Divergent selection during speciation of Lake Malawi cichlid fishes inferred from parallel radiations in nuptial coloration

Evolutionary diversification of pigment pattern in Danio fishes: differential fms dependence and stripe loss in D. albolineatus

The developmental bases for species differences in adult phenotypes remain largely unknown. An emerging system for studying such variation is the adult pigment pattern expressed by Danio fishes. These patterns result from several classes of pigment cells including black melanophores and yellow xanthophores, which differentiate during metamorphosis from latent stem cells of presumptive neural crest origin. In the zebrafish D. rerio, alternating light and dark horizontal stripes develop, in part, owing to interactions between melanophores and cells of the xanthophore lineage that depend on the fms receptor tyrosine kinase; zebrafish fms mutants lack xanthophores and have disrupted melanophore stripes. By contrast, the closely related species D. albolineatus exhibits a uniform pattern of melanophores, and previous interspecific complementation tests identified fms as a potential contributor to this difference between species. Here, we survey additional species and demonstrate marked variation in the fms-dependence of hybrid pigment patterns, suggesting interspecific variation in the fms pathway or fms requirements during pigment pattern formation. We next examine the cellular bases for the evolutionary loss of stripes in D. albolineatus and test the simplest model to explain this transformation, a loss of fms activity in D. albolineatus relative to D. rerio. Within D. albolineatus, we demonstrate increased rates of melanophore death and decreased melanophore migration, different from wild-type D. rerio but similar to fms mutant D. rerio. Yet, we also find persistent fms expression in D. albolineatus and enhanced xanthophore development compared with wild-type D. rerio, and in stark contrast to fms mutant D. rerio. These findings exclude the simplest model in which stripe loss in D. albolineatus results from a loss of fms-dependent xanthophores and their interactions with melanophores. Rather, our results suggest an alternative model in which evolutionary changes in pigment cell interactions themselves have contributed to stripe loss, and we test this model by manipulating melanophore numbers in interspecific hybrids. Together, these data suggest evolutionary changes in the fms pathway or fms requirements, and identify changes in cellular interactions as a likely mechanism of evolutionary change in Danio pigment patterns.

Pigment pattern evolution by differential deployment of neural crest and post-embryonic melanophore lineages in Danio fishes

Latent precursors or stem cells of neural crest origin are present in a variety of post-embryonic tissues. Although these cells are of biomedical interest for roles in human health and disease, their potential evolutionary significance has been underappreciated. As a first step towards elucidating the contributions of such cells to the evolution of vertebrate form, we investigated the relative roles of neural crest cells and post-embryonic latent precursors during the evolutionary diversification of adult pigment patterns in Danio fishes. These pigment patterns result from the numbers and arrangements of embryonic melanophores that are derived from embryonic neural crest cells, as well as from post-embryonic metamorphic melanophores that are derived from latent precursors of presumptive neural crest origin. In the zebrafish D. rerio, a pattern of melanophore stripes arises during the larval-to-adult transformation by the recruitment of metamorphic melanophores from latent precursors. Using a comparative approach in the context of new phylogenetic data, we show that adult pigment patterns in five additional species also arise from metamorphic melanophores, identifying this as an ancestral mode of adult pigment pattern development. By contrast, superficially similar adult stripes of D. nigrofasciatus (a sister species to D. rerio) arise by the reorganization of melanophores that differentiated at embryonic stages, with a diminished contribution from metamorphic melanophores. Genetic mosaic and molecular marker analyses reveal evolutionary changes that are extrinsic to D. nigrofasciatus melanophore lineages, including a dramatic reduction of metamorphic melanophore precursors. Finally, interspecific complementation tests identify a candidate genetic pathway for contributing to the evolutionary reduction in metamorphic melanophores and the increased contribution of early larval melanophores to D. nigrofasciatus adult pigment pattern development. These results demonstrate an important role for latent precursors in the diversification of pigment patterns across danios. More generally, differences in the deployment of post-embryonic neural crest-derived stem cells or their specified progeny may contribute substantially to the evolutionary diversification of adult form in vertebrates, particularly in species that undergo a metamorphosis.

