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20-chromosome toads

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Guttural toads; photo by Petra Karstedt, from wikipedia.

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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).

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).

Southern panther toad Amietophrynus pantherinus; image by Serban Proches, from wikipedia.

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.

Guttural toad; image by Damien Boilley, from wikipedia.

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…

For previous articles on hyloid anurans see…

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 American Museum 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.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Andreas Johansson 4:51 am 09/12/2011

    I’m getting the impression that chromosome count is rather more evolutionary stable among these toads than among, say, mammals. If that’s correct, any idea as to the reason?

    Link to this
  2. 2. victorg 11:17 pm 09/12/2011

    There’s a similar chromosome-count clade in Hylidae (I believe they’re all Dendropsophus now?):

    Chek AA, Lougheed SC, Bogart JP, Boag PT: Perception and history: molecular phylogeny of a diverse group of neotropical frogs, the 30-chromosome Hyla (Anura: Hylidae). Mol Phylogenet Evol 2001, 18:370-385 http://post.queensu.ca/~lough/Chek_30_chromosome.pdf

    Link to this
  3. 3. imhennessy 10:15 pm 09/14/2011

    I’m also curious about both the stability and importance of chromosome count. I understand that there are some mammals with different chromosome counts which can produce viable crosses, although they’re in the minority. If I’m recalling correctly, they may include some of those fascinating cattle you’ve been writing about of late.

    I always enjoy finding a new article by you. Thanks.

    Link to this
  4. 4. CS Shelton 9:49 pm 09/15/2011

    Oh wow… The reversing toad interests me. If it’s true that it’s part of the 20 chromosome clade and did revert to 22, then this question occurs: Was the chromosome that split the same one that originally fused, and if not, did it split along the original fusion line?

    I’m sure I’m not using the correct terminology, but I hope you get my meaning.

    Link to this
  5. 5. Christopher Taylor 11:34 pm 09/15/2011

    Finally given in and registered. Note to self: never actually read the terms and conditions they ask you to agree to for these things. Seriously, it’s better for your gorge that you simply don’t know.

    Anywho, in response to imhennessy, what I’ve read suggests that the main controlling factor on interfertility of organisms with different chromosome numbers isn’t the chromosome number per se, it’s whether or not the chromosomes can line up appropriately during meiosis for each of the daughter zygotes to get the proper half-complement of chromosomes. Compare this case, where small chromosome number differences create different species, with this case, where large chromosome differences do not result in speciation (sorry about the self-promotion).

    If this inference is correct, then, looking at karyotype differences alone, hybrids between humans and chimpanzees should not only be viable, they’d quite probably be fertile.

    Link to this
  6. 6. Christopher Taylor 10:50 am 09/16/2011

    Did my comment go into moderation, or has it vanished?

    Link to this
  7. 7. Christopher Taylor 10:51 am 09/16/2011

    Sorry, I see it’s back now.

    Link to this
  8. 8. Spookpadda 7:57 am 03/12/2012

    Just discovered this now – great blog, thanks.

    Toad chromosome counts are extremely stable, which is what makes the 20-chromosome group, and the possibility of reversion, interesting. I’m not sure why they are so stable or whether there is much gene swapping among chromosomes between speciation events. They are yet to be “discovered” by practitioners of chromosome painting.

    Actually the 20-chromosome group is far from well resolved and all analyses to date have woeful taxon sampling – missing out the most interesting ones. There are some well resolved clades such as the garmani – rangeri cluster and another cluster around gutturalis – xeros but then there are some enigmatic species such as Amietophrynus maculatus, A. regularis and A. pardalis / pantherinus which float around mysteriously. Several major constituents have not yet been karyotyped, in particularly most forest lineages of central and west Africa and the Nile valley, with the possibility of other reversions (A. mauritanicus) or additional lineages that are not actually part of this complex.

    One problem in the analysis of this group is that many of these species are very similar to each other, even those that are not close relatives (such as A. gutturalis vs. A. regularis). In some cases different studies have assigned molecular data to the same species name that seem actually to be from different species in different clades. A related problem is that several species have extremely wide distributions, and vary across the continent, in genetics, appearance and calls, with the possibility of cryptic species. In other cases, such as A. brauni and A. garmani, very closely related species look very different, due to adaptation to forest vs. savanna habitats, and have previously been assigned to totally different phenetic clusters.

    I think you are overplaying the issue of hybridisation. Crosses between A. rangeri and A. gutturalis are quite common but seem generally to be sterile and behaviourally anomolous (rangeri quacks, gutturalis snores, the hybrids are totally confused or quiet). This is related to habitat change but these species do overlap in their natural habitat and there has probably been some degree of natural hybridisation over many millenia. Hybridisation is often postulated among other groups but rarely confirmed, and almost always very limited, which is not too surprising given the difficulty in just identifying different species. I’m not aware of any significant genetic introgression among species in this group (although it would not be too surprising). As almost always the greatest conservation issue is simply loss of habitat, probably most acutely for the local endemics of the Ethiopian and Kenyan highlands.

    You make a couple of jarring statements. There’s nothing unusual about the parotoids of A. garmani (or at least A. poweri because no-one has recently worked in Somalia to pin down the real A. garmani). Ignoring minor differences they resemble closely those in A. rangeri or A. regularis. A. lemairii on the other hand is a very unusual toad and is likely as not to be confused with a ranoid. Southern panther toad is an appalling misnomer for the Western Leopard Toad, A. pantherinus – an adoption from the awful Frank and Ramus names based on literal translation of the latin (leopards have spots, panthers don’t). A. superciliosus is worth mention as a big weird forest species that doesn’t fit the mode of this group. Perhaps the most unusual species though may be A. perreti, a local endemic to granite inselbergs in Nigeria, with a suctorial tadpole that lives in seepage films on these rock mountains (http://www.arkive.org/nigerian-toad/bufo-perreti/image-G33027.html). There has not been any new information published on this species since the 1960s and it has not been included in any recent analyses.

    I don’t think that you can really say that the Southern African species are still understudied. There is a wealth of information out there on them. The most recent book is Du Preez & Carruthers (2009) “A complete guide to the Frogs of Southern Africa”. This must surely be one of the best and most comprehensive frog guides on earth (following on from a wealth of previous guides for this region).

    Apart from the 20 chromosome toads, and the associated genera, a similar sampling problem affects that other weird assortment of Bufonoids from Africa – the Nectophrynoides / Nectophryne / Werneria / Wolterstorffina / Didynamipus “group”. No molecular study to date has had adequate taxon sampling to sort out their relationships to each other or to the global phylogeny. There is suggestive evidence that these form a deep and diverse clade but I think it is too early, with too few genera included, to accept this as fact.

    Link to this

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