Genome sequences of Tropheus moorii and Petrochromis trewavasae, two eco-morphologically divergent cichlid fishes endemic to Lake Tanganyika.

With more than 1000 species, East African cichlid fishes represent the fastest and most species-rich vertebrate radiation known, providing an ideal model to tackle molecular mechanisms underlying recurrent adaptive diversification. We add high-quality genome reconstructions for two phylogenetic key species of a lineage that diverged about ~ 3-9 million years ago (mya), representing the earliest split of the so-called modern haplochromines that seeded additional radiations such as those in Lake Malawi and Victoria. Along with the annotated genomes we analysed discriminating genomic features of the study species, each representing an extreme trophic morphology, one being an algae browser and the other an algae grazer. The genomes of Tropheus moorii (TM) and Petrochromis trewavasae (PT) comprise 911 and 918 Mbp with 40,300 and 39,600 predicted genes, respectively. Our DNA sequence data are based on 5 and 6 individuals of TM and PT, and the transcriptomic sequences of one individual per species and sex, respectively. Concerning variation, on average we observed 1 variant per 220 bp (interspecific), and 1 variant per 2540 bp (PT vs PT)/1561 bp (TM vs TM) (intraspecific). GO enrichment analysis of gene regions affected by variants revealed several candidates which may influence phenotype modifications related to facial and jaw morphology, such as genes belonging to the Hedgehog pathway (SHH, SMO, WNT9A) and the BMP and GLI families.

Evolution of body shape in sympatric versus non-sympatric Tropheus populations of Lake Tanganyika.

Allopatric speciation often yields ecologically equivalent sister species, so that their secondary admixis enforces competition. The shores of Lake Tanganyika harbor about 120 distinct populations of the cichlid genus Tropheus, but only some are sympatric. When alone, Tropheus occupies a relatively broad depth zone, but in sympatry, fish segregate by depth. To assess the effects of competition, we studied the partial co-occurrence of Tropheus moorii 'Kaiser' and 'Kirschfleck' with Tropheus polli. A previous study demonstrated via standardized breeding experiments that some observed differences between Tropheus 'Kaiser' living alone and in sympatry with T. polli have a genetic basis despite large-scale phenotypic plasticity. Using geometric morphometrics and neutral genetic markers, we now investigated whether sympatric populations differ consistently in body shape from populations living alone and if the differences are adaptive. We found significant differences in mean shape between non-sympatric and sympatric populations, whereas all sympatric populations of both color morphs clustered together in shape space. Sympatric populations had a relatively smaller head, smaller eyes and a more anterior insertion of the pectoral fin than non-sympatric populations. Genetically, however, non-sympatric and sympatric 'Kaiser' populations clustered together to the exclusion of 'Kirschfleck'. Genetic distances, but not morphological distances, were correlated with geographic distances. Within- and between-population covariance matrices for T. moorii populations deviated from proportionality. It is thus likely that natural selection acts on both phenotypic plasticity and heritable traits and that both factors contribute to the observed shape differences. The consistency of the pattern in five populations suggests ecological character displacement.

Water-level fluctuations and metapopulation dynamics as drivers of genetic diversity in populations of three Tanganyikan cichlid fish species.

Understanding how genetic variation is generated and maintained in natural populations, and how this process unfolds in a changing environment, remains a central issue in biological research. In this work, we analysed patterns of genetic diversity from several populations of three cichlid species from Lake Tanganyika in parallel, using the mitochondrial DNA control region. We sampled populations inhabiting the littoral rocky habitats in both very deep and very shallow areas of the lake. We hypothesized that the former would constitute relatively older, more stable and genetically more diverse populations, because they should have been less severely affected by the well-documented episodes of dramatic water-level fluctuations. In agreement with our predictions, populations of all three species sampled in very shallow shorelines showed traces of stronger population growth than populations of the same species inhabiting deep shorelines. However, contrary to our working hypothesis, we found a significant trend towards increased genetic diversity in the younger, demographically less stable populations inhabiting shallow areas, in comparison with the older and more stable populations inhabiting the deep shorelines. We interpret this finding as the result of the establishment of metapopulation dynamics in the former shorelines, by the frequent perturbation and reshuffling of individuals between populations due to the lake-level fluctuations. The repeated succession of periods of allopatric separation and secondary contact is likely to have further increased the rapid pace of speciation in lacustrine cichlids.

