20-chromosome toads

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

More toads! (for previous articles in the series – required reading if you’re really interested – see the links below). In the previous article I introduced the idea that a large number of (mostly) poorly known African toads might be close relatives: they generally group together in cladograms, and – even when they don’t – they occupy the same general ‘section’ of the cladogram. While studies do differ on various details of the toad cladogram (see the previous article), several of them find ‘the African clade’ to be the sister-group to the rather larger clade that includes the majority of Eurasian toad lineages (Pramuk 2006, Van Bocxlaer et al. 2009, 2010). To remind you where we are in the toad tree as a whole, that previously used, highly simplified cladogram is shown below.

Here I’ll follow the topology recovered by Van Bocxlaer et al. (2010): Amietophrynus Frost et al., 2006 is sister to a clade that includes a Capensibufo Grandison, 1980 + Vandijkophrynus Frost et al., 2006 clade and a Mertensophryne Tihen, 1960 + Poyntonophrynus Frost et al., 2006 clade. There isn’t a great deal of natural history information on these toads - typical comments pertaining to several of the species are that “More fieldwork is required”, “This species would be very rewarding to study”, and “Some very useful fieldwork could be done by anyone with a tape recorder and a tadpole net during the rainy season when these frogs are breeding” (all quotes from Alan Channing’s 2001 Amphibians of Central and Southern Africa).

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A reasonably large, ecologically diverse group of African toads have 20 chromosomes (as opposed to 22). This appears to be a unique character and all of these toads have hence sometimes been referred to as the 20-chromosome toads. Frost et al. (2006) proposed that these should be recognised as a genus and coined the new name Amietophrynus for them (the name honours herpetologist Jean-Louis Amiet). At least some of the many ‘species groups’ included within Amietophrynus are probably monophyletic but substantial further study is needed to resolve the relationships here. Adding complexity to the whole ‘20 chromosome’ thing is the fact that the members of at least one of these species groups – known conventionally as the ‘Bufo pardalis group’ – seems to have undergone a reversal back to 22 chromosomes (Cunningham & Cherry 2004).

Many 20-chromosome toads are not all that different in overall appearance from Eurasian species, though some (like Garman’s toad A. garmani and the Kisolo toad A. kisoloensis, both of which are extremely widespread) appear distinctive in having elongate, strongly convex parotoids. Some species (like Ranger’s toad A. rangeri) have a ridge of glands that extend from the corner of the mouth to above the arm. In others (like Lemaire’s toad A. lemairii), the parotoids continue posteriorly along the animal’s back, reaching about level with the knees. A few species within the group are very unusual in being strongly aquatic. The forest-dwelling Kisolo toad A. kisoloensis of central-eastern Africa is large (females can have an SVL of 87 mm), rather smooth skinned, and with extensively webbed digits. Breeding males turn beautiful bright yellow during breeding but are otherwise olive greenish.

The secretions produced by the parotoid glands of some species are known to be highly toxic to other animals – Channing (2001) reports that individuals of other anuran species placed in the same bag as a Guttural toad A. gutturalis will die due to poisoning, and that “mammals of up to 5 kg will vomit and die if they absorb small amounts of the secretion” (p. 76). Epinephrine (= adrenaline) is partly to blame.

Vocal sacs are absent in some species (like the Somber toad A. funereus) raising, as usual, the question of how these toads find each other during the breeding season.

Some are strikingly marked, one of the best examples being the Southern panther toad A. pantherinus (a species that has recently undergone substantial decline due to urban encroachment, habitat loss and a large number of deaths on the roads). Some 20-chromosome toads are grassland specialists (a good example is the Asmara toad A. asmarae), others (like the Oban toad A. camerunensis) are creatures of tropical forested environments, and others are generalists that occur in diverse habitats (examples include A. djohongensis from Cameroon and Nigeria and the east African Keith’s toad A. kerinyagae).

One conservation problem seemingly affecting various species is hybridisation caused by habitat change. It seems that species previously segregated as stream-breeders or pond-breeders are now meeting up due to the creation of channels and dams and now hybridising, apparently for the first time. Ranger’s toad and the Guttural toad are now hybridising at several sites in South Africa, for example (Channing 2001). Seeing as I consulted Channing (2001) so many times while writing this article, here’s a picture of its front cover. Get it if you can - it includes great colour photos of loads of ‘African clade’ toad species.

And that’s where we’ll end for now. More toads real soon.

For previous articles in the Tet Zoo toads series see...

• Toadtastic - the invasion begins!

• Bidder's organ and the holy quest for synapomorphies

• Our sex lives in words and pictures (or, On the reproductive biology of the Bufonidae)

• Skulls, crests, snouts and giant poison glands: the heads of toads

• Toads of the world: first, (some) toads of the north

• The Natterjack, its life and times

• The resurrection of Anaxyrus

• South America, land of toads, part I: harlequins, redbellies and plump toads

• South America, land of toads part II: tree toads, Truebella, Frostius... oh, and did I mention the COMMUNAL NESTS?

• Blomberg’s toad and its omosternum-bearing buddies

• The toads series comes to SciAm: because Africa has toads too

For previous articles on hyloid anurans see...

• Britain's lost tree frogs: sigh, not another 'neglected native'

• Ghost frogs, hyloids, arcifery.. what more could you want?

• Green-boned glass frogs, monkey frogs, toothless toads

• It's the Helmeted water toad!

• Horn-headed biting frogs and pouches and false teeth

• More wide-mouthed South American horned frogs

• We need MORE FROGS

Refs - -

Channing, A. 2001. Amphibians of Central and Southern Africa. Cornell University Press, Ithaca & London.

Cunningham M, & Cherry MI (2004). Molecular systematics of African 20-chromosome toads (Anura: Bufonidae). Molecular phylogenetics and evolution, 32 (3), 671-85 PMID: 15288046

Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., De Sá, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green, D. M. & Wheeler, W. C. 2006. The amphibian tree of life. Bulletin of the AmericanMuseum of Natural History 297, 1-370.

Pramuk, J. 2006. Phylogeny of South American Bufo (Anura: Bufonidae) inferred from combined evidence. Zoological Journal of the Linnean Society 146, 407-452.

Van Bocxlaer, I., Biju, S. D., Loader, S. P. & Bossuyt, F. 2009. Toad radiation reveals into-India dispersal as a source of endemism in the Wester Ghats-Sri Lanka biodiversity hotspot. BMC Evolutionary Biology 2009, 9:131 doi:10.1186/1471-2148-9-131

- ., Loader, S. P., Roelants, K., Biju, S. D., Menegon, M. & Bossuyt, F. 2010. Gradual adaptation toward a range-expansion phenotype initiated the global radiation of toads. Science 327, 679-662.