Three endemic species/morphs of haplochromine cichlid (above) are evolving in Lake Massoko, Tanzania, allowing us to investigate the genetic basis of sympatric speciation.
Tilapia Diversity in Tanzania. Tilapias are big, largely vegetarian cichlid fish that are farmed in warm freshwater throughout the world. Tilapia farming is reckoned to be worth $7.6bn a year, and growing fast. Many other farmed fish, like salmon, have to be few on other fish caught from the seas, but tilapia are much more sustainable and can be fed mainly on plant material and agricultural waste. They are the fish of the future and it will be important to conserve unique native genetic resources, for future strain improvement, just as the World has made great efforts to preserve native diversity of crop plants. Tilapias originally come from Africa, and Tanzania is a particular hotspot for native species (more than 25). However, these unique species are at risk from the very success of tilapias in fish farming. The fast growing 'Nile Tilapia' has become established in Lake Victoria and this species has been deliberately stocked in fish ponds all over the country (as in many other parts of Africa). From these ponds, it often escapes into natural river systems, and indeed has been deliberately introduced into a number of lakes and reservoirs. It is not clear what effects this will have on the native species, but in South Africa, they are very worried that it is replacing the native Mozambique tilapia. Nile Tilapia seems to be very prone to hybridise with native tilapia, so there may soon be very few pure native populations left. This is certainly happening in Tanzania too. We are partners in a project led by Martin Genner at Bristol and Ben Ngatunga (Tanzania Fisheries Research Institute) to survey various lakes and rivers in Tanzania and find out about the remaining native strains and make recommendations about how to conserve them. This is funded by a Royal Society/Leverhulme Trust Grant (2014-2017). We are hoping to get additional funding to look at the genomes of these tilapias.
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Ecological Speciation of Cichlid Fishes in Lake Massoko. We have found 3 distinct morphs (or species?) of haplochromine cichlid fishes in Lake Massoko, a tiny crater lake in southern Tanzania. The lake is 700m in diameter and completely isolated from other water bodies. The lake is over 40m deep, but seems to be deoxygenated below about 25m, so the fish either live in open water or in a very narrow ring round the edge. There are catfish (Clarias gariepinus) and 2 kinds of tilapia (Oreochromis squamipinnis, Tilapia rendalli) in the lake. But, we are most interested in the haplochromines, which are derived from the widespread Astatotilapia calliptera which lives in streams nearby. In the shallow water, there is a form that grows to a large size and has yellow males, rather like the stream-living ancestor. In deep water there is another big form, but with dark blue males. They differ in body shape, diet and genetically, too. There is also a 3rd form that matures at small sizes and seems to breed at all depths. We have seen schools of small Astatotilapia feeding in midwater, which is very unusual behaviour for members of this genus. These are probably the dwarf form. We have funding from the Leverhulme Trust (2014-2017) to sequence the full genomes of 200 male Astatotilapia from this lake and to compare genetic variants with differences in anatomical structures that might be related to the different niches that these fish are adapting to exploit. We will also be studying the mate preferences of females in laboratory trials at Bangor University. We are working with Martin Genner at Bristol, Milan Malinsky and Eric Miska at Cambridge, Richard Challis at Edinburgh and Richard Durbin at the Sanger Institute.
Introduction of alien tilapias for fish farming threatens unique wild relatives across Africa. This fish (above), collected from the wild in Tanzania in 2014, is probably a hybrid between the alien Nile Tilapia (striped tail) and the native Wami Tilapia (blotches on the flanks), Oreochromis urolepis.
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Parallel Evolution in Crater Lakes: Lake Massoko is not the only crater lake in southern Tanzania, and many of the other lakes nearby contain closely related haplochromine cichlids. Several other lakes seem to contain forms with yellowish males living in shallow water, and forms with darker males living in deeper water. This may represent different stages of a similar kind of speciation. We have started to look at morphological differences among the different forms within and been lakes, beginning with a masters project by Emma Tiernan. Dr Henrik Kusche, who has been awarded a DAAD postdoctoral fellowship from Germany, plans to investigate this in more depth, additionally looking at differences in ecology and diet among the morphotypes. We hope to follow this up with detailed molecular studies. Preliminary analysis by Richard Challis, using a grant from the Fisheries Society of the British Isles, is supporting the idea that these forms are diverging in situ within each crater lake, allowing us to look at replicated speciation events.
Right: pale and dark male crater lake cichlid morphotypes (clockwise from top left): Lake Itamba, Lake Ilamba, Lake Kingiri, Lake Massoko.
Right: pale and dark male crater lake cichlid morphotypes (clockwise from top left): Lake Itamba, Lake Ilamba, Lake Kingiri, Lake Massoko.