Complete mitochondrial DNA replacement in a Lake Tanganyika cichlid fish.

We used nuclear and mitochondrial DNA (mtDNA) sequences from specimens collected throughout Lake Tanganyika to clarify the evolutionary relationship between Lamprologus callipterus and Neolamprologus fasciatus. The nuclear data support the reciprocal monophyly of these two shell-breeding lamprologine cichlids. However, mtDNA sequences show that (i) L. callipterus includes two divergent and geographically disjunct (North-South) mtDNA lineages; and that (ii) N. fasciatus individuals cluster in a lineage sister group to the northern lineage of L. callipterus. The two mtDNA lineages of L. callipterus diverged c. 684 kya to 1.2 Ma, coinciding with a major water level low stand in Lake Tanganyika, which divided the lake into isolated sub-lakes. This suggests that the two mtDNA lineages originated as the result of the separation of L. callipterus populations in different sub-basins. The incongruent phylogenetic position of N. fasciatus can best be explained by an ancient unidirectional introgression from L. callipterus into N. fasciatus. Remarkably, our data indicate that this event resulted in the complete mtDNA replacement in N. fasciatus. Our data suggest that hybridization occurred soon after the divergence of the two L. callipterus mtDNA lineages, probably still during the water level low stand, and that subsequently the invading mtDNA lineage spread throughout the lake.

A single mitochondrial haplotype and nuclear genetic differentiation in sympatric colour morphs of a riverine cichlid fish.

Some of the diversity of lacustrine cichlid fishes has been ascribed to sympatric divergence, whereas diversification in rivers is generally driven by vicariance and geographic isolation. In the riverine Pseudocrenilabrus philander species complex, several morphologically highly distinct populations are restricted to particular river systems, sinkholes and springs in southern Africa. One of these populations consists of a prevalent yellow morph in sympatry with a less frequent blue morph, and no individuals bear intermediate phenotypes. Genetic variation in microsatellites and AFLP markers was very low in both morphs and one single mtDNA haplotype was fixed in all samples, indicating a very young evolutionary age and small effective population size. Nevertheless, the nuclear markers detected low but significant differentiation between the two morphs. The data suggest recent and perhaps sympatric divergence in the riverine habitat.

Speciation via introgressive hybridization in East African cichlids?

Speciation caused by introgressive hybridization occurs frequently in plants but its importance remains controversial in animal evolution. Here we report a case of introgressive hybridization between two ancient and genetically distinct species of Lake Tanganyika cichlids that led to the formation of a new species. Neolamprologus marunguensis contains mtDNA haplotypes from both parental species varying on average by 12.4% in the first section of the control region and by 5.2% in a segment of the cytochrome b gene. All individuals have almost identical DNA sequences in the flanking regions of the single-copy nuclear DNA locus TmoM27, and show a mosaic of alleles derived from both parental lineages in six microsatellite loci. Hence, our finding displays another mode of speciation in cichlid fishes. The increase of genetic and phenotypic diversity due to hybridization may contribute to the uniquely rapid pace of speciation in cichlids.

Population structure in two sympatric species of the Lake Tanganyika cichlid tribe Eretmodini: evidence for introgression.

