Aardwolves and cats share surprising chromosome similarities, study finds

In the first detailed chromosome map of the aardwolf, Texas A&M University researchers have discovered that, despite millions of years of separate evolution, the chromosomes of aardwolves, other hyenas and even domestic cats have remained remarkably similar.

The aardwolf, a termite‑eating mammal native to Africa, is the least‑studied member of the hyena family. Until now, the only chromosome information on the aardwolf was its chromosome count — 40 — which was discovered in the 1960s.

The study, published in the Journal of Heredity, was led by Dr. Terje Raudsepp, a professor in the Texas A&M College of Veterinary Medicine and Biomedical Sciences’ (VMBS) Department of Veterinary Integrative Biosciences, in collaboration with Twin Pine Conservation Institute (TPCI).

The new study provides the first detailed look at the species’ whole chromosomes, or karyotype, and contributes to broader efforts to build chromosome‑level references for mammals. So far, only about 1,200 of roughly 6,000 mammal species have detailed chromosome maps.

“This chromosome‑level study shows why adding more mammalian species is important,” Raudsepp said. “You never know when a species might become extinct.”

Although earlier studies have sequenced the aardwolf’s genome, its DNA remains divided among roughly 5,600 fragments. By mapping the animal’s chromosomes, the new study gives researchers a guide for putting those pieces in order.

“Now that we have been able to compare the chromosomes of the aardwolf and spotted hyenas, there are specific differences to look for in a future genome sequence,” said Mayra Mendoza, a doctoral student in Raudsepp’s lab and a co-first author of the study.

What the chromosome map shows

To obtain the aardwolf’s chromosomes, the team grew cells from a skin sample sent by TPCI and waited for them to divide — the only moment when chromosomes appear as distinct rods under a microscope.

Using a staining technique that highlights each chromosome with a unique, barcode‑like pattern of light and dark stripes, the team compared the structure of aardwolf chromosomes with those of spotted hyenas, the aardwolf’s close relative.

Side by side, the two species’ chromosomes looked nearly identical.

“The center aligns the same way, and the banding pattern is unchanged — only three pairs show something different,” Mendoza said. “Later, we confirmed that these regions may be unique to the aardwolf or perhaps to another hyena lineage.”

The stability makes sense, as aardwolves and spotted hyenas diverged only about 10.6 to 11.6 million years ago, a relatively short span in evolutionary terms.

The surprise, though, came when the team compared aardwolves with domestic cats using a chromosome-painting technique, which allows researchers to compare chromosomes at the DNA sequence level. The two species, which look nothing alike today, split from their common ancestor roughly three times earlier than aardwolves and spotted hyenas. Yet the comparison revealed a similarly stable chromosome pattern.

“It was quite surprising. The colors are very strong, clear and pretty,” Mendoza said. “The two species are so far apart, yet the colors don’t look like they belong to two different species.”

Among the 19 chromosomes in cats, 12 match up one-to-one with a single chromosome in aardwolves. The remaining seven are more scattered, with each of those corresponding to material spread across different aardwolf chromosomes.

“From the original lineage, there haven’t been major chromosome shuffles or rearrangements in the evolutionary process,” Mendoza said. “If you ask someone which animals hyenas are most closely related to, they’ll likely say dogs, but hyenas are more closely related to cats.”

Cats were also a practical choice for comparison because Dr. Roscoe Stanyon, one of the study’s collaborators, had already developed a specialized set of DNA markers for each cat chromosome. Developing these tools can take years, so having them readily available made cats an ideal species for comparison, according to Raudsepp.

Researchers found additional evidence of this evolutionary stability by examining two features of the aardwolf’s chromosomes.

The first was a nucleolus organizer region, or NOR, a stretch of chromosome containing genes that help cells produce proteins. The researchers found that NOR is in the same location in aardwolves, other hyenas and cats.

“NORs are extremely variable across different species,” Raudsepp said. “But in hyenas and in cats — including domestic cats, lions, tigers and cheetahs — they all have just one chromosome pair that contains the NOR.”

The other was telomeres, the protective caps normally found at the ends of chromosomes. This provided another way to assess whether the aardwolf’s chromosomes had undergone major structural changes during evolution.

“Occasionally, over the course of evolution, telomeric sequences appear in the middle of chromosomes, in addition to the ends. These internal telomeric sites can signal past chromosomal rearrangements,” Raudsepp said. “But the bottom line is that in the aardwolf, we did not find any telomeric sequences away from chromosome ends.”

One sample opens the door to future research

The small skin sample sent by TPCI has now become a living collection of aardwolf cells that can be maintained in Raudsepp’s laboratory and used for other types of genetic testing.  

“A single good cell line opens the door to multiple kinds of research,” Raudsepp said.

Because Raudsepp’s lab already had the tools needed to study chromosomes, Mendoza and undergraduate researcher Ellie Kolb, the paper’s first author, pursued chromosome-level analysis while whole-genome sequencing remained on hold. Their small side project ultimately provided a more detailed look at how the aardwolf’s DNA is organized and showed why chromosome mapping remains valuable even as genome-sequencing technology advances.

“Guiding undergraduates who were devoted to the project helped me grow just as much as the science itself,” Mendoza said. “Watching the project take shape through a collective push—from the undergraduate, graduate and principal investigator levels—was the most rewarding part for me.”

By Ye (Vivian) Chen, Texas A&M University College of Veterinary Medicine and Biomedical Sciences