Fish with promiscuous females have evolved 'super sperm' to deal with competition

Published: Tuesday, January 20, 2009, 11:44 [IST]

London, January 20 (ANI): A new study led by Canadian researchers has lent more force to the belief that the evolution of 'super sperm' depends upon competition.

While it is known that sperm from promiscuous chimps move faster than those from relatively monogamous gorillas, Ontario-based researchers at McMaster University in Hamilton have for the first time shown the same pattern among fish too.

Research leader Sigal Balshine and her colleagues studied a variety of species of cichlid fish living in Lake Tanganyika in central Africa.

The researchers chose cichlids for the study because they form a major family of fish, with species employing the full range of mating behaviours - from monogamy to "sperm shopping", in which a female gets several males to ejaculate into her mouth, which is where she carries her eggs.

In accordance with their promiscuity, 29 species of fish were ranked from zero to four.

The research team later recorded sperm characteristics, such as size and speed.

They corrected for the degree of relatedness between species, which might otherwise suggest a relationship between mating behaviour and sperm characteristics that did not exist, by constructing a "family tree" from cichlid mitochondrial DNA sequences.

The researchers observed that the tougher the competition among sperm, the faster, bigger, more numerous, and longer-lived they become.

According to them, sperm of strictly monogamous fish were small and slow, at around 50 microns per second.

"But as you move up the scale, species have more competitive ejaculates, with the most promiscuous producing 'superman' sperm - they were almost twice as fast, larger, there was more of them, and they lived longer," New Scientist magazine quote team member John Fitzpatrick, from the University of Western Australia in Perth, as saying.

"It demonstrates for the first time that sperm traits such as speed and size can evolve together, and that those traits are related to the risk of sperm competition with rival males. Elegant," said Mats Olsson, of the University of Wollongong in New South Wales, Australia.

The Balshine team also found that the ancestral species was monogamous, with tiny gonads, and small sperm, and that more promiscuous behaviour preceded the evolution of more competitive sperm.

A research article on the team's study has been published in the journal Proceedings of the National Academies of Science. (ANI)

 

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