Evolution of colour patterns in East African cichlid fish

African cichlid fishes have undergone outbursts of explosive speciation in several lakes, accompanied by rapid radiations in coloration and ecology. Little is known about the evolutionary forces that triggered these events but a hypothesis, published by Wallace Dominey in 1984, has figured prominently. It states that the evolution of colour patterns is driven by sexual selection and that these colour patterns are important in interspecific mate choice, a combination which holds the potential for rapid speciation. Here we present phylogenetic analyses that describe major events in colour evolution and test predictions yielded by Dominey's hypothesis. We assembled information on stripe patterns and the presence or absence of nuptial coloration from more than 700 cichlid species representing more than 90 taxa for which molecular phylogenetic hypotheses were available. We show that sexual selection is most likely the selection force that made male nuptial coloration arise and evolve quickly. In contrast, stripe patterns, though phylogenetically not conserved either, are constrained ecologically. The evolution of vertical bar patterns is associated with structurally complex habitats, such as rocky substrates or vegetation. The evolution of a horizontal stripe is associated with a piscivorous feeding mode. Horizontal stripes are also associated with shoaling behaviour. Strength of sexual selection, measured in terms of the mating system (weak in monogamous, strong in promiscuous species), has no detectable effects on stripe pattern evolution. In promiscuous species the frequency of difference between sister species in nuptial hue is higher than in pair bonding and harem forming species, but the frequency of difference in stripe pattern is lower. We argue that differences between the two components of coloration in their exposure to natural selection explain their very different evolutionary behaviour. Finally, we suggest that habitat-mediated selection upon chromomotor flexibility, a special form of phenotypic plasticity found in the river-dwelling outgroups of the lake-dwelling cichlids, explains the rapid and recurrent ecology-associated radiation of stripe patterns in lake environments, a new hypothesis that yields experimentally testable predictions.     

Molecular Characterization of Two Endothelin Pathways in East African Cichlid Fishes

The adaptive radiations of cichlid fishes in East Africa have been associated with the acquisition of evolutionary novelties as well as the ecological opportunities existing in the East African Great lakes. Two remarkable evolutionary innovations are the pharyngeal jaw apparatus, found in all cichlid species, and the anal fin egg-spots of mouthbrooding cichlids. Based on their conserved functions during the development of both the jaw apparatus and pigmentation, the endothelin ligands and receptors form a putative link between these naturally and sexually selected traits. Here we study the evolutionary history of four members of two endothelin pathways (Edn1/EdnrAa and Edn3b/EdnrB1a) to elucidate their possible roles during the evolution and development of key innovations in East African cichlids species. The analyses performed on partial sequences (ca. 6,000 bp per taxon) show that all four endothelin family members evolved under purifying selection, although both ligands are characterized by an accelerated rate of protein evolution in comparison to the receptors. In accordance with earlier findings, we show that the mature protein sequence of Edn1 and Edn3 are highly conserved, also in cichlids, whereas the preproendothelin parts are variable indicating relaxed selective constraints. In the receptors, nonsynonymous substitutions were mainly found in the ligand-binding domains suggesting functional divergence. Gene expression assays with Real-Time PCR indeed reveal that the two studied endothelin pathways are expressed in the cichlid pharyngeal jaw and in the haplochromine egg-spot (among other pigment-cell containing tissues), suggesting their involvement during morphogenesis of naturally and sexually selected traits in cichlids.     

Intrasexual competition among females and the stabilization of a conspicuous colour polymorphism in a Lake Victoria cichlid fish

The maintenance of colour polymorphisms within populations has been a long-standing interest in evolutionary ecology. African cichlid fish contain some of the most striking known cases of this phenomenon. Intrasexual selection can be negative frequency dependent when males bias aggression towards phenotypically similar rivals, stabilizing male colour polymorphisms. We propose that where females are territorial and competitive, aggression biases in females may also promote coexistence of female morphs. We studied a polymorphic population of the cichlid fish Neochromis omnicaeruleus from Lake Victoria, in which three distinct female colour morphs coexist: one plain brown and two blotched morphs. Using simulated intruder choice tests in the laboratory, we show that wild-caught females of each morph bias aggression towards females of their own morph, suggesting that females of all three morphs may have an advantage when their morph is locally the least abundant. This mechanism may contribute to the establishment and stabilization of colour polymorphisms. Next, by crossing the morphs, we generated sisters belonging to different colour morphs. We find no sign of aggression bias in these sisters, making pleiotropy unlikely to explain the association between colour and aggression bias in wild fish, which is maintained in the face of gene flow. We conclude that female-female aggression may be one important force for stabilizing colour polymorphism in cichlid fish.     

