The molecular secret behind the giant lips of African cichlids

Over the years, aquarists and biologists have marveled at the dramatically thick lips of several African cichlid species. These unique fish, found in lakes Victoria, Malawi, and Tanganyika, have puzzled researchers who seek to understand how and why such large lips evolved.

This fascination has led to fresh investigations by scientists comparing lip structures, tissue composition, and gene expression in these fish.

The research effort is led by Dr. Nagatoshi Machii from the Tokyo Institute of Technology, whose group has examined microscopic details and molecular data to pinpoint what drives lip enlargement in certain cichlids.

Cichlid lips and parallel evolution

Researchers have documented that cichlids across different lakes display similar traits, even though they are not closely related. Such repeated emergence of shared features is known as parallel evolution, a process often fueled by environmental pressures.

“Parallel evolution is evidence of natural selection that drives adaptive solutions to the environment,” wrote Jonathan Losos, an evolutionary biologist based at Harvard.

Lakes as natural laboratories

The isolated ecosystems of Lake Victoria, Malawi, and Tanganyika act like nature’s own research stations. Each lake evolved its own line of cichlid species, yet many developed the same traits independently – a perfect setup to study parallel evolution.

By comparing species across these lakes, scientists can spot patterns that might be missed in a single location. It’s a rare opportunity to test how evolution repeats itself under similar environmental conditions but with different starting points.

Unmasking the lip-building proteins

Detailed analyses indicate that thick cichlid lips contain a proteoglycan-rich layer (a structural element found in various tissues, including bone, cartilage, tendons, and skin, that play crucial roles in tissue organization, hydration, and mechanical properties) not seen in normal-lipped fish.

Proteoglycans are large molecules with attached sugar chains, sometimes linked to extra water retention.

One study on cichlids with enlarged lips showed that certain proteins, including chondroitin sulfate proteoglycans, were in high abundance. This composition may give these hefty lips their signature plumpness by increasing connective tissue volume.

Lip growth starts young

The team found that differences in gene expression begin earlier than expected. Even in juvenile cichlids, before their lips fully thicken, the genes responsible for building extracellular matrix components are already more active in species that develop larger lips.

This suggests the groundwork for lip hypertrophy is laid early in life and then reinforced as the fish matures. The continuous expression of these genes throughout development points to a tightly regulated system that ensures these lip traits persist across generations.

Genes behind oversized cichlid lips

Scientists also noted changes in genes involved in the Wnt signaling network (a complex network of proteins that plays a critical role in how cells grow, divide, move, and develop.

It’s especially important during embryonic development, but it also helps maintain and repair tissues in adults). This pathway influences a wide array of growth and developmental processes in many animals.

Some cichlids with oversized lips showed heightened expression of Wnt-related genes at both juvenile and adult stages. This suggests the pathway may stay active over time, nudging tissues to expand and accumulate more extracellular material.

Tissue connection mirrors human biology

Recent findings have drawn an unexpected link between the oversized lips of cichlids and conditions seen in humans. The same proteoglycans found in fish lips are also involved in human skin disorders like keloids, where excess connective tissue causes raised, fibrous scars.

In both fish and humans, these molecules seem to attract and hold large amounts of water, helping tissues expand. Researchers believe that understanding how these proteins behave in cichlids may offer new insights into how connective tissues grow – or overgrow – in other animals, including us.

Possible role in cichlid feeding

Biologists have long debated the ecological benefits of bulky lips. Past work suggests these lips might help fish probe rocky crevices or minimize tissue damage as they forage. 

Another line of research hints that extra lip surface could boost contact with environmental cues. This might aid in locating prey, which would be especially useful in the maze-like habitats of the African Great Lakes.

Why not all cichlids have big lips

Despite the shared environments of the East African Great Lakes, not every cichlid species ends up with hypertrophied lips. That’s because this trait appears to be polygenic, meaning it depends on many genes working together – each with a small influence.

Environmental pressures like foraging style or diet likely interact with genetic tendencies. In species where the benefit of thick lips isn’t strong enough, the genes driving lip growth may remain quiet or develop differently, even if the molecular tools are present.

Future research directions

Genetic studies have pinpointed multiple points in the cichlid genome that may influence lip thickness. Each locus seemingly offers a small push toward fuller lips, forming a broad and complex mechanism for the trait. 

Investigations into parallels between enlarged fish lips and certain human connective tissue conditions have also started. Some scientists hope comparisons might clarify how collagen and other structural molecules work in different organisms over many generations.

The study is published in the journal eLife.

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