Emus, those towering birds native to Australia, are a familiar sight with their long legs and rapid ground speed. But looking at their sturdy build and powerful legs, a question often arises: Can emus fly? The simple answer is no, emus are flightless birds. However, the reasons behind their inability to take to the skies are far more complex and fascinating, rooted in evolutionary biology and recent genetic discoveries.
To truly understand why emus are grounded, we need to delve into the world of ratites – a group of flightless birds that includes ostriches, kiwis, rheas, cassowaries, and tinamous, along with the extinct moa and elephant birds. Intriguingly, only tinamous retain the ability to fly, making the study of ratites crucial to understanding the evolution of flightlessness in birds.
adult cassowary
Recent research published in Science sheds light on the genetic mechanisms behind this phenomenon, pointing towards mutations in regulatory DNA as a key factor in grounding these birds. This groundbreaking study challenges previous assumptions and opens new avenues for understanding how major evolutionary changes occur.
The Role of Regulatory DNA in Flightlessness
For a long time, scientists have debated whether significant evolutionary shifts, like losing the ability to fly, are primarily driven by changes in protein-making genes or by alterations in regulatory DNA. Regulatory DNA doesn’t contain instructions for building proteins directly. Instead, it acts like a conductor, controlling when and where genes are switched on or off. Think of it as the subtle tweaks in the genetic code that dictate how genes are expressed, influencing traits like wing size and muscle development – crucial for flight.
Camille Berthelot, an evolutionary geneticist at INSERM in Paris, explains that mutations in protein-coding genes can have widespread and potentially damaging effects because a single protein might be involved in numerous bodily functions. On the other hand, regulatory DNA offers a more nuanced and localized way to influence gene activity. Changes in regulatory DNA can fine-tune gene expression in specific tissues, making it a more flexible and less disruptive pathway for evolutionary experimentation.
However, pinpointing the role of regulatory DNA in evolution has been challenging. Unlike protein-coding genes that often share recognizable patterns, regulatory DNA segments are diverse and can vary significantly between species.
Genetic Evidence: Unlocking the Emu Flightless Mystery
To overcome this challenge, evolutionary biologist Scott Edwards from Harvard University and his team embarked on a comprehensive genetic study. They sequenced the genomes of 11 bird species, including eight flightless ratites. By comparing these genomes with existing genomes of flying birds, they searched for conserved regulatory DNA regions – stretches of DNA that remained relatively unchanged across bird evolution, indicating important functions.
Within these conserved regions, they identified 2,355 regulatory DNA segments that showed an accumulation of mutations specifically in ratites, but not in flying bird lineages. This accelerated mutation rate suggests that these regulatory DNA segments were evolving faster in flightless birds and potentially losing their original functions related to flight.
Further analysis revealed that many of these mutated regulatory DNA segments were located near genes involved in limb development. This strongly suggests a link between these mutations and the development of smaller wings, a hallmark of flightless birds like emus.
To test this hypothesis, the researchers focused on a specific regulatory DNA segment called an enhancer, which controls gene activation. They compared the enhancer from a flying tinamou with the same enhancer from a flightless rhea. The tinamou enhancer effectively activated a gene in developing chicken wings, while the rhea enhancer failed to do so. This experiment provides direct evidence that changes in regulatory DNA enhancers can indeed disrupt wing development and contribute to flightlessness in ratites.
Why Emus Can’t Fly: A Matter of Evolutionary Trade-offs
The research suggests that the ancestors of ratites, including emus, were likely capable of flight. Over time, these birds independently lost the ability to fly, possibly multiple times within the ratite lineage. This loss of flight is not necessarily a disadvantage; in fact, it can be an evolutionary adaptation to specific environments.
Flight is energetically expensive. Birds need powerful flight muscles and lightweight skeletons, demanding significant resources. For large terrestrial birds like emus, ostriches, and cassowaries, flight may have become less crucial for survival. Instead, they evolved to thrive on the ground, developing powerful legs for running and defense. Their size and speed provide protection from predators, and their diet is readily available on the ground.
The loss of flight in emus is likely a result of these evolutionary trade-offs, driven by changes in regulatory DNA that subtly altered gene expression, leading to reduced wing size and flight muscle development over generations.
Beyond Emus: Implications for Evolution
This study highlights the significant role of regulatory DNA in shaping evolutionary change. It demonstrates that even subtle mutations in these control regions of the genome can have profound effects on an organism’s traits, leading to major evolutionary adaptations like the loss of flight.
While protein-coding gene mutations are undoubtedly important in evolution, this research emphasizes that regulatory DNA is a crucial player, offering a flexible and nuanced mechanism for evolutionary change. Understanding the intricacies of regulatory DNA is key to unraveling the genetic basis of diversity in the natural world and understanding how closely related species can evolve such different characteristics. The case of the emu and other flightless birds provides a compelling example of how these “tweaks” in our genetic blueprints can lead to remarkable evolutionary transformations.
