Differential use of multiple genetic sex determination systems in divergent ecomorphs of an African crater lake cichlid

African cichlid fishes not only exhibit remarkably high rates of speciation but also have some of the fastest evolving sex determination systems in vertebrates. However, little is known empirically in cichlids about the genetic mechanisms generating new sex-determining variants, what forces dictate their fate, the demographic scales at which they evolve, and whether they are related to speciation. To address these questions, we looked for sex-associated loci in full genome data from 647 individuals of Astatotilapia calliptera from Lake Masoko, a small isolated crater lake in Tanzania, which contains two distinct ecomorphs of the species. We identified three separate XY systems on recombining chromosomes. Two Y alleles derive from mutations that increase expression of the gonadal soma-derived factor gene (gsdf) on chromosome 7; the first is a tandem duplication of the entire gene observed throughout much of the Lake Malawi haplochromine cichlid radiation to which A. calliptera belongs, and the second is a 5 kb insertion directly upstream of gsdf. Both the latter variant and another 700 bp insertion on chromosome 19 responsible for the third Y allele arose from transposable element insertions. Males belonging to the Masoko deep-water benthic ecomorph are determined exclusively by the gsdf duplication, whereas all three Y alleles are used in the Masoko littoral ecomorph, in which they appear to act antagonistically among males with different amounts of benthic admixture. This antagonism in the face of ongoing admixture may be important for sustaining multifactorial sex determination in Lake Masoko. In addition to identifying the molecular basis of three coexisting sex determining alleles, these results demonstrate that genetic interactions between Y alleles and genetic background can potentially affect fitness and adaptive evolution.

Astatotilapia species (Teleostei, Cichlidae) from Malawi, Mozambique and Tanzania, excluding the basin of Lake Victoria.

The haplochromine cichlid fauna of the rivers and smaller lakes of Tanzania and SE Africa are of key importance in understanding the origins and inter-relationships of the great lake cichlid radiations of Malawi, Tanganyika and Victoria. Prior to formal taxonomic investigations, here we present the results of investigations of the type specimens, identification of type localities and superficial characterisation of freshly-collected material. The type locality of Astatotilapia bloyeti (Sauvage, 1883) is identified as the Mkondowa River, near Kilosa. This species appears to be the only haplochromine found in the Wami system. It is concluded that junior synonyms of A. bloyeti include A. strigigena (Pfeffer, 1893), A. kilossana (Steindachner, 1915) and A. paludinosa Greenwood, 1980. Astatotilapia sparsidens (Hilgendorf, 1905) is closely related and may constitute a junior synonym or a sister taxon. The range of A. bloyeti includes the Pangani system. The type locality for Astatotilapia gigliolii (Pfeffer, 1896) is identified as the Kingani River, or southern Ruvu, where it appears to be the only haplochromine in the river system. Junior synonyms include Astatotilapia vollmeringi (Steindachner, 1915) from the Great Ruaha River at Kidatu and Astatotilapia tweddlei Jackson, 1985 from Lakes Chilwa and Chiuta. The species is also reported from the lower part of the Rufiji system, the Ruvuma system and from other lakes and rivers in between. A few specimens were also collected in and around a fish farm at Songea, in the upper reaches of the Ruhuhu system (Lake Malawi catchment). Apart from this record, the only Astatotilapia species in the Lake Malawi catchment is A. calliptera. Four undescribed Astatotilapia species are identified from the Rufiji system, while other possibly undescribed taxa from the basins of Lake Tanganyika and Rukwa are discussed.