Patterns of genetic differentiation were analysed and compared in two sympatric species of the endemic Lake Tanganyika cichlid tribe Eretmodini by means of mitochondrial DNA (mtDNA) sequences of the control region and six microsatellite DNA loci. The sample area covers a total of 138 km of mostly uninterrupted rocky shoreline in the Democratic Republic of Congo and includes the entire distribution range of Tanganicodus cf. irsacae that stretches over a distance of 35 km. Both markers detected significant genetic differentiation within and between the two species. T. cf. irsacae contained lower overall genetic variation than Eretmoduscyanostictus, possibly due to its more restricted range of distribution and its smaller effective population sizes. Complete fixation of Tanganicodus mtDNA haplotypes was observed in Eretmodus at two localities, while at two other localities some Tanganicodus individuals possessed Eretmodus mtDNA haplotypes. Taking into account the relatively large average sequence divergence of 6.2% between the two species, as well as the geographical distribution of mtDNA haplotypes in the lake, the observed pattern is more likely to be a consequence of asymmetric introgression than of shared ancestral polymorphism. As there is significant population differentiation between sympatric Tanganicodus and Eretmodus populations, the events of introgressions may have happened after secondary contact, but our data provide no evidence for ongoing gene flow and suggest that both species are reproductively isolated at present time.

Lake level fluctuations synchronize genetic divergences of cichlid fishes in African lakes.

Water level fluctuations are important modulators of speciation processes in tropical lakes, in that they temporarily form or break down barriers to gene flow among adjacent populations and/or incipient species. Time estimates of the most recent major lowstands of the three African Great Lakes are thus crucial to infer the relative timescales of explosive speciation events in cichlid species flocks. Our approach combines geological evidence with genetic divergence data of cichlid fishes from the three Great East African Lakes derived from the fastest-evolving mtDNA segment. Thereby, we show for each of the three lakes that individuals sampled from several populations which are currently isolated by long geographic distances and/or deep water form clusters of equally closely related haplotypes. The distribution of identical or equally closely related haplotypes in a lake basin allows delineation of the extent of lake level fluctuations. Our data suggest that the same climatic phenomenon synchronized the onset of genetic divergence of lineages in all three species flocks, such that their most recent evolutionary history seems to be linked to the same external modulators of adaptive radiation. A calibration of the molecular clock of the control region was elaborated by gauging the age of the Lake Malawi species flock through the divergence among the utaka-cichlid and the mbuna-cichlid lineages to minimally 570,000 years and maximally 1 Myr. This suggests that the low-lake-level period which established the observed patterns of genetic relatedness dates back less than 57,000 years, probably even to 17,000-12,400 years ago, when Lake Victoria dried up and Lakes Malawi and Tanganyika were also low. A rapid rise of all three lakes about 11,000 years ago established the large-scale population subdivisions observed today. Over that period of time, a multitude of species originated in Lakes Malawi and Victoria with an impressive degree of morphological and ecological differentiation, whereas the Tanganyikan taxa that were exposed to the same habitat changes hardly diverged ecologically and morphologically. Our findings also show that patterns of genetic divergences of stenotopic organisms provide valuable feedback on geological and sedimentological time estimates for lake level changes.

A new molecular phylogenetic hypothesis for the evolution of freshwater eels.

Phylogenetic analysis of a segment of the mitochondrial 16S rDNA of eight Anguilla species from the Indo-Pacific region and from the North Atlantic revealed that the genus Anguilla appears to be surprisingly young, based upon the small observed maximum genetic distance of 4.8% and the high degree of morphological similarity among the species. The placement of A. marmorata as the most ancestral lineage suggests that the genus is likely to have originated in the Indo-Malayian region, from which it quickly spread. Two Pacific species, A. obscura and A. japonica, branched next. A. japonica was placed as sister group to all remaining species, which formed three clades: the first comprising A. australis, the second A. reinhardti and A. mossambica, and the third A. anguilla and A. rostrata. All analyzed specimens of A. rostrata originating from southern New Jersey to Nova Scotia had identical mitotypes, while five mitochondrial genotypes were found in Europe differing by zero to two substitutions. The two Atlantic eel species are very closely related; all surveyed specimens of A. anguilla differ by three to five substitutions from their American allies, corroborating the existence of two distinct biological species. This was also confirmed by restriction analysis of a 350-bp segment of the cytochrome b, in which American specimens were distinct in sharing a single diagnostic restriction site of HinfI. Our results suggest little to no gene flow between the two nominal Atlantic eel species.

