Scott Pitnick’s fascination with sperm derives from their “incredibly unique biology.” Sperm have fast adaptability, diversity and the ability to live within a foreign environment, such as the female reproductive tract.

“My job description as an evolutionary biologist is to explain diversity. And so when I see all the myriad forms that a sperm can take, that gets me very excited,” said Pitnick, a biology professor at Syracuse University.

Pitnick’s research team recently published a study on May 26 stating that a male fruit fly’s unusually large sperm serve as a strong sexual ornament compared to multiple species.

In addition to Pitnick, the study was co-authored by William Starmer, an emeritus professor of biology at SU; John Belote, a biology professor at SU; Nalini Puniamoorthy, a postdoctorate fellow at SU; Mollie Manier, an assistant professor of biology at The George Washington University and Stefan Lüpold, a group leader in the Department of Evolutionary Biology and Environmental Studies at the University of Zurich.

The study used an index to compare how strong sexual selection has been in driving evolution of sexual ornaments and sexual weapons across various species — including earwig forceps, lizard dewlaps, stag beetle mandibles, pheasant spurs and tails. From one to six — with one being the weakest and six being the strongest — fruit fly sperm had a 5.52 rating, while others fell between 1.55 and 3.75.

In general, Pitnick said, males have few restrictions on how much sperm they can generate in an ejaculation. This allows males, including humans, to create hundreds of thousands of small sperm in their sex organs.

Females, on the other hand, have a limited amount of sex cells, or eggs, which makes them more selective about with whom they mate, Pitnick said. In order to attract females and produce offspring, males of some species develop traits, such as a peacock’s bright plumage and long feathers that give them an advantage in a competition against other males.

“Everything we understand about the fundamental nature of sex differences follows from the general rule that sperm are cheap and eggs are costly,” Pitnick said.

However, when Pitnick was a Ph.D. student at Arizona State University, he became aware of Drosophila bifurca, a fruit fly species in which the males make fewer, but larger sperm.

After graduating in 1992, Pitnick wrote a paper stating these male fruit flies produce sperm as long as 5.8 centimeters, or 2.28 inches. That discovery, he said, was counterintuitive to both the theoretical expectations and the dogma about the fundamental nature of sex difference.

“Now we had this conundrum where we have males of some species that have very female-like reproductive strategies that produce very few costly sperm,” he said. “That’s a problem that requires explanation.”

After years of research, Pitnick’s team found that male fruit flies produce fewer, but larger sperm because they have an advantage in fertilization of eggs as a consequence of the females’ evolving reproductive tracts that favor longer sperm, Pitnick said.

He added that it is a noteworthy feature that even though females cannot see sperm, they exert their influence in sexual preference after mating using morphological — and perhaps also molecular — characteristics of their reproductive tract.

The study was important to his research team, Pitnick said, because the data that went into it had been collected for more than 15 years.

Said Pitnick: “It is a complex story.”

Published on June 22, 2016 at 11:43 am

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