What can we learn from promiscuous bugs and crocodile studs?

Newswise — You may think human sex is bizarre enough. But elsewhere in the animal kingdom, features like competition between sperm and semen that influences behavior conspire to make it even weirder. A special issue of the journal Molecular & Cellular Proteomics highlights recent discoveries in the reproductive biology of species from insects to crocodiles. Here are some highlights:

Intrepid researchers harvest “croc sperm” to understand evolution

Sperm get their start in the testes. But in most mammals, they can’t fertilize an egg without going through several stages of maturation after leaving the testes. A key step is capacitation, a biochemical change that happens after sperm reach the uterus. Without it, they can’t break through and fertilize an egg. Researchers from the University of Newcastle in Australia have found evidence that reptile sperm also does some growing up after leaving the testes, overturning the long-held belief that post-testicular maturation is a trait unique to mammals.

They got their samples from an alarming source. “Semen used throughout this study was collected by digital massage from mature saltwater crocodiles during the breeding season,” authors led by Professor Brett Nixon wrote, adding parenthetically that “mature” means the crocodiles were 3 or more meters long.

In an earlier study, the team had found that a molecule that triggers capacitation can make sperm in these samples swim faster. Now, they used proteomics to determine that that molecule acts through signaling proteins, including several with known roles in mammalian sperm capacitation, giving strong circumstantial evidence that capacitation occurs in non-mammal animals.

Given that shared traits usually come from a common ancestor, the team says that post-testicular maturation appears to have evolved earlier than scientists had previously thought. Besides shedding light on the evolution of sex, their work may also help crocodile breeders, who raise the animals for conservation and to produce leather, to pioneer crocodile IVF.

Ant queens keep the peace between dueling males’ sperm

In many species, males compete for a chance to mate with a female. But in ants, each queen mates with many males early in life, then stores their sperm for years at a time. This opens up a new battleground for the males: stored semen can compete to fertilize an egg over vast periods of reproductive time.

Ant semen has evolved to contain proteins that jockey for evolutionary advantage by incapacitating rival males’ sperm. But for the female ant, this kind of sexual selection is counterproductive. She mates at a young age and needs to hold on to that sperm for a reproductive lifetime that may last years.

Ryan Dosselli and colleagues at universities in Australia, Denmark and the United States found that in leaf-cutter ants, whose seminal fluid contains proteases to harm rival sperm, the female’s reproductive tract starts secreting counter-proteins soon after insemination to preserve sperm for as long as possible.

Using mosquito semen ... to prevent reproduction?!

As the world warms and the range of disease-carrying mosquitoes grows, scientists are testing novel ways to control mosquito populations. One strategy is to release sterile male mosquitoes; it works because female mosquitos usually don’t mate more than once. Researchers are still working out why.

Scientists have previously found a peptide in mosquito semen that can change a female’s behavior, making her less interested in finding a second mate. But that peptide’s effects are short-lived. What stops a female from getting back on the mating scene in the longer term?

Other components of semen are known to be involved, but their identities are not yet known. Ethan Degner and colleagues at Cornell and Syracuse University are investigating which of the proteins made in mosquitoes’ tiny testes are actually transferred to the female reproductive tract.

The team used transcriptomics to survey gene expression in male reproductive tissues and proteomics to identify transferred proteins. They raised male and female mosquitoes on food that contained different stable isotopes of nitrogen, let them mate, and then compared the proteins found in isolated sperm with the proteins in a female mosquito’s reproductive tract after mating. 

The sperm and seminal fluid proteome the researchers defined gives them and others a starting point to look for more proteins that contribute to females’ lifelong turnoff. If they can figure out which of these proteins are important for suppressing re-mating in the long term, they may find a way to supercharge sterile males, making the females they mate with even less likely to have any offspring. This could decrease mosquito population sizes, perhaps protecting people from exposure to mosquito-borne diseases.

Animal sex: Not as weird as it seems?

As strange as animals’ reproductive habits may be, according to MCP Associate Editor Tim Karr, a professor at the Biodesign Institute at Arizona State University, they’re also fundamentally the same across the Tree of Life.

“Sperm and egg are the only cell types (whose) function is universal throughout the animal kingdom,” Karr, who edited the special issue, said. “From a cell-centric perspective sperm and egg represent our deepest links to the evolution of eukaryotic life on the planet.”

###

These and other research articles on reproductive proteomics were published in a special issue of the journal Molecular & Cellular Proteomics (MCP), available here: https://www.mcponline.org/content/18/Supplement_1

MCP showcases research into proteomes, large-scale sets of proteins from different organisms or biological contexts. The journal publishes work that describes the structural and functional properties of proteins and their expression, particularly with respect to developmental time courses. Emphasis is placed on determining how the presence or absence of proteins affect biological responses, and how the interaction of proteins with their cellular partners influences their functions.