Baby white wallaby harboring hopping DNA

Kyoto, Japan - Kyoto University has discovered for the first time a genetic cause of albinism, or loss of pigmentation, in the small kangaroo-like wallaby.

Wallabies naturally have a brown or gray coat, which is what geneticists call a wild-type gene expression, or phenotype. Research team leader Akihiko Koga noticed that cases of white baby wallabies being born to wild-type mothers were reported from different countries.

"I wondered if there was a recent common ancestor as the source of albinism," says Koga.

The opportunity to investigate this hypothesis arrived on one fateful day in 2015 when a pale-skinned wallaby was born to a normal colored mother at Noichi Zoo in Kochi, Japan.

The team examined the gene for the enzyme tyrosinase or TYR, known for its role in producing melanin, the natural skin pigment. In the TYR gene, they discovered that an extra DNA fragment led to the mutation, that is the loss of the last two-thirds portion of the gene protein.

This is akin to copying and pasting a word inside a tweet before posting it, causing an equal length of text at the end of the tweet to exceed the word count; this figuratively "pushes" the essential word off the end, rendering the tweet incomplete and confusing.

The mutation was caused by a copy of the genetic material of a retrovirus, such as HIV, and inserted into the host wallaby. This copy is called an endogenous retrovirus, which was detected using a simple polymerase chain reaction, or PCR, test which is currently in common use for determining viral infections, such as sars-covid.

Easier said than done as the mutant gene was elusive and challenging to find in mammalian albinos, including primates and racoon dogs. But the team succeeded with a marsupial.

"We found this inserted fragment in a wallaby, which we fittingly named walb," mentions Koga.

The walb gene insertion is likely to have occurred by one of the following three processes: 1) endogenization, which is infection by an exogenous virus -- derived from another organism; 2) reinfection by an earlier generation virus; or 3) retrotransposition, defined as a relocation of an endogenous retrovirus within a cell.

While the process has not yet been identified at this point, Koga's team has deduced that it is a recent evolutionary event. Such mutant genes tend to remove themselves from the family tree if its genetic expression is disadvantageous for the wallaby's survival in the wild.

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The paper "An endogenous retrovirus presumed to have been endogenized or relocated recently in a marsupial, the red-necked wallaby" appeared 14 January 2022 in Genome, with doi: 10.1139/gen-2021-0047

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at both undergraduate and graduate levels is complemented by numerous research centers, as well as facilities and offices around Japan and the world. For more information please see: http://www.kyoto-u.ac.jp/en