Colour diversification in Malawi cichlids: evidence for adaptation,reinforcement or sexual selection?

The mechanisms responsible for the explosive radiations of haplochromine cichlid fishes in Africa's great lakes remain controversial. Since species thought to be closely related often differ most apparendy in male breeding colours, I examined patterns of male colour variation in rock-dwelling cichlids from Lake Malawi to test whether initial divergence between species is likely to have been caused by adaptation to differing habitats, by selection against hybridization (reinforcement or character displacement), or by sexual selection. I found that all significant variation in overall male colour occurs within, not between, species complexes, in contrast to variation in habitat and behavioural traits, which differed significandy between complexes. Male colour does not vary significandy with habitat characteristics such as water depth, and sympatric species differ no more in colour than allopatric ones. These results fail to provide support for adaptation or reinforcement as initial causes of colour divergence, and are consistent with the possibility that divergence may instead have been caused by sexual selection, but a direct test of sexual selection is not yet possible.     

African cichlid fish: a model system in adaptive radiation research

The African cichlid fish radiations are the most diverse extant animal radiations and provide a unique system to test predictions of speciation and adaptive radiation theory. The past few years have seen major advances in the phylogenetics, evolutionary biogeography and ecology of cichlid fish. Most of this work has concentrated on the most diverse radiations. Unfortunately, a large number of small radiations and 'non-radiations' have been overlooked, potentially limiting the contribution of the cichlid system to our understanding of speciation and adaptive radiation. I have reviewed the literature to identify 33 intralacustrine radiations and 76 failed radiations. For as many as possible I collected information on lake size, age and phylogenetic relationships. I use these data to address two questions: (i) whether the rate of speciation and the resulting species richness are related to temporal and spatial variation in ecological opportunity and (ii) whether the likelihood of undergoing adaptive radiation is similar for different African cichlid lineages. The former is a key prediction of the ecological theory of adaptive radiation that has been presumed true but remains untested for cichlid radiations. The second is based on the hypothesis that the propensity of cichlids to radiate is due to a key evolutionary innovation shared by all African cichlids. The evidence suggests that speciation rate declines through time as niches get filled up during adaptive radiation: young radiations and early stages of old radiations are characterized by high rates of speciation, whereas at least 0.5 Myr into a radiation speciation becomes a lot less frequent. The number of species in cichlid radiations increases with lake size, supporting the prediction that species diversity increases with habitat heterogeneity, but also with opportunity for isolation by distance. Finally, the data suggest that the propensity to radiate within lakes is a derived property that evolved during the evolutionary history of some African cichlids, and the appearance of which does not coincide with the appearance of proposed key innovations in morphology and life history.     

Cichlid genomics and phenotypic diversity in a comparative context

Cichlid fishes provide an excellent natural system for integrating studies of genomics and adaptive radiation. Cichlids are unique in comprising a substantial fraction of all vertebrate species, possessing unique jaw structures, displaying an exceptional range of breeding systems, and exhibiting rampant phenotypic convergence. The rate of divergence in cichlid jaws, teeth, color patterns, visual systems, reproductive biology, and mating behaviors is unparalleled among vertebrates. I discuss ways rapid divergence in cichlids and other adaptive radiations make understanding the genomic basis of adaptive divergence more tractable. Then, I briefly overview some major findings and insights into vertebrate adaptation that have been gained through cichlid genetic studies. Finally, I discuss the extensive evolutionary replication provided by cichlid adaptive radiations and their potential for studies of genotype-to-phenotype mapping.     

Intraspecific sexual selection on a speciation trait, male coloration, in the Lake Victoria cichlid Pundamilia nyererei

The haplochromine cichlids of Lake Victoria constitute a classical example of explosive speciation. Extensive intra- and interspecific variation in male nuptial coloration and female mating preferences, in the absence of postzygotic isolation between species, has inspired the hypothesis that sexual selection has been a driving force in the origin of this species flock. This hypothesis rests on the premise that the phenotypic traits that underlie behavioural reproductive isolation between sister species diverged under sexual selection within a species. We test this premise in a Lake Victoria cichlid, by using laboratory experiments and field observations. We report that a male colour trait, which has previously been shown to be important for behavioural reproductive isolation between this species and a close relative, is under directional sexual selection by female mate choice within this species. This is consistent with the hypothesis that female choice has driven the divergence in male coloration between the two species. We also find that male territoriality is vital for male reproductive success and that multiple mating by females is common.     

