The making of a toad, through sex and cloning

Bufo baturae mating. © Matthias Stöck.

At a certain age, children want to know the answer to one simple question: how are babies made? And the answer seems simple enough, too (if sometimes a bit awkward): one sperm plus one egg equals a baby.

But this simple scenario breaks down quickly if we look around the animal kingdom (I won’t even get into the weird stuff plants do). There are tons of different ways of making babies, ones that use sperm and eggs, and others that just skip the sperm altogether. Some of the weirdest baby-making strategies come from animals that were originally a mix between two different species called hybrids.

Three at a time

Some time long ago, two toad species came together in a moment of whatever passes for passion among toads. The result is the Batura  (Bufo baturae), a toad that lives high up in the mountains of Pakistan. Batura toads are triploid. This means they have 3 sets of chromosomes, unlike humans that only have 2. Being triploid is tricky: you can’t divide an odd number of chromosomes by 2 to make an egg or sperm, which means you can’t reproduce sexually with a triploid mom and dad to get a triploid baby.

Batura toads, however, were reported 9 years ago to do just that. A paper this month in the Proceedings of the Royal Society B by Matthias Stock and colleagues, finally solves the mystery about how this is possible.

The making of a triploid

Triploid species are not a new thing, so how do they normally reproduce? There are many different strategies that have been discovered. A triploid species might be entirely female and reproduce asexually, so that all her daughters are genetic clones of her. Some all-female hybrid species will still mate with males from one of the original parent species, but the sperm are only used as a trigger for starting development; the baby doesn’t get any of its genes from “dad”.

Male B. baturae. © Matthias Stöck.

Other strategies also use males from one of the original species as the dad, using a part of his genome to make more females, but not other males. What makes the Batura toads so interesting is that hybrids mate with hybrids to make both males and females, all of which are triploid, through a mixture of cloning and sex.

The previous paper in 2002 found that these toads have 2 distinct sets of chromosomes, one from each of the original parent species that came together to make the hybrids. For simplicity, I’ll just call one set of chromosomes “A” and the other one “B”. All of the toads have one “A” set and 2 “B” sets.

How Batura toads reproduce. The female passes on a set of A chromosomes that she got from her mother. These are identical in all the offspring from this female. The female’s 2 sets of B chromosomes recombine to make a new, reshuffled B set. Each B set gets paired with a copy of the A set and put into an egg. The male throws away all of his A chromosomes, reshuffles the B’s, and each of his sperm carry one set of B chromosomes .The resulting offspring are all triploid like the parents, with an unchanged set of A chromosomes and a brand new set of B’s.

The authors of the paper looked at patterns of inheritance of 15 DNA sequences in the toad. They found that all sequences that came from the “A” set were only passed to offspring from the mother. So, even though males had “A” chromosomes, they never put them into their sperm. That’s where the cloning part comes in: all the “A” chromosomes are clones.

But what about the sex?

Just like in humans, the “B” sets of chromosomes recombine in the toads. There are 11 chromosomes in each set, so after recombination the egg or sperm of the toad will carry 11 chromosomes with a gene variant combination never seen before. Each toad gets one set of “B” from its mom and one from its dad.

Of course, we still don’t really understand how the egg or sperm cell knows that it should make a second copy of the “A” set (so that 2 eggs can be made) or to simply throw them out (to make sperm).

The future of the Batura toad chromosomes

This mode of reproduction has never been seen before, and maybe won’t be seen again. This species of toad is fairly young, so it hasn’t had enough time to accumulate too many damaging mutations in the “A” chromosomes. But just like the Y chromosome in humans and many other mammals, we would expect to start seeing these chromosomes shrivel up as they accumulate junk. Recombination is usually the cure for harmful mutations, but since each toad only has one copy, there is nothing for them to recombine with.

There is also the interesting possibility that one or more of the “A” chromosomes could evolve a sex-determining gene (if it hasn’t started to already) and become the basis for a female-determining system. If this happened, males would stop inheriting the “A” chromosomes altogether and they would only exist in females. I guess I’ll have to check back in a few million years and see if this comes true :)

The “B” chromosomes on the other hand should stay going strong. They’re getting a regular check-up each generation to fix any bad mutations. Also, unlike the “A” set, they get passed through males, so any mutations that negatively affect males aren’t able to accumulate.

Other crazy sex stories

Wasmannia auropunctata (worker female). She’s sterile, but at least she has a mom and dad.

Some of the weirdest sex strategies exist in social insects, like ants. In the little fire ant Wasmannia auropunctata, both males and females reproduce asexually. It’s easy to see how this works in females: they simply take their whole genome and pack it into an egg to make a new queen female. Since the egg already has 2 sets of chromosomes, it doesn’t need to get fertilized. (Worker females, however, are produced by normal sexual reproduction).

If queens don’t carry any genes from the males, which all have only one set of chromosomes, then does that mean males never pass on any genes, except to sterile workers? What’s in it for the males? In this crazy ant species, when making a male ant, the sperm actually displace or destroy the queen’s DNA and replace it with their own. How this happens is still a mystery. The consequence is that queens and males are never related at all. I guess it’s one way to avoid inbreeding.

A wood ant species. Image via Wikipedia.

Another weird sex story comes from a hybrid species of wood ants. These ants, like many ants, are haplo-diploid. Females have 2 sets of chromosomes and males have only one. Somehow, the female is able to separate her 2 sets by whether she inherited them from her mother or father. The ones she inherited from her mother will get put into eggs that are fertilized to make daughters. The chromosomes from her father will be used to make males. So, in this species, the males have no fathers, but they are genetically identical to their grandfathers.

Stock, M., Ustinova, J., Betto-Colliard, C., Schartl, M., Moritz, C., & Perrin, N. (2011). Simultaneous Mendelian and clonal genome transmission in a sexually reproducing, all-triploid vertebrate Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.1738

Fournier, D., Estoup, A., Orivel, J., Foucaud, J., Jourdan, H., Breton, J., & Keller, L. (2005). Clonal reproduction by males and females in the little fire ant Nature, 435 (7046), 1230-1234 DOI: 10.1038/nature03705

Kulmuni, J., Seifert, B., & Pamilo, P. (2010). Segregation distortion causes large-scale differences between male and female genomes in hybrid ants Proceedings of the National Academy of Sciences, 107 (16), 7371-7376 DOI: 10.1073/pnas.0912409107

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One thought on “The making of a toad, through sex and cloning

  1. Pingback: Sperm are Cool #1: Naked Mole Rats | Molecular Love (and other facts of life)

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