Researchers uncover genetic explanation for missing bird phallus mystery
By Marilee Griffin
Researchers at the UF Genetics Institute have discovered how, at the genetic level, different bird species either develop or do not develop phalluses, a finding that may help scientists better understand how embryonic growth and development are regulated and how evolutionary changes arise. They reported their findings in the journal Current Biology.
“We’ve been able to uncover the developmental basis of this evolutionary event,” said Martin J. Cohn, Ph.D., a developmental biologist and senior author of the paper. “We now have a better understanding of how cell death is controlled and how organ growth is regulated. Examining genital development in different species can provide insights that might not be learned by studying a single species.”
In nature, when cell death or cell division occurs improperly due to gene deregulation, the result can be tumors, diseases, birth defects and cancer. Malformations of the reproductive organs such as the external genitalia are among the most common birth defects in humans, Cohn said.
“Despite the high incidence of birth defects affecting the genitalia, genital development is not well-understood at the molecular genetic level,” he said. “By studying how evolution of genetic pathways led to changes in anatomical form, we can also ask whether some of the malformations and diseases that we see in humans might involve these same genetic programs.”
Today, less than 3 percent of all birds have phalluses, even though their ancestors had external genitalia. Why one of the world’s most successful and diverse animals lost an organ thought to be essential to breeding is uncertain.
The researchers’ findings show that during embryonic development, chicks begin to form the precursor of the phallus, a genital tubercle. However, a gene that causes cell death then activates, and the tubercle regresses. By successfully disrupting the activity of this gene, the researchers proved it is responsible for the anatomical change.
The genes they studied are also involved in mammalian genital development, said Cohn, a Howard Hughes Medical Institute scientist and a professor in the department of molecular genetics and microbiology at the UF College of Medicine. Like humans, birds breed through internal fertilization. Unlike most animals that use this method to produce offspring, however, most birds don’t have a phallus for delivering sperm into the female body; rather, both male and females have an opening called a cloaca that they press together in a “cloacal kiss.”
“It’s such a strange evolutionary event,” said Cohn, who has studied limb development and limb reduction in animals such as snakes and whales. “Here’s arguably the most important structure for internal fertilization, which should be under tremendously strong natural selection. And yet most birds have lost their external genitalia and they still reproduce through internal fertilization and are incredibly successful.”