Male mating preferences pre-date the origin of a female trait polymorphism in an incipient species complex of Lake Victoria cichlids

Disruptive sexual selection on colour patterns has been suggested as a major cause of diversification in the cichlid species flock of Lake Victoria. In Neochromis omnicaeruleus, a colour and sex determination polymorphism is associated with a polymorphism in male and female mating preferences. Theoretical work on this incipient species complex found conditions for rapid sympatric speciation by selection on sex determination and sexual selection on male and female colour patterns, under restrictive assumptions. Here we test the biological plausibility of a key assumption of such models, namely, the existence of a male preference against a novel female colour morph before its appearance in the population. We show that most males in a population that lacks the colour polymorphism exhibit a strong mating preference against the novel female colour morph and that reinforcement is not a likely explanation for the origin of such male preferences. Our results show that a specific condition required for the combined action of selection on sex determination and sexual selection to drive sympatric speciation is biologically justified. Finally, we suggest that Lake Victoria cichlids might share an ancestral female recognition scheme, predisposing colour monomorphic populations/species to similar evolutionary pathways leading to divergence of colour morphs in sympatry.     

The genetic architecture of male colour differences between a sympatric Lake Malawi cichlid species pair

The genetic basis of traits involved in reproductive isolation is a key parameter in models of sympatric speciation by sexual selection, a potential mechanism driving the explosive radiation of East African cichlids. Analysis of hybrid crosses between two sympatric Lake Malawi cichlid species, representing the extremes of the extant colour distribution, generated Castle-Wright estimates of four to seven loci controlling colour differences. Segregation patterns deviated from a purely additive model with a significant contribution from dominance, and possibly also epistasis. Evidence was found for a strong influence of autosomal loci. As departures from simple additive variation could effect the operation of models of sympatric speciation, dominance and epistasis should not be neglected.     

Temporal Patterns of Diversification across Global Cichlid Biodiversity (Acanthomorpha: Cichlidae)

The contrasting distribution of species diversity across the major lineages of cichlids makes them an ideal group for investigating macroevolutionary processes. In this study, we investigate whether different rates of diversification may explain the disparity in species richness across cichlid lineages globally. We present the most taxonomically robust time-calibrated hypothesis of cichlid evolutionary relationships to date. We then utilize this temporal framework to investigate whether both species-rich and depauperate lineages are associated with rapid shifts in diversification rates and if exceptional species richness can be explained by clade age alone. A single significant rapid rate shift increase is detected within the evolutionary history of the African subfamily Pseudocrenilabrinae, which includes the haplochromins of the East African Great Lakes. Several lineages from the subfamilies Pseudocrenilabrinae (Australotilapiini, Oreochromini) and Cichlinae (Heroini) exhibit exceptional species richness given their clade age, a net rate of diversification, and relative rates of extinction, indicating that clade age alone is not a sufficient explanation for their increased diversity. Our results indicate that the Neotropical Cichlinae includes lineages that have not experienced a significant rapid burst in diversification when compared to certain African lineages (rift lake). Neotropical cichlids have remained comparatively understudied with regard to macroevolutionary patterns relative to African lineages, and our results indicate that of Neotropical lineages, the tribe Heroini may have an elevated rate of diversification in contrast to other Neotropical cichlids. These findings provide insight into our understanding of the diversification patterns across taxonomically disparate lineages in this diverse clade of freshwater fishes and one of the most species-rich families of vertebrates.     

Recent speciation between sympatric Tanganyikan cichlid colour morphs

Lake Tanganyika, Africa's oldest lake, harbours an impressive diversity of cichlid fishes. Although diversification in its radiating groups is thought to have been initially rapid, cichlids from Lake Tanganyika show little evidence for ongoing speciation. In contrast, examples of recent divergence among sympatric colour morphs are well known in haplochromine cichlids from Lakes Malawi and Victoria. Here, we report genetic evidence for recent divergence between two sympatric Tanganyikan cichlid colour morphs. These Petrochromis morphs share mitochondrial haplotypes, yet microsatellite loci reveal that their sympatric populations form distinct genetic groups. Nuclear divergence between the two morphs is equivalent to that which arises geographically within one of the morphs over short distances and is substantially smaller than that among other sympatric species in this genus. These patterns suggest that these morphs diverged only recently, yet that barriers to gene flow exist which prevent extensive admixture despite their sympatric distribution. The morphs studied here provide an unusual example of active diversification in Lake Tanganyika's generally ancient cichlid fauna and enable comparisons of speciation processes between Lake Tanganyika and other African lakes.     