Multiple recurrent evolution of trophic types in northeastern Atlantic and Mediterranean seabreams (Sparidae, Percoidei).

Seabreams are among the most valuable fish, not only for small-scale and semiindustrial fisheries but also for aquaculture throughout the Mediterranean. Nevertheless, their phylogenetic relationships are not at all clear. The current taxonomy is based solely on trophic morphology and rests on the assumption that each trophic type evolved only once from a less specialized ancestral condition. We analyzed a 486-bp segment of the mitochondrial 16S rDNA of all 24 seabream species described for the northeastern Atlantic and the Mediterranean to elucidate their generic and subfamily-level relationships. Three major mitochondrial lineages, each comprising species of different feeding strategy and dentition, were found that do not agree with the present taxonomic assignments. Most of the investigated genera were resolved paraphyletically, indicating that the structure and arrangement of oral teeth must have repeatedly evolved from a less specialized ancestral condition. Further, the genus Sparus was resolved as distantly related to the genus Pagrus, in that it was assigned to a different major mitochondrial lineage. Oblada melanura was consistently placed within the Diplodus radiation as sister group to Diplodus puntazzo. Our phylogenetic hypothesis thus suggests multiple independent origins of similar trophic specializations within the Sparidae and indicates that the currently recognized three or four subfamilies need to be redefined.

Mitochondrial DNA reveals cryptic oligochaete species differing in cadmium resistance.

Species of the family Tubificidae represent a major faunal element in benthic freshwater communities throughout the world. Some of them are considered particularly tolerant of the influence of toxicants such as cadmium. One of the most abundant species, "Tubifex tubifex," is frequently used as an indicator of environmental pollution, despite considerable taxonomic problems caused by phenotypic plasticity and genetic heterogeneity. Our study provides a phylogeny of "T. tubifex" based on a segment of the mitochondrial 16S rDNA and presents a rapid PCR-based method of genotype screening which was then applied in cadmium toxicity studies on natural populations. Phylogenetic analysis identified five major mitochondrial lineages, some of them separated by large genetic distances (up to 13%) but morphologically indistinguishable, thus highly suggestive of the existence of cryptic species. All lineages were present at different frequencies in the European river populations studied, with a tendency of the more resistant lineages to occur at higher frequencies in the more tolerant populations. In fact, lineage-specific toxicity experiments showed that individuals of different mitochondrial lineages consistently varied in cadmium resistance, suggesting that in benthic oligochaetes, evolution seems to proceed predominantly through natural selection acting on physiological, rather than morphological, characters. In consequence, toxicological studies involving "T. tubifex" as a monitoring or test organism should allow for the possibility of genetic inhomogeneity of this mudworm group by combining both toxicological and genetic methods.

Ecological parallelism and cryptic species in the genus Ophiothrix derived from mitochondrial DNA sequences.

We addressed the long-standing problem of species assignment of two nominal species of the genus Ophiothrix (Echinodermata, Ophiuroidea) by phylogenetic analysis of a segment of the mitochondrial 16S rDNA. Our phylogeny identified two distinct mitochondrial lineages that do not correspond to the present species assignments. Individuals of the endemic Mediterranean species O. quinquemaculata were clustered with individuals of O. fragilis in both mitochondrial lineages. We thus suggest that these taxa are not biological species but ecotypes. Differences between the two ecotypes in morphological and physiological characteristics may be explained by adaptation to environmental conditions at different water depths. Despite the observed ecomorphological variability within each of the two major mitochondrial lineages, the large genetic distance (9.0-12.0%) between them does suggest the existence of two distinct biological species. Their reproductive isolation could result from differences in reproductive strategy rather than by ecological and/or morphological differentiation.

Mitochondrial phylogeny of the genus Regulus and implications on the evolution of breeding behavior in sylvioid songbirds.

