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There’s more to songbird sex than we thought. It seems that all of them have an extra chromosome in their sex cells that isn’t seen elsewhere in their bodies. The extra DNA could help explain why songbirds account for roughly 5000 of the 10,000 known bird species.

Songbirds typically have 40 pairs of chromosomes in most cells of their body. But in the late 1990s, biologists studying zebra finches made an odd discovery. Some of the cells involved in sexual reproduction, called germline cells, .

At the time zebra finches were considered a genetic oddity, says Pavel Borodin at Russia’s Institute of Cytology and Genetics in Siberia, and few researchers bothered to investigate any further. But now Borodin and his colleagues have discovered other songbirds have a 41^st chromosome pair too.

They examined the chromosomes of 14 species scattered across the family tree of songbirds, including finches, skylarks and canaries, along with chromosomes from eight non-songbird species.

All 14 songbirds carried the extra chromosome in their germline cells, but none of the non-song birds did. Borodin says this suggests the extra chromosome may have first appeared in the last common ancestor of all songbirds, which lived about 35 million years ago and as such is probably present in all modern songbirds.

Rapidly evolving

Borodin thinks the 41^st chromosome may help during sexual reproduction by providing more protein and nutrients for developing sperm and egg cells. Two studies published last year that looked at the in back up the idea. They found evidence that it contains additional copies of protein-generating genes found elsewhere in the genome.

Because that protein boost isn’t needed elsewhere in the body, natural selection has eradicated the extra chromosome from other body cells. “We don’t know how, but it is eliminated,” says Borodin. “It’s not just switched off – it’s physically absent.”

Whether the extra genes are important in additional ways is unclear. Some songbirds have a vocal learning ability similar to that underpinning human speech. Borodin and his colleagues found fragments of a vocal learning gene on the extra chromosome, but because those fragments are probably non-functioning they think it’s unlikely that this is a sign that the extra chromosome boosts the birds’ vocal learning ability.

Alexander Suh at Uppsala University, Sweden – who co-authored one of last year’s studies on the zebra finch chromosome – says the extra chromosome could be significant for songbird evolution.

His team discovered that the extra chromosome is evolving rapidly. He says this means it might be driving the evolution of new songbird species, which could explain why there is such an extraordinary diversity of songbird species on Earth today.