tribolium (flour beetle), and are working on more. The two species of Drosophila are of particular interest, he noted, because they are sibling species that hybridize, and watching the behavior of their sperm may provide some clues about the process of speciation.
"The work is stunning," said Hosken. "A picture is worth a thousand words, and there it is in front of us."
**__Related stories:__***linkurl:A downside to female promiscuity;http://www.the-scientist.com/blog/display/55799/ [25th June 2009]*linkurl:Ancient bivalve had huge sperm;http://www.the-scientist.com/blog/display/55768/ [18th June 2009]*linkurl:Sperm size matters;http://www.the-scientist.com/article/display/20846/ [8th November 2002]
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