Evidence for sympatric speciation by host shift in the sea

The genetic divergence and evolution of new species within the geographic range of a single population (sympatric speciation) contrasts with the well-established doctrine that speciation occurs when populations become geographically isolated (allopatric speciation). Although there is considerable theoretical support for sympatric speciation, this mode of diversification remains controversial, at least in part because there are few well-supported examples. We use a combination of molecular, ecological, and biogeographical data to build a case for sympatric speciation by host shift in a new species of coral-dwelling fish (genus Gobiodon). We propose that competition for preferred coral habitats drives host shifts in Gobiodon and that the high diversity of corals provides the source of novel, unoccupied habitats. Disruptive selection in conjunction with strong host fidelity could promote rapid reproductive isolation and ultimately lead to species divergence. Our hypothesis is analogous to sympatric speciation by host shift in phytophagous insects except that we propose a primary role for intraspecific competition in the process of speciation. The fundamental similarity between these fishes and insects is a specialized and intimate relationship with their hosts that makes them ideal candidates for speciation by host shift.

Reticulate sympatric speciation in Cameroonian crater lake cichlids

BACKGROUND: Traditionally the rapid origin of megadiverse species flocks of extremely closely related species is explained by the combinatory action of three factors: Disruptive natural selection, disruptive sexual selection and partial isolation by distance. However, recent empirical data and theoretical advances suggest that the diversity of complex species assemblages is based at least partially on the hybridization of numerous ancestral allopatric lineages that formed hybrids upon invasion of new environments. That reticulate speciation within species flocks may occur under sympatric conditions after the primary formation of species has been proposed but not been tested critically. RESULTS: We reconstructed the phylogeny of a complex cichlid species flock confined to the tiny Cameroonian crater lake Barombi Mbo using both mitochondrial and nuclear (AFLP) data. The nuclear phylogeny confirms previous findings which suggested the monophyly and sympatric origin of the flock. However, discordant intra-flock phylogenies reconstructed from mitochondrial and nuclear data suggest strongly that secondary hybridization among lineages that primarily diverged under sympatric conditions had occurred. Using canonical phylogenetic ordination and tree-based tests we infer that hybridization of two ancient lineages resulted in the formation of a new and ecologically highly distinct species, Pungu maclareni. CONCLUSIONS: Our findings show that sympatric hybrid speciation is able to contribute significantly to the evolution of complex species assemblages even without the prior formation of hybrids derived from allopatrically differentiated lineages.

Male-male competition and nuptial-colour displacement as a diversifying force in Lake Victoria cichlid fishes

We propose a new mechanism for diversification of male nuptial-colour patterns in the rapidly speciating cichlid fishes of Lake Victoria. Sympatric closely related species often display nuptial colours at opposite ends of the spectrum with males either blue or yellow to red. Colour polymorphisms within single populations are common too. We propose that competition between males for breeding sites promotes such colour diversification, and thereby speciation. We hypothesize that male aggression is primarily directed towards males of the common colour, and that rare colour morphs enjoy a negatively frequency-dependent fitness advantage. We test our hypothesis with a large dataset on the distributions and nuptial colorations of 52 species on 47 habitat islands in Lake Victoria, and with a smaller dataset on the within-spawning-site distributions of males with different coloration. We report that territories of males of the same colour are negatively associated on the spawning site, and that the distribution of closely related species over habitat islands is determined by nuptial coloration in the fashion predicted by our hypothesis. Whereas among unrelated species those with similar nuptial colour are positively associated, among closely related species those with similar colour are negatively associated and those with different colour are positively associated. This implies that negatively frequency-dependent selection on nuptial coloration among closely related species is a sufficiently strong force to override other effects on species distributions. We suggest that male-male competition is an important and previously neglected agent of diversification among haplochromine cichlid fishes.

Laboratory mating trials indicate incipient speciation by sexual selection among populations of the cichlid fish Pseudotropheus zebra from Lake Malawi

It has been suggested that sexual selection may have played a major role in the rapid evolution of hundreds of species of cichlid fishes in Lake Malawi. We report the results of a laboratory test of assortative mating among Lake Malawi cichlid fishes from five closely related geographical populations differing in male courtship colour. Paternity of clutches was tested using microsatellite DNA typing of offspring. Out of 1955 offspring typed, 1296 (66.3%) were sired by the male from the same population as the female, which is more than three times the rate expected if females do not differentiate among males of the different populations (20%). This result indicates that mate preferences of geographical races are strongly differentiated, consistent with the races representing incipient geographical species diverging under sexual selection exerted by female preferences for different male courtship colours.