We tested four hypotheses about the relationships of the kinglets (genus Regulus)to seven closely related genera of the songbird superfamily Sylvioidea using mitochondrial DNA sequences. The kinglets were suggested to be closely related to the tits (Parus) or to the Old World Warblers (Phylloscopus) and were also suggested to constitute the, or at least one of the, most ancestral splits among the sylvioids. Our phylogenetic analysis grouped the kinglets as the sister group of a clade comprising Parus and Phylloscopus and including the genera Sylvia, Aegithalos, and Leptopoecile. Two of the taxa were placed more ancestral to the kinglets: Sitta and Certhia. We also identified the endemic kinglet species from the Canary Islands s the sister group of R. regulus. The superimposition of breeding behavior on the phylogeny suggests that hole nesting is ancestral and various other patterns of nest construction have evolved from it. The placement of Parus implies that hole nesting in the Paridae is likely to have originated secondarily. Further, Leptopoecile and Aegithalos, two genera for which a helper system of elder offspring in breeding was described, were resolved as a clade.

Cryptic species in marine polychaete and their independent introduction from North America to Europe.

The vast body of ballast water carried across oceans by freight ships represents a major source for the introduction of foreign species into marine ecosystems. The worm Marenzelleria viridis, originally found only in North America, appeared in estuaries of the North Sea in 1979 and 6 years later also in the Baltic, where it has developed into a major faunal element. Two competing hypotheses are discussed here: either both populations owe their presence to a single introductory event in the North Sea, or each population originated from a separate introduction. Our phylogeographic analysis of Baltic, North Sea and American Marenzelleria, based on mitochondrial 16S rDNA sequences (326-bp segment) of 98 individuals from 17 localities on the North American, North Sea, and Baltic coasts not only favors the two-event hypothesis, but also separates the locations of origin for the introductions. Eighteen mitochondrial genotypes were identified altogether. In agreement with allozyme data, three lineages were identified: genotypes assigned to the same lineage differed from each other by up to 5 point mutations, and those assigned to different lineages differed by up to 17. The existence of three morphologically indistinguishable, and thus cryptic, species is therefore suggested. The individuals from the Baltic Sea probably originated from the Atlantic coast of the United States between Chesapeake Bay and Georgia, and the North Sea populations may stem from the U.S. coast region north of Chesapeake Bay to Nova Scotia. Despite their similar morphologies, the two European Marenzelleria species may differ ecologically with respect to their preference for habitat salinity. Assuming that transport via ballast water occurs quite frequently, we hypothesize that both European cryptic species of Marenzelleria may originally have been introduced to both the North Sea and the Baltic Sea but that neither of them was able to proliferate in both water bodies owing to their differential physiological performances at high and low salinities.

Phylogenetic relationships of Central European wolf spiders (Araneae: lycosidae) inferred from 12S ribosomal DNA sequences.

We have analyzed a sequence dataset of a portion of mitochondrial 12S rRNA gene of the ribosomal small subunit for 27 species of the family Lycosidae (wolf spiders) from Central Europe, belonging to six genera (Alopecosa, Arctosa, Pardosa, Pirata, Trochosa, and Xerolycosa) and four subfamilies (Evippinae, Lycosinae, Pardosinae and Venoniinae). Phylogenetic analyses were performed in two steps and corroborate the monophyly of all the genera analyzed with strong bootstrap support. In the first step focusing on the most ancestral splits the genus Pirata consistently emerged as the most ancestral branch, followed by the two genera Arctosa and Xerolycosa, with conflicting branching order, however. The second step of analysis placed Xerolycosa more ancestral than Arctosa. Arctosa appeared as sister group to the genera Alopecosa, Trochosa, and Pardosa. The palearctic genus Xerolycosa was not yet included in previous studies derived from morphological characters, but its placement based on mtDNA sequences is in good agreement to that according to current diagnostic morphological features. Further, the single representative of the genus Arctosa examined in our study was placed at a more ancestral position than in a previous investigation based on phenotypic characters. The superimposition of the currently used diagnostic phenotypic characters on the DNA-based phylogeny shows that both character sets are widely congruent; only 3 out of 16 phenotypic characters were resolved as homoplasious, suggesting their parallel evolution and/or reversal. Among the three different styles of predation found in the Lycosids, tube builders appear to be the most ancestral from which burrow dwellers descended and from which two groups of vagrant hunters evolved in parallel.

