Morphometric differentiation among haplochromine cichlid fish species of a satellite lake of Lake Victoria

Lake Victoria holds a young but species‐rich assemblage of cichlid fishes, which form a monophyletic assemblage with additional species from surrounding water bodies, termed the Lake Victoria superflock. Lake Victoria is surrounded by smaller lakes that are somewhat disconnected from the main lake. Lake Kanyaboli is such a small lake, having markedly reduced species diversity, in part comprised of Lake Victoria species and endemics. Here, we studied the modern haplochromine component of the cichlid fauna, represented by Lipochromis maxillaris, Astatotilapia nubila, Xystichromis phytophagus and Astatotilapia sp. ‘Bigeye’, as well as a number of unidentified modern haplochromine specimens. We used landmark‐based geometric morphometrics to study the degree of morphological divergence among those young entities. Twenty landmarks and 14 interlandmark distances were used for shape analysis. Multivariate analysis revealed significant differences between all four species, but principal component analysis and canonical variate analysis did not clearly discriminate between A. nubila and X. phytophagus, demonstrating great overall morphological similarity despite clear dietary differences. Besides coloration there was sexual dimorphism in body proportions, so that only male individuals were analysed further. In all four species, the observed similarities and differences in body shape conform to the type of ecological specialization of the fish. Most unidentified specimens overlapped the range of A. nubila and X. phytophagus, while the assignment test based on the canonical variate analysis suggested 70% of the three overlapping entities as separate units. To test their reproductive distinctness and to demonstrate potential hybridization, nuclear genetic data are needed.     

Phylogeny of the Lake Tanganyika cichlid species flock and its relationship to the Central and East African haplochromine cichlid fish faunas

Lake Tanganyika, the oldest of the East African Great Lakes, harbors the ecologically, morphologically, and behaviorally most complex of all assemblages of cichlid fishes, consisting of about 200 described species. The evolutionary old age of the cichlid assemblage, its extreme degree of morphological differentiation, the lack of species with intermediate morphologies, and the rapidity of lineage formation have made evolutionary reconstruction difficult. The number and origin of seeding lineages, particularly the possible contribution of riverine haplochromine cichlids to endemic lacustrine lineages, remains unclear. Our phylogenetic analyses, based on mitochondrial DNA sequences of three gene segments of 49 species (25% of all described species, up to 2,400 bp each), yield robust phylogenies that provide new insights into the Lake Tanganyika adaptive radiation as well as into the origin of the Central- and East-African haplochromine faunas. Our data suggest that eight ancient African lineages may have seeded the Tanganyikan cichlid radiation. One of these seeding lineages, probably comprising substrate spawning Lamprologus-like species, diversified into six lineages that evolved mouthbrooding during the initial stage of the radiation. All analyzed haplochromines from surrounding rivers and lakes seem to have evolved within the radiating Tanganyikan lineages. Thus, our findings contradict the current hypothesis that ancestral riverine haplochromines colonized Lake Tanganyika to give rise to at least part of its spectacular endemic cichlid species assemblage. Instead, the early phases of the Tanganyikan radiation affected Central and East African rivers and lakes. The haplochromines may have evolved in the Tanganyikan basin before the lake became a hydrologically and ecologically closed system and then secondarily colonized surrounding rivers. Apparently, therefore, the current diversity of Central and East African haplochromines represents a relatively young and polyphyletic fauna that evolved from or in parallel to lineages now endemic to Lake Tanganyika.     

Genetic distinction of four haplochromine cichlid fish species in a satellite lake of Lake Victoria,East Africa

Lake Victoria is famous for its in evolutionary terms young but species‐rich assemblage of cichlid fishes. This ‘superflock’ also includes additional species from adjacent water systems. Lake Victoria is surrounded by several smaller lakes that are connected to the main water body of Lake Victoria only through swampy areas. Lake Kanyaboli is one such lake, harbouring a much poorer species diversity, mostly comprised of Lake Victoria endemics, some of which are now considered extirpated from the main lake. The focus of this study was on the modern haplochromine component of the cichlid fauna, represented by Lipochromis maxillaris, Astatotilapia nubila, Xystichromis phytophagus and Astatotilapia sp. ‘Bigeye’, as well as a number of morphologically distinct haplochromine specimens that could not be assigned to any of the recognized species. We used five microsatellite markers to distinguish these five taxa. Genetically, L. maxillaris was clearly differentiated from all other taxa, and A. sp. ‘Bigeye’ was moderately differentiated from the remaining three. Astatotilapia nubila, X. phytophagus and the unidentified specimens constituted a partially overlapping cluster. As each of the clusters had several (5–14) private alleles, extremely recent divergence is suggested. As all taxa except for A. sp. ‘Bigeye’ and the unidentified specimens also occur or at least occurred in Lake Victoria, it is likely that they evolved as part of the Lake Victoria superflock, while A. sp. ‘Bigeye’ and the unidentified specimens may have currently evolved in situ. The observation of slightly distinct albeit overlapping body shapes and the extremely close genetic relationship between three of the five taxa are fully compatible and in support of the hybrid swarm theory of adaptive radiation.     