Learned social preference in zebrafish

How social aggregations arise and persist is central to our understanding of evolution, behavior, and psychology. When social groups arise within a species, evolutionary divergence and speciation can result. To understand this diversifying role of social behavior, we must examine the internal and external influences that lead to nonrandom assortment of phenotypes. Many fishes form aggregations called shoals that reduce predation risk while enhancing foraging and reproductive success. Thus, shoaling is adaptive, and signals that maintain shoals are likely to evolve under selection. Given the diversity of pigment patterns among Danio fishes, visual signals might be especially important in mediating social behaviors in this group. Our understanding of pigment pattern development in the zebrafish D. rerio allows integrative analyses of how molecular variation leads to morphological variation among individuals and how morphological variation influences social interactions. Here, we use the zebrafish pigment mutant nacre/mitfa to test roles for genetic and environmental determinants in the development of shoaling preference. We demonstrate that individuals discriminate between shoals having different pigment pattern phenotypes and that early experience determines shoaling preference. These results suggest a role for social learning in pigment pattern diversification in danios.

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.

The species flocks of East African cichlid fishes: recent advances in molecular phylogenetics and population genetics

With more than 3,000 species, the fish family Cichlidae is one of the most species-rich families of vertebrates. Cichlids occur in southern and central America, Africa, Madagascar, and India. The hotspot of their biodiversity is East Africa, where they form adaptive radiations composed of hundreds of endemic species in several lakes of various sizes and ages. The unparalleled species richness of East African cichlids has been something of a conundrum for evolutionary biologists and ecologists, since it has been in doubt whether these hundreds of species arose by allopatric speciation or whether it is necessary to invoke somewhat less traditional models of speciation, such as micro-allopatric, peripatric, or even sympatric speciation or evolution through sexual selection mediated by female choice. Ernst Mayr's analyses of these evolutionary uniquely diverse species assemblages have contributed to a more direct approach to this problem and have led to a deeper understanding of the patterns and processes that caused the formation of these huge groups of species. We review here recent molecular data on population differentiation and phylogenetics, which have helped to unravel, to some extent, the patterns and processes that led to the formation and ecological maintenance of cichlid species flocks. It is becoming apparent that sexually selected traits do play an important role in speciation in micro-allopatric or even sympatric settings. Species richness seems to be roughly correlated with the surface area, but not the age, of the lakes. We observe that the oldest lineages of a species flock of cichlids are often less species-rich and live in the open water or deepwater habitats. While the species flocks of the Lake Malawai and the Lake Victoria areas were shown to be monophyletic, the cichlid assemblage of Lake Tanganyika seems to consist of several independent species flocks. Cichlids emerge as an evolutionary model system in which many fundamental questions in evolution and ecology can be tested successfully, yet for other fish species flocks the relative importance of alternative mechanisms of speciation is likely to differ from that in cichlid fish.

AFLPs RESOLVE PHYLOGENY AND REVEAL MITOCHONDRIAL INTROGRESSION WITHIN A SPECIES FLOCK OF AFRICAN ELECTRIC FISH (MORMYROIDEA: TELEOSTEI)

Estimating species phylogeny from a single gene tree can be especially problematic for studies of species flocks in which diversification has been rapid. Here we compare a phylogenetic hypothesis derived from cytochrome b (cyt b) sequences with another based on amplified fragment length polymorphisms (AFLP) for 60 specimens of a monophyletic riverine species flock of mormyrid electric fishes collected in Gabon, west-central Africa. We analyze the aligned cyt b sequences by Wagner parsimony and AFLP data generated from 10 primer combinations using neighbor-joining from a Nei-Li distance matrix, Wagner parsimony, and Dollo parsimony. The different analysis methods yield AFLP tree topologies with few conflicting nodes. Recovered basal relationships in the group are similar between cyt b and AFLP analyses, but differ substantially at many of the more derived nodes. More of the clades recovered with the AFLP characters are consistent with the morphological characters used to designate operational taxonomic units in this group. These results support our hypothesis that the mitochondrial gene tree differs from the overall species phylogeny due at least in part to mitochondrial introgession among lineages. Mapping the two forms of electric organ found in this group onto the AFLP tree suggests that posteriorly innervated electrocytes with nonpenetrating stalks have independently evolved from anteriorly innervated, penetrating-stalk electrocytes at least three times.