Molecular phylogeny analysis of fiddler crabs: test of the hypothesis of increasing behavioral complexity in evolution.

The current phylogenetic hypothesis for the evolution and biogeography of fiddler crabs relies on the assumption that complex behavioral traits are assumed to also be evolutionary derived. Indo-west Pacific fiddler crabs have simpler reproductive social behavior and are more marine and were thought to be ancestral to the more behaviorally complex and more terrestrial American species. It was also hypothesized that the evolution of more complex social and reproductive behavior was associated with the colonization of the higher intertidal zones. Our phylogenetic analysis, based upon a set of independent molecular characters, however, demonstrates how widely entrenched ideas about evolution and biogeography led to a reasonable, but apparently incorrect, conclusion about the evolutionary trends within this pantropical group of crustaceans. Species bearing the set of "derived traits" are phylogenetically ancestral, suggesting an alternative evolutionary scenario: the evolution of reproductive behavioral complexity in fiddler crabs may have arisen multiple times during their evolution. The evolution of behavioral complexity may have arisen by coopting of a series of other adaptations for high intertidal living and antipredator escape. A calibration of rates of molecular evolution from populations on either side of the Isthmus of Panama suggest a sequence divergence rate for 16S rRNA of 0.9% per million years. The divergence between the ancestral clade and derived forms is estimated to be approximately 22 million years ago, whereas the divergence between the American and Indo-west Pacific is estimated to be approximately 17 million years ago.

Genetic divergence and speciation in an extremely young species flock in Mexico formed by the genus Cyprinodon (Cyprinodontidae, Teleostei).

The lake Laguna Chichancanab contains one of the evolutionarily youngest species flocks known, composed of five endemic species of the genus Cyprinodon. The presumed sister species Cyprinodon artifrons still exists in marine coastal habitats. Sequences of the mitochondrial control region were obtained for C. artifrons (genes of 17 individuals sequenced) and for all five endemic species (genes of 32 individuals sequenced). While nine different haplotypes were found in C. artifrons, only a single haplotype was shared with the species from the laguna. Four additional haplotypes were found among the endemics. These four were most closely related to that single haplotype shared with C. artifrons. The deprivation in haplotypes among the endemics points to a small founder population or to bottlenecking. The distribution pattern of overlapping haplotypes in four of the five species suggests incomplete lineage sorting or hybridization among them. Complete reproductive isolation is only assumed for one species, C. maya, in which a single and unique haplotype was found.

Mitochondrial phylogeny of the Lamprologini, the major substrate spawning lineage of cichild fishes from Lake Tanganyika in eastern Africa.

