Frog Species Breaks Record for Most Sex Chromosomes in a Vertebrate

By Ryan F. Mandelbaum on at

You may have learned in school that humans have 23 pairs of chromosomes: 22 “autosomes” and their partners, which contain pretty much the same genes and in the same order, plus one pair of sex chromosomes that lead to the differentiation in sexual traits. But enough about us. The Amazonian Leptodactylus pentadactylus (smoky jungle frog) has six pairs of sex chromosomes.

Image: Analise Zocher/Flickr

A team of researchers in Brazil made this realisation with several lines of evidence, noting that this frog’s sex chromosomes require a whole different way of producing offspring. Plus, it’s record-breaking.

“The number of sex chromosomes is larger than the number of autosomes found, and these data represent the largest number of multiple sex chromosomes ever found among vertebrate species.” the authors write in the study published recently in the journal Chromosoma. This breaks the record previously held by the platypus, which has 10 sex chromosomes, 5 X and 5 Y.

The researchers point out that sex determination systems have evolved separately in different groups. In humans, offspring normally receive two pairs of X chromosomes or an X and a Y. The presence of a Y chromosome releases a hormone telling the body to develop a penis and testes (and so on) instead of a vagina and ovaries. In birds and other species it works the opposite way: there’s either ZZ or ZW, where the W signifies that the bird is going to lay eggs.

But things work way differently with these frogs. Scientists analysed their DNA and found six “X” and six “Y” chromosomes in males, and twelve “X” chromosomes in females. They also inserted a special fluorescent probe that binds to the DNA, and watched the cells get ready to replicate.

Again, remember us humans: Most of our cells have 46 chromosomes (23 pairs). Sperm and egg cells are formed through a process called “meiosis,” which results in those pairs separating. Babies develop when the sperm cell and the egg cell combine, each bringing 23 single chromosomes to the party (the egg has an “X,” and the sperm can have an “X” or a “Y”).

So how does it work with six pairs of sex chromosomes? In males, the Xs and Ys align in an “astonishing stable ring-shaped meiotic chain” of chromosomes, which arranges itself so that when the cells divide, the correct gametes form with the correct chromosomes.

“This is really outside what we think is ‘normal’ meiosis,” researcher Tony Gamble from Marquette University in Wisconsin told me in an email. “I’m excited about the paper because it shows just how incredibly weird sex chromosomes can be. Just when you think you’ve seen the weirdest thing imaginable—like the platypus and its 5X5Y system—along comes something even weirder. It also suggest this meiotic chain system may not be as rare as we think and should encourage scientists to explore sex chromosomes in a greater diversity of species.”

He noted that the paper was mostly observational—the challenge will be actually figuring out the evolution of these systems. “Understanding how these systems evolve and are maintained is a different story though and will likely prove much harder to figure out.”

Dang, evolution. You continue to confuse. [Chromosoma]

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