Inheritance of female mating preference in a sympatric sibling species pair of Lake Victoria cichlids: implications for speciation

Female mate choice has often been proposed to play an important role in cases of rapid speciation, in particular in the explosively evolved haplochromine cichlid species flocks of the Great Lakes of East Africa. Little, if anything, is known in cichlid radiations about the heritability of female mating preferences. Entirely sympatric distribution, large ecological overlap and conspicuous differences in male nuptial coloration, and female preferences for these, make the sister species Pundamilia pundamilia and P. nyererei from Lake Victoria an ideally suited species pair to test assumptions on the genetics of mating preferences made in models of sympatric speciation. Female mate choice is necessary and sufficient to maintain reproductive isolation between these species, and it is perhaps not unlikely therefore, that female mate choice has been important during speciation. A prerequisite for this, which had remained untested in African cichlid fish, is that variation in female mating preferences is heritable. We investigated mating preferences of females of these sister species and their hybrids to test this assumption of most sympatric speciation models, and to further test the assumption of some models of sympatric speciation by sexual selection that female preference is a single-gene trait. We find that the differences in female mating preferences between the sister species are heritable, possibly with quite high heritabilities, and that few but probably more than one genetic loci contribute to this behavioural speciation trait with no apparent dominance. We discuss these results in the light of speciation models and the debate about the explosive radiation of cichlid fishes in Lake Victoria.     

Quantitative Genetic Analyses of Male Color Pattern and Female Mate Choice in a Pair of Cichlid Fishes of Lake Malawi,East Africa

The traits involved in sexual selection, such as male secondary sexual characteristics and female mate choice, often co-evolve which can promote population differentiation. However, the genetic architecture of these phenotypes can influence their evolvability and thereby affect the divergence of species. The extraordinary diversity of East African cichlid fishes is often attributed to strong sexual selection and thus this system provides an excellent model to test predictions regarding the genetic architecture of sexually selected traits that contribute to reproductive isolation. In particular, theory predicts that rapid speciation is facilitated when male sexual traits and female mating preferences are controlled by a limited number of linked genes. However, few studies have examined the genetic basis of male secondary sexual traits and female mating preferences in cichlids and none have investigated the genetic architecture of both jointly. In this study, we artificially hybridized a pair of behaviorally isolated cichlid fishes from Lake Malawi and quantified both melanistic color pattern and female mate choice. We investigated the genetic architecture of both phenotypes using quantitative genetic analyses. Our results suggest that 1) many non-additively acting genetic factors