Activation of cell death gene triggered loss of phallus in bird evolution
“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, said Cohn.
For example, hypospadias, a congenital deformity of the urethra, now affects approximately 1 in 125 males born in the United States, according to the Centers for Disease Control and Prevention. During the past 40 years, that figure has increased dramatically for unknown reasons, 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.”
Genitalia are perhaps the fastest-evolving organ, he said. If you want to distinguish species based on anatomy, one of the most variable characteristics is genital anatomy. For example, the common ancestor of land fowl and waterfowl had a phallus, but once the two groups split and began diversifying, different anatomical changes occurred in each lineage. Waterfowl such as ducks and geese kept the external genitalia of their ancestors while land fowl such as chickens and quails lost theirs.
There are many hypotheses to explain why land fowl reduced their phalluses during evolution, said lead author Ana Herrera, a graduate student in the department of molecular genetics and microbiology. One theory suggests that since the gene that stops the chick’s tubercle from forming is also involved in the development of other structures such as the beak, feathers and limbs, the disappearance of the phallus was simply a byproduct of one of these other structures changing. Another theory cites sexual selection, wherein females were more apt to choose mates with smaller genitalia, so the trait stabilized in the population.
Whatever the reason, once a gene called Bmp4 activates in the tip of the genital tubercle during embryonic development, genital outgrowth stops in chickens. The gene turns on a cell suicide program that causes the tubercle to regress, Cohn said. In birds that develop phalluses, like ducks and emus, Bmp4 is not activated in this region and the phallus continues to grow.
“Programmed cell death is an evolutionarily ancient process,” he said. “It’s a normal part of development. That’s how you control form and regulate growth. It’s how you make joints between your bones and remove the webbing between your embryonic fingers. Cell death is an important part of sculpting the embryo.”
To test whether the Bmp4 gene was indeed responsible for sculpting away the chicks’ genital tubercles, Cohn and Herrera’s team blocked the gene to see if it rescued the developing phallus.
For the experiment, they used a native protein known to inhibit Bmp signaling. They loaded tiny beads with this protein and microsurgically implanted the beads into the tubercles of chick embryos. Where they placed the inhibiting protein, the cells survived and genital growth continued.
In a reverse experiment, they used the same process to add Bmp protein to the genital tubercles of duck embryos – birds with external genitalia – and successfully induced cell death.
“This approach not only allows us to understand how evolution works, but it also gives us new tools for future studies,” Cohn said.