Polymorphic microsatellite loci optimised for studies on the convict cichlid fish (<Emphasis Type="Italic">Amatitlania siquia</Emphasis>)

The neotropical convict cichlid fish (Amatitlania siquia) is an important model species for the study of animal behaviour. It is therefore surprising that no genetic markers are available for this species. Here, we have optimised four polymorphic microsatellite loci for use with this fish, where each locus comprises between 9 and 28 alleles. We demonstrate how these markers can be used to genotype fry (young) and for the analyses of genetic structure and prevalence of interfamilial mixing within biparental broods of wild convict cichlids. These microsatellites will facilitate future research, hitherto not possible, on the molecular, behavioural and evolutionary ecology of the convict cichlid and possibly other closely related species.     

What big lips are good for: on the adaptive function of repeatedly evolved hypertrophied lips of cichlid fishes

Linking phenotypic traits to an adaptive ecological function is a major goal of evolutionary biology. However, this task is challenging and has been accomplished in only a handful of species and ecological model systems. The repeatedly evolved adaptive radiations of cichlid fishes are composed of an enormously diverse set of species that differ in trophic morphology, body shape, coloration, and behaviour. Ecological guilds of species with conspicuously hypertrophied lips have evolved in parallel in all major cichlid radiations and are characterized by large lips and pointed and narrow heads. In the present study, we experimentally tested the adaptive significance of this set of conspicuous traits by comparing the success of hypertrophied‐lipped and closely‐related thin‐lipped endemic Lake Victoria cichlids in a novel foraging assay. The hypertrophied‐lipped species (Haplochromis chilotes) was clearly more successful in exploiting food resources from narrow crevices and the observed difference in foraging success increased more at narrower angles. Furthermore, pronounced differences in exploratory behaviour between the species suggest that the evolution of hypertrophied‐lipped species involved the co‐evolution of a suite of traits that include foraging behaviour. The repeated evolution of hypertrophied‐lip morphology in conjunction with a narrow and pointed head shape in cichlids represents an evolutionary innovation that facilitates foraging in rocky crevices, thus allowing access to a novel niche. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 448–455.     

Colour-assortative mating among populations of Tropheus moorii, a cichlid fish from Lake Tanganyika, East Africa

The species flocks of cichlid fishes in the East African Lakes Tanganyika, Malawi and Victoria are prime examples of adaptive radiation and explosive speciation. Several hundreds of endemic species have evolved in each of the lakes over the past several thousands to a few millions years. Sexual selection via colour-assortative mating has often been proposed as a probable causal factor for initiating and maintaining reproductive isolation. Here, we report the consequences of human-mediated admixis among differentially coloured populations of the endemic cichlid fish Tropheus moorii from several localities that have accidentally been put in sympatry in a small harbour bay in the very south of Lake Tanganyika. We analysed the phenotypes (coloration) and genotypes (mitochondrial control region and five microsatellite loci) of almost 500 individuals, sampled over 3 consecutive years. Maximum-likelihood-based parenthood analyses and Bayesian inference of population structure revealed that significantly more juveniles are the product of within-colour-morph matings than could be expected under the assumption of random mating. Our results clearly indicate a marked degree of assortative mating with respect to the different colour morphs. Therefore, we postulate that sexual selection based on social interactions and female mate choice has played an important role in the formation and maintenance of the different colour morphs in Tropheus, and is probably common in other maternally mouthbrooding cichlids as well.     

Incipient speciation in sympatric Nicaraguan crater lake cichlid fishes: sexual selection versus ecological diversification.