Lake Tanganyika harbors the oldest, morphologically and behaviorally most diverse flock of cichlid species. While the cichlids in Lakes Malawi and Victoria breed their eggs exclusively by buccal incubation (termed "mouthbrooding"), the Tanganyikan cichlid fauna comprise mouthbrooding and substrate-spawning lineages (fish spawn on rocks, and never orally incubate eggs or wrigglers). The substrate-spawning tribe Lamprologini appears to occupy a key position that might allow one to elucidate the origin of the Tanganyika flock, because five riverine (therefore nonendemic) species from the Zaire River system have been assigned to this tribe, in addition to the lake's endemic species, which make up almost 50% of all 171 species known from this lake (Poll 1986). From 16 species (18 individuals) of the tribe Lamprologini, a 402-bp segment of the mitochondrial cytochrome b gene was sequenced, and, from 25 lamprologine species (35 individuals), sequences from the mitochondrial control region were obtained. To place the Lamprologini into a larger phylogenetic framework, orthologous sequences were obtained from eight nonlamprologine Tanganyikan cichlid species (13 individuals). The Lamprologini are monophyletic, and a clade of six Tanganyikan lineages of mouthbrooders, representing five tribes (Poll 1986), appears to be their sister group. Comparisons of sequence divergences of the control region indicate that the Lamprologini may be older than the endemic Tanganyikan tribe Ectodini, and short basal branches might suggest a rapid formation of lineages at an early stage of the Tanganyika radiation. It is interesting that three analyzed riverine members of the tribe form a monophyletic group; however, they are not the most ancestral branch of the Lamprologini. This might indicate that they are derived from an endemic lamprologine ancestor that left Lake Tanganyika by entering the Zaire River system. These riverine species may not have seeded the Tanganyikan radiation, as currently thought, but may have recently recolonized the river after a long period of isolation, as soon as the lake was connected to the Zaire River again about 2 Mya. Neolamprologus moorii, endemic to Lake Tanganyika, appears to represent the most basal clade of the Lamprologini. Complex breeding behavior, involving the usage of gastropod shells and associated with dwarfism, is likely to have evolved in parallel in several lineages among the Lamprologini. The tribe Lamprologini may be in need of revision, since several genera appear to be polyphyletic.

Mitochondrial phylogeny of the endemic mouthbrooding lineages of cichlid fishes from Lake Tanganyika in eastern Africa.

Of the three cichlid species flocks in eastern Africa, Lake Tanganyika harbors the oldest species assemblage, which is also the most diverse morphologically and behaviorally. For 12 species (20 individuals) of 12 genera of the tribe Ectodini, 852 bp from two segments (cytochrome b and control region) of the mitochondrial genome were sequenced. In addition, orthologous sequences were obtained from eight species (11 individuals) representing other mouthbrooding lineages from Lake Tanganyika. Comparisons of sequence divergences revealed that the single Tanganyikan tribe Ectodini appears to be approximately five times older than the whole Lake Malawi cichlid species flock, suggesting that the radiation of the Tanganyikan mouthbrooding lineages took place long before the species flocks of Lakes Malawi and Victoria evolved. Seven of nine surveyed tribes of Tanganyikan cichlids appear to be approximately equally divergent, and this seems to corroborate the hypothesis of a rapid simultaneous formation of lineages at an early stage in the history of the Lake Tanganyika species flock. The close genetic relationship between the endemic Tropheus lineage and a nonendemic "Haplochromine," Astatotilapia burtoni, indicates that members of the tribe Tropheini may be the sister group of the cichlid flocks of Lakes Malawi and Victoria. The phylogenetic analyses demonstrate the monophyly of the Ectodini and identify the Cyprichromini as their sister group among the Tanganyikan cichlids. Within the tribe Ectodini the molecular data suggest both a branching pattern different than that previously proposed and a subdivision of the Ectodini into four clades, instead of the two originally described. The previously postulated model of morphological transformations believed to be responsible for the drastically different types of ecological specialization found among the Ectodini might therefore be in need of reinterpretation. Characters immediately related to foraging and nutrition seem to be particularly prone to homoplasy, even among members of a single lineage of cichlid fishes.

Genetic divergence, speciation and morphological stasis in a lineage of African cichlid fishes.

Since their discovery at the turn of the century, the species assemblages of cichlid fishes in the East African Lakes Victoria, Malawi and Tanganyika have fascinated evolutionary biologists. Many models have attempted to account for the 'explosive' evolution of several hundred species within these lakes. Here we report a case of surprisingly large genetic divergence among populations of the endemic Tropheus lineage of Lake Tanganyika. This lineage of six species contains twice as much genetic variation as the entire morphologically highly diverse cichlid assemblage of Lake Malawi and six times more variation than the Lake Victoria species flock. Although it is highly variable in coloration, this group of species and its closest relatives have not undergone appreciable morphological change. The observed geographic pattern of genetic variation suggests that major lake level fluctuations affected the distribution and speciation of this lineage of cichlid fishes.