Identification of a novel sex determining chromosome in cichlid fishes that acts as XY or ZW in different lineages

Sex determination systems are highly conserved among most vertebrates with genetic sex determination, but can be variable and evolve rapidly in some. Here, we study sex determination in a clade with exceptionally high sex chromosome turnover rates. We identify the sex determining chromosomes in three interspecific crosses of haplochromine cichlid fishes from Lakes Victoria and Malawi. We find evidence for different sex determiners in each cross. In the Malawi cross and one Victoria cross the same chromosome is sex-linked but while females are the heterogametic sex in the Malawi species, males are the heterogametic sex in the Victoria species. This chromosome has not previously been reported to be sex determining in cichlids, increasing the number of different chromosomes shown to be sex determining in cichlids to 12. All Lake Victoria species of our crosses are less than 15,000 years divergent, and we identified different sex determiners among them. Our study provides further evidence for the diversity and evolutionary flexibility of sex determination in cichlids, factors which might contribute to their rapid adaptive radiations.

A new species of Diplotaxodon (Cichliformes: Cichlidae) from Lake Malaŵi

A species of haplochromine cichlid fish of the genus Diplotaxodon Trewavas, endemic to Lake Malaŵi is described: Diplotaxodon dentatus, new species. All eight type specimens were trawled together off Thumbi East Island in the Southeastern arm of the lake at 73 meters in 1985. They were initially identified as D. argenteus because the teeth on the oral jaws were fully exposed with a closed mouth. The shorter snout length of D. dentatus (26.6–29.2 % HL) clearly separates it from D. argenteus (31.7–34.2 % HL). A plot of a principal components analysis further supports the separation of D. dentatus from D. argenteus.

The coincidence of ecological opportunity with hybridization explains rapid adaptive radiation in Lake Mweru cichlid fishes

The process of adaptive radiation was classically hypothesized to require isolation of a lineage from its source (no gene flow) and from related species (no competition). Alternatively, hybridization between species may generate genetic variation that facilitates adaptive radiation. Here we study haplochromine cichlid assemblages in two African Great Lakes to test these hypotheses. Greater biotic isolation (fewer lineages) predicts fewer constraints by competition and hence more ecological opportunity in Lake Bangweulu, whereas opportunity for hybridization predicts increased genetic potential in Lake Mweru. In Lake Bangweulu, we find no evidence for hybridization but also no adaptive radiation. We show that the Bangweulu lineages also colonized Lake Mweru, where they hybridized with Congolese lineages and then underwent multiple adaptive radiations that are strikingly complementary in ecology and morphology. Our data suggest that the presence of several related lineages does not necessarily prevent adaptive radiation, although it constrains the trajectories of morphological diversification. It might instead facilitate adaptive radiation when hybridization generates genetic variation, without which radiation may start much later, progress more slowly or never occur.

Description of five new rheophilic Orthochromis species (Teleostei: Cichlidae) from the Upper Congo drainage in Zambia and the Democratic Republic of the Congo

Five new rheophilic haplochromine cichlid species are described from the Upper Congo drainage of Zambia and the Democratic Republic of the Congo: Orthochromis mporokoso sp. nov. and O. katumbii sp. nov. from the Bangwelu-Mweru ecoregion, O. kimpala sp. nov. and O. gecki sp. nov. from the Upper Lualaba ecoregion, and O. indermauri sp. nov. from the Lufubu River of the Lake Tanganyika ecoregion. Orthochromis kimpala sp. nov, O. gecki sp. nov., and O. indermauri sp. nov. are distinguished from all currently valid species of the genus Orthochromis Greenwood 1954, except for O. torrenticola (Thys van den Audenaerde 1963), by the presence of eggspots or eggspot-like maculae on the anal fin (vs. no eggspots). The three species can be easily distinguished from O. torrenticola by having three anal spines (vs. four anal spines). Moreover, all five new species can be individually distinguished from all currently known rheophilic taxa placed in the genera Orthochromis, Schwetzochromis Poll 1948 and the rheophilic species of the genus Haplochromis Hilgendorf 1888 (e.g. H. bakongo Thys van den Audenaerde 1964, H. snoeksi Wamuini Lunkayilako & Vreven 2010, H. vanheusdeni Schedel et al. 2014) either based on meristic values, morphometric distances and colouration patterns, or on a combination of them.