The growing body of empirical evidence for sympatric speciation has been complemented by recent theoretical treatments that have identified evolutionary conditions conducive to speciation in sympatry. The Neotropical Midas cichlid (Amphilophus citrinellum) fits both of the key characteristics of these models, with strong assortative mating on the basis of a colour polymorphism coupled with trophic and ecological differentiation derived from a polymorphism in their pharyngeal jaws. We used microsatellite markers and a 480 bp fragment of the mitochondrial DNA control region to study four polymorphic populations of the Midas cichlid from three crater lakes and one large lake in Nicaragua in an investigation of incipient sympatric speciation. All populations were strongly genetically differentiated on the basis of geography. We identified strong genetic separation based on colour polymorphism for populations from Lake Nicaragua and one crater lake (Lake Apoyo), but failed to find significant genetic structuring based on trophic differences and ecological niche separation in any of the four populations studied. These data support the idea that sexual selection through assortative mating contributes more strongly or earlier during speciation in sympatry than ecological separation in these cichlids. The long-term persistence of divergent cichlid ecotypes (as measured by the percentage sequence divergence between populations) in Central American crater lakes, despite a lack of fixed genetic differentiation, differs strikingly from the patterns of extremely rapid speciation in the cichlids in Africa, including its crater lakes. It is unclear whether extrinsic environmental factors or intrinsic biological differences, e.g. in the degree of phenotypic plasticity, promote different mechanisms and thereby rates of speciation of cichlid fishes from the Old and New Worlds.     

Sympatric speciation and extinction driven by environment dependent sexual selection

A theoretical model is studied to investigate the possibility of sympatric speciation driven by sexual selection and ecological diversification. In particular we focus on the rock-dwelling haplochromine cichlid species in Lake Victoria. The high speciation rate in these cichlids has been explained by their apparent ability to specialize rapidly to a large diversity of feeding niches. Seehausen and colleagues however, demonstrated the importance of sexual selection in maintaining reproductive barriers between species. Our individual-orientated model integrates both niche differentiation and a Fisherian runaway process, which is limited by visibility constraints. The model shows rapid sympatric speciation or extinction of species, depending on the strength of sexual selection.     

Evolution: love thy neighbour

Five neighbouring populations of a cichlid fish from Lake Malawi differ in male courtship colour. Females show strong mate preferences for male coloration from their own populations. This suggests that sexual selection is an important factor contributing to speciation in cichlids.     

Geometric morphometrics applied to viscerocranial bones in three populations of the Lake Tanganyika cichlid fish Tropheus moorii

Lake Tanganyika comprises the oldest and most diverse species flock of cichlid fishes. Many species are subdivided into numerous populations, often classified as geographical races, colour morphs or sister species. Unlike younger species flocks, in which speciation is accompanied by eco-morphological diversification that of Lake Tanganyika is at a mature stage with little further morphological change, most probably caused by stabilizing selection. This study addresses body shape differences among three genetically distinct but morphologically similar populations of Tropheus  moorii , by focusing on bony structures of the cichlid head. We test by means of geometric morphological methods whether shape changes in the cichlid head are based on particular osteological differences. Specimens were disarticulated enzymatically, and standardized digital images of the disarticulated bony elements were taken. A landmark system was established for the dentary, articular, premaxilla, quadrate and the preopercle. Only the dentary shows significant differentiation among the three populations. Since all three populations live in similar cobble habitats and occupy the same trophic niche, the observed difference in the shape of the dentary can either be explained by different directional selection due to subtle habitat differences, or by neutral drift constrained by borders enforced through stabilizing selection. Lack of difference might indicate stabilizing selection on bone shape.     

Positive selection and gene conversion in SPP120, a fertilization-related gene, during the East African cichlid fish radiation

The ability to infer historical natural selection from sequence data aides in finding genes that might be important in adaptation and the formation of new species. As the fastest evolving and largest known vertebrate radiation, the cichlid fish of the African Great Lakes exhibit a wide range of recent morphological diversification. We used DNA databases, mostly of expressed sequence tags, to find candidate orthologous coding sequences from 2 tribes of cichlids and, using an automated procedure, scanned these sequence pairs for high dN/dS, the signal of positive selection and protein adaptation. The results included vertebrate genes commonly found to be under selection (e.g., major histocompatibility complex [MHC] loci) as well as genes known to be important specifically in the cichlid radiation (e.g., long-wave-sensitive opsins). Further investigation focused on a gene encoding a fertilization-related protein, SPP120, which was previously known only from cichlids. Using maximum likelihood analysis on novel SPP120 cDNA sequences from a range of African cichlids, we demonstrate the influence of positive selection in a specific subregion of the protein. We also show that SPP120 is a tandemly arranged, multicopy gene evolving with occasional interlocus gene conversion. A search of the Medaka genome database also revealed a tandem arrangement of multiple SPP120 copies and evolutionary rate differences between Medaka gene subregions mirroring those found for cichlids. Combined, these results suggest that SPP120 has been under repeated diversifying selection for over 100 Myr.