Aardwolf's first chromosome map shows hyenas kept most of the layout they share with cats

That cat match comes to about 63 percent of the cat's chromosomes ^[2]. The spotted hyena fit is closer. The aardwolf's 40 chromosomes form 20 pairs ^[3]. Mayra Mendoza, a doctoral student in Raudsepp's lab and a co-first author, said that "only three pairs show something different" ^[7][10]. "The center aligns the same way, and the banding pattern is unchanged," she said ^[10].

The hyena half of the result was expected. Aardwolves and spotted hyenas diverged about 10.6 to 11.6 million years ago, a short span in evolutionary terms ^[12]. Cats and aardwolves split from their common ancestor roughly three times earlier ^[13]. Taken literally, that multiple puts the split at about 32 to 35 million years ^[4]. "The two species are so far apart, yet the colors don't look like they belong to two different species," Mendoza said ^[14]. She added: "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." ^[16]

The method shaped what the team could compare. Chromosomes show up as distinct rods only while a cell is dividing, so the team grew cells from a skin sample sent by Twin Pine Conservation Institute and waited ^[8]. For the hyena comparison they used a stain that marks each chromosome with a barcode of light and dark stripes ^[9]. For the cat they used chromosome painting, which compares chromosomes at the DNA sequence level, according to the phys.org account ^[13]. Choosing the cat was partly a matter of convenience. Roscoe Stanyon, a collaborator, had already built DNA markers for each cat chromosome, and such tools can take years to develop, according to Raudsepp ^[17]. The account describes one skin sample and does not say whether other animals were karyotyped ^[8].

The team also checked the nucleolus organizer region, a stretch of chromosome carrying genes that help cells produce proteins ^[18]. "NORs are extremely variable across different species," Raudsepp said ^[19]. The study treats its shared position in aardwolves, other hyenas and cats as additional evidence of stability ^[18].

The map's first use is assembly. Earlier studies sequenced the aardwolf genome, but the DNA is still in roughly 5,600 fragments, and the chromosome map is a guide for putting them in order ^[6]. "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," Mendoza said ^[7]. The thing this doesn't tell you is what sits in those three differing pairs. That will have to come from the sequence ^[7]. Mendoza was guarded about where the regions came from: "Later, we confirmed that these regions may be unique to the aardwolf or perhaps to another hyena lineage." ^[11]

About 4,800 of the roughly 6,000 mammal species still have no detailed chromosome map ^[1]. "This chromosome-level study shows why adding more mammalian species is important," Raudsepp said. "You never know when a species might become extinct." ^[5] I think the aardwolf result supports that case on narrower grounds than extinction risk. Stable chromosomes make a comparison with a relative that already has marker sets cheap and informative. Even so, seven cat chromosomes split and three hyena pairs changed ^[15][10]. Using either relative's map as a proxy would have missed those differences. The study covers the hyena family and one cat, and it cannot say how stable chromosomes have been in other mammal lineages.