A BAC library of the East African haplochromine cichlid fish Astatotilapia burtoni

A BAC library was constructed from Astatotilapia burtoni, a haplochromine cichlid that is found in Lake Tanganyika, East Africa, and its surrounding rivers. The library was generated from genomic DNA of blood cells and comprises 96,768 individual clones. Its median insert size is 150 kb and the coverage is expected to represent about 14 genome equivalents. The coverage evaluation was based on genome size estimates that were obtained by flow cytometry. In addition, hybridization screens with five probes largely corroborate the above coverage estimate, although the number of clones ranged from 5 to 22 authenticated clones per single copy probe. The BAC library described here is expected to be useful to the scientific community interested in cichlid genomics as an important resource to gain new insights into the rapid evolution of the great species diversity of haplochromine cichlid fishes.     

Age and spread of the haplochromine cichlid fishes in Africa

The Haplochromini are by far the most species-rich cichlid fish tribe that originated along with the so-called primary radiation of the Lake Tanganyika cichlid species flock, i.e. at the same time during which the majority of the endemic Lake Tanganyika cichlid tribes emerged. Unlike the other tribes, the haplochromines are not restricted to Lake Tanganyika but distributed throughout Africa, except for the northwestern part of the continent. Haplochromine cichlids seeded the adaptive radiation of cichlid fishes in Lakes Malawi, Kivu, Victoria, Turkana, as well as in the now extinct paleo-Lake Makgadikgadi. Here we present a comprehensive phylogenetic and phylogeographic analysis of haplochromine cichlids that is based upon DNA sequences of two mitochondrial gene segments of riverine taxa covering all major African biogeographic regions where haplochromines are found. Our analysis revealed that six lineages of haplochromines originated within a short period of time, about 5.3-4.4 MYA. These haplochromine lineages show a highly complex phylogeographic pattern, probably severely influenced by climate- and/or geology-induced changes of the environment, with river capture events most likely playing an important role for species dispersal.     

Parallel evolution in Ugandan crater lakes: repeated evolution of limnetic body shapes in haplochromine cichlid fish

Background

The enormous diversity found in East African cichlid fishes in terms of morphology, coloration, and behavior have made them a model for the study of speciation and adaptive evolution. In particular, haplochromine cichlids, by far the most species-rich lineage of cichlids, are a well-known textbook example for parallel evolution. Southwestern Uganda is an area of high tectonic activity, and is home to numerous crater lakes. Many Ugandan crater lakes were colonized, apparently independently, by a single lineage of haplochromine cichlids. Thereby, this system could be considered a natural experiment in which one can study the interaction between geographical isolation and natural selection promoting phenotypic diversification.

Results

We sampled 13 crater lakes and six potentially-ancestral older lakes and, using both mitochondrial and microsatellite markers, discovered strong genetic and morphological differentiation whereby (a) geographically close lakes tend to be genetically more similar and (b) three different geographic areas seem to have been colonized by three independent waves of colonization from the same source population. Using a geometric morphometric approach, we found that body shape elongation (i.e. a limnetic morphology) evolved repeatedly from the ancestral deeper-bodied benthic morphology in the clear and deep crater lake habitats.

Conclusions

A pattern of strong genetic and morphological differentiation was observed in the Ugandan crater lakes. Our data suggest that body shape changes have repeatedly evolved into a more limnetic-like form in several Ugandan crater lakes after independent waves of colonization from the same source population. The observed morphological changes in crater lake cichlids are likely to result from a common selective regime.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0287-3) contains supplementary material, which is available to authorized users.     

Pollen competition as an asymmetric reproductive barrier between two closely related Silene species

Reproductive barriers are important determinants of gene flow between divergent populations or species. We studied pollen competition as a post‐mating reproductive barrier between Silene dioica and S. latifolia. Gene flow between these species is extensive, but early‐generation hybrids are rare. In an experiment with conspecific, heterospecific and 50 : 50 mixed pollinations in the two species, pollination treatments did not significantly affect seed set and seed weight. However, molecular determination of siring success after mixed pollinations showed that fewer than expected hybrids were produced in S. latifolia (18% hybrids) but not in S. dioica (51% hybrids). This constitutes an asymmetric post‐mating reproductive barrier and likely contributes to the rarity of early‐generation hybrids. Our study shows that pollen competition can be an effective barrier to hybridization between closely related species that likely acts in concert with other reproductive barriers.     

Lateralization in response to social stimuli in a cooperatively breeding cichlid fish

Cerebral lateralization, an evolutionarily ancient and widespread phenomenon among vertebrates, is thought to bestow cognitive advantages. The advantages of lateralization at the individual-level do not necessarily require that the entire population share the same pattern of lateralization. In fact, directional bias in lateralization may lead to behavioural predictability and enhanced predator success or prey evasion. Recent theory has suggested that population-level lateralization may be favored if individuals are better able to perform coordinated behaviours, providing a distinct advantage in cooperative contexts. Here we test whether the highly social, cooperatively breeding cichlid fish Neolamprologus pulcher shows lateralized responses to a social stimulus. We found population-level biases in males; on average male N. pulcher use their right eye/left hemisphere to view their mirror image. Individual females had a preferred hemisphere, but these preferences appeared not to be directionally aligned among females. We discuss these results in the context of coordinated social behaviour and suggest future research directions.     

Divergent protein coding regions in otherwise closely related androgen-regulated mRNAs

Rat seminal vesicles serve as a model system for studying androgen action. We have sequenced and compared full length cDNAs for two major proteins (S and F) synthesised and secreted under hormonal control. Overall, mRNAS and mRNAF share 57% nucleotide sequence homology suggesting that their genes arose by duplication of a common ancestor. However, the mRNAs display a highly unusual regional distribution of sequence homology, with the untranslated regions (UTRs) being substantially more homologous than the protein-coding regions (PCRs). Detailed analysis of nucleotide substitutions at synonymous and replacement sites shows that the PCRs have evolved very rapidly. Evolutionary conservation of the UTRs is no higher than that of UTRs generally and thus provides no evidence of a specific regulatory role for the UTRs in androgen action. The primary sequences of proteins S and F have diverged so rapidly that they are the best examples of neutrally evolving proteins for which comparative nucleotide sequence data are available. However, despite their rapid divergence, the predicted higher order structures for both proteins consist largely of non-regular conformation. This is discussed in terms of their roles as structural components of the rodent copulatory plug.     

The chemosensory responses of young cichlid fish to parents and predators

The chemosensory responses of the fry of the Midas cichlid, Cichlasoma citrinellum (Pisces: Cichlidae), were tested in a Y-maze during their first 11 days of free-swimming. Fry preferred all conspecific females to blank water but did not distinguish between their own mother, another mother or a non-parental, adult female. Fry preferred females to males. Similar results were obtained for males. Fry preferred their siblings and (unrelated) juveniles to blank water. However, while they failed to show a preference between siblings and their mother, they preferred their mother to juveniles. Fry were able to any of three heterospecifics (C. managuense, C. nigrofasciatum and the allopatric Sarotherodon mossambicus). The chemosensory responses demonstrated in the Y-maze are hypothesized to aid the fry in staying near the centre of the guarded territory and away from predators, particularly under murky or nocturnal conditions.     

Divergent selection and phenotypic plasticity during incipient speciation in Lake Victoria cichlid fish

Divergent selection acting on several different traits that cause multidimensional shifts are supposed to promote speciation, but the outcome of this process is highly dependent on the balance between the strength of selection vs. gene flow. Here, we studied a pair of sister species of Lake Victoria cichlids at a location where they hybridize and tested the hypothesis that divergent selection acting on several traits can maintain phenotypic differentiation despite gene flow. To explore the possible role of selection we tested for correlations between phenotypes and environment and compared phenotypic divergence (P_ST) with that based on neutral markers (F_ST). We found indications for disruptive selection acting on male breeding colour and divergent selection acting on several morphological traits. By performing common garden experiments we also separated the environmental and heritable components of divergence and found evidence for phenotypic plasticity in some morphological traits contributing to species differences.     

Population-structure and genetic diversity in a haplochromine cichlid fish [corrected] of a satellite lake of Lake Victoria

The approximately 500 species of the cichlid fish species flock of Lake Victoria, East Africa, have evolved in a record-setting 100,000 years and represent one of the largest adaptive radiations. We examined the population structure of the endangered cichlid species Xystichromis phytophagus from Lake Kanyaboli, a satellite lake to Lake Victoria in the Kenyan Yala wetlands. Two sets of molecular markers were analysed--sequences of the mitochondrial control region as well as six microsatellite loci--and revealed surprisingly high levels of genetic variability in this species. Mitochondrial DNA sequences failed to detect population structuring among the three sample populations. A model-based population assignment test based on microsatellite data revealed that the three populations most probably aggregate into a larger panmictic population. However, values of population pairwise FST indicated moderate levels of genetic differentiation for one population. Eleven distinct mitochondrial haplotypes were found among 205 specimens of X. phytophagus, a relatively high number compared to the total number of 54 haplotypes that were recovered from hundreds of specimens of the entire cichlid species flock of Lake Victoria. Most of the X. phytophagus mitochondrial DNA haplotypes were absent from the main Lake Victoria, corroborating the putative importance of satellite lakes as refugia for haplochromine cichlids that went extinct from the main lake in the last decades and possibly during the Late Pleistocene desiccation of Lake Victoria.     

On the age and origin of the species flock of haplochromine cichlid fishes of Lake Victoria

Recent suggestions concerning the age and origin of the flock of haplochromine cichlid fishes in Lake Victoria (East Africa) are considered. These accept as proven the suggestion that Lake Victoria dried out completely in the Late Pleistocene, was dry for several thousand years, and refilled ca. 12400 years ago. Apart from the fact that other geophysical evidence contradicts this claim, its biological implications, which do likewise, have never been considered by those who have accepted it. Like those of all previous authors who have seized on the presence of the haplochromine flock of perhaps more than 600 species as evidence of extremely rapid evolution since the lake allegedly refilled, the account completely overlooks the fact that any such desiccation must have eliminated not only the haplochromine cichlids but the entire biota of the lake. Nevertheless, its present fauna not only includes the haplochromines but many other endemic organisms that would not be expected, and whose presence and history demand an explanation if the lake did indeed dry out. No such explanation has been offered, nor does such seem possible. The recent interpretation of events is questioned and rejected.     

Characterization of Vibrio anguillarum and closely related species isolated from farmed fish in Norway.

A total of 264 bacterial strains tentatively or definitely classified as Vibrio anguillarum were examined. The strains were isolated from diseased or healthy Norwegian fish after routine autopsy. With the exception of five isolates from wild saithe (Pollachius virens), the strains originated from nine different species of farmed fish. The bacteria were subjected to morphological, physiological, and biochemical studies, numerical taxonomical analyses, serotyping by slide agglutination and enzyme-linked immunosorbent assay, DNA-plasmid profiling, and in vitro antimicrobial drug susceptibility testing. The results of the microbiological studies were correlated to anamnestic information. The bacterial strains were identified as V. anguillarum serovar O1 (n = 132), serovar O2 (n = 89), serovar O4 (n = 2), serovar O8 (n = 1), and not typeable (n = 1) as well as Vibrio splendidus biovar I (n = 36) and biovar II (n = 1), Vibrio tubiashii (n = 1), and Vibrio fischerii (n = 1). V. anguillarum serovar O1 or O2 was isolated in 176 out of 179 cases of clinical vibriosis in Atlantic salmon (Salmo salar). V. anguillarum serovar O1 was the only serovar isolated from salmonid fish species other than Atlantic salmon, while V. anguillarum serovar O2 was isolated from all marine fish suffering from vibriosis. A 48-Mda plasmid was isolated from all V. anguillarum serovar O1 isolates examined. Serovar O2 isolates did not harbor any plasmids. Resistance against commonly used antibiotic compounds was not demonstrated among V. anguillarum isolates. Neither V. splendidus biovar I nor other V. anguillarum-related species appeared to be of clinical importance among salmonid fish.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recombination rate variation in closely related species

Despite their importance to successful meiosis and various evolutionary processes, meiotic recombination rates sometimes vary within species or between closely related species. For example, humans and chimpanzees share virtually no recombination hotspot locations in the surveyed portion of the genomes. However, conservation of recombination rates between closely related species has also been documented, raising an apparent contradiction. Here, we evaluate how and why conflicting patterns of recombination rate conservation and divergence may be observed, with particular emphasis on features that affect recombination, and the scale and method with which recombination is surveyed. Additionally, we review recent studies identifying features influencing fine-scale and broad-scale recombination patterns and informing how quickly recombination rates evolve, how changes in recombination impact selection and evolution in natural populations, and more broadly, which forces influence genome evolution.     

Immune activation is inversely related to, but does not cause variation in androgen levels in a cichlid fish species

Studies on birds and mammals indicate that sexual traits may signal superior health because active immunity, like inflammatory responses to infections, is suppressive to the production of androgens that facilitate the expression of these traits. Here we test this possible pathway for honest signaling in a teleost species, Sarotherodon galilaeus, by activating the immune system with sheep red blood cells (SRBC), which is a non-pathogenic T- and B-cell stimulating antigen. Two weeks after the start of treatment adult males injected with SRBC showed a significant increase in antibody production in comparison with control males. The variation in specific antibody production was negatively related with variation in both testosterone and 11-ketotestosterone levels. This suggests that investment in immune protection is incompatible with increased activity of the hypothalamic-pituitary-gonadal axis. However, opposite to our expectation no difference in androgen levels was found between placebo and SRBC treatment suggesting that immune activation did not cause androgen suppression in our studied species.