| Popis |
The endemic Lake Tanganyika cichlid genus Tropheus Boulenger 1898 consists of highly stenotypic rock dwellers, whose limited capacity for dispersal gave rise to over a hundred colour morphs. This made Tropheus an important model for evolutionary research. Yet, the genus’ taxonomy is confusing with most species being ill-defined and with many populations that cannot be unambiguously assigned to a valid species. As such, a revision of the genus is needed. Notwithstanding its high chromatic and genetic differentiation, the general morphology of Tropheus was, until recently, reported to be static. This hypothesis of morphological stasis was investigated in Tropheus duboisi Marlier 1959, an early diverged Tropheus that displays only minor intra-specific differences in colour pattern and is less stenotypic than its congeners. The results, however, revealed significant morphological differentiation between geographically isolated populations. This indicates that problems for a morphological revision of the genus not only result from the low degree of morphological differentiation between species, but also from the presence of intra-specific differentiation. One of the long-standing problems in Tropheus is the similarity between T. annectens Boulenger 1900 and T. polli Axelrod 1977, which are sometimes regarded as synonyms. Using museum specimens and recently collected material and a multidisciplinary approach, this problem could be resolved and both species are synonymised. In T. annectens as well, intra-specific differentiation is large. Surprisingly, the morphological differences found between different populations of T. annectens are mirrored by the differences encountered between populations of another, sympatric, Tropheus species: T. brichardi Nelissen & Thys van den Audenaerde 1975. Here, sympatric populations of different species are more alike in body proportions than allopatric populations of the same species. This finding suggests either a strong environmental effect on morphology or possible hybridisation. It also shows that, at least for Tropheus, body proportions might not be good characteristics to distinguish between species. A morphological study is performed that includes over a thousand Tropheus specimens from more than 150 locations around the lakeshore. This shows that in Tropheus, seven species could be identified. Three of these: T. moorii Boulenger 1898, T. brichardi and the hitherto undescribed species T. sp. ‘black’ have a large distribution and a large amount of inter-population chromatic variation. The other Tropheus species have more restricted distributions and are less variable in terms of colour pattern. At most parts of the lakeshore, species could be unambiguously defined using a few meristic or morphometric characteristics. At other regions, species boundaries were blurred by recent and ancient hybridisation events. This is the case at the edges of species’ distributions and at shallower parts of the Lake that have dried out relatively recently. For populations that could not be assigned easily to a nominal species, hybridisation was confirmed by incongruences between nuclear and mitochondrial phylogenetic reconstructions. Interestingly, hybrid zones present in Tropheus were mirrored by those of other Lake Tanganyika fishes, such as Mastacembelidae.
|
