Why fruit fly sperm are giant

May 25, 2016, University of Zurich
Mating Drosophila fruit flies. Credit: Image: Stefan Luepold, University of Zurich

In the animal kingdom, sperm usually are considerably smaller than eggs, which means that males can produce far more of them. Large numbers of tiny sperm can increase the probability of successful fertilization, especially when females mate with several males. This is because the competition among sperm from different males to fertilize the few eggs increases as sperm become more abundant. This sperm competition spurs sexual selection after mating, favoring the best sperm in the female reproductive tract.

Therefore, it is astonishing that of certain animal species produce only very few, but gigantic sperm. "The record holder is Drosophila bifurca: Although this fruit fly is only a few millimeters long, its sperm reach an impressive length of almost six centimeters," says Stefan Luepold, an evolutionary biologist at the University of Zurich. However, such examples contradict the common understanding of sexual selection because fewer sperm vying to fertilize an egg should relax the selection pressure on sperm to be more successful than their competitors. If only a few sperm are transferred to females as in fruit flies, selection - and thus also the evolution of longer sperm - is expected to be weakened or even halted.

Sexual selection has a major impact on sperm length

A study lead by Stefan Luepold and published in the journal Nature now provides the first explanation for the development of such . Together with Scott Pitnick and other scientists from Syracuse University and George Washington University in the USA, he managed to demonstrate that sexual selection has a major influence on the evolution of sperm length in fruit flies. The scientists combined experimental, quantitative genetic and comparative studies on sexual traits in various Drosophila species.

The study revealed that, on the one hand, different characteristics and processes of sperm uptake, storage and use in the favor longer sperm. The longer the sperm become, however, the fewer of them can be produced and transferred. Consequently, females have to mate more frequently to ensure fertilization of their eggs. And each mating creates an opportunity for sexual selection via . "In fruit flies, for instance, longer sperm are really good at displacing their competitors from the female reproductive tract, which gives them an advantage in the competition for fertilization. Sexual selection thus favors longer sperm," explains Stefan Luepold, first author of the study.

Females prefer larger males with more sperm

On the other hand, sperm length is influenced by female preferences prior to mating. Small males can invest less in and therefore use up their sperm reserves after only few copulations. Only large, healthy males that are favored by females can afford to produce more sperm despite the increased energetic costs of longer sperm. As a result, only large males benefit from the frequent mating opportunities. Thanks to this increased reproductive success, the genes for longer sperm are able to spread in the population, which ultimately drives the evolution of longer sperm. These complex relationships can maintain, if not intensify, sexual selection even when only very few sperm end up competing for fertilization.

These new findings reveal that the general understanding of needs to be broadened. The evolution of sperm is ultimately based on similar processes as other male sexual traits such as horns to frighten away rivals and ornaments to attract females. "Compared to these and numerous other exaggerated sexual traits, fruit-fly are probably the most extreme example in the animal kingdom," finds Luepold. In the case of Drosophila bifurca, they are around 20 times longer than the male itself and thus transmitted as tightly coiled balls.

Explore further: Size matters when it comes to sperm dominance

More information: Stefan Lüpold et al, How sexual selection can drive the evolution of costly sperm ornamentation, Nature (2016). DOI: 10.1038/nature18005

Related Stories

Size matters when it comes to sperm dominance

November 18, 2015

A tiny fruit fly has the bizarre distinction of possessing the longest sperm of any animal—20 times the length of its own body and 1,000 times that of human sperm.

Recommended for you

Scientists study puncture performance of cactus spines

November 20, 2018

Beware the jumping cholla, Cylindropuntia fulgida. This shrubby, branching cactus will—if provoked by touching—anchor its splayed spines in the flesh of the offender. The barbed spines grip so tightly that a segment of ...

Traffic noise stresses out frogs, but some have adapted

November 20, 2018

Frogs from noisy ponds near highways have altered stress and immune profiles compared to frogs from more quiet ponds—changes that reduce the negative effects of traffic noise on the amphibians. According to a new study, ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

neiorah
5 / 5 (1) May 25, 2016
Are you sure about the measurements bc centimeters is bigger than millimeters. I can see it if the fly was about 60 mm long and the sperm was 6 centimeters bc they would be the same length. if not then the sperm is at least ten times the length of the fruit fly or longer. Amazing..
dan42day
not rated yet May 25, 2016
In the case of Drosophila bifurca, they are around 20 times longer than the male itself and thus transmitted as tightly coiled balls.


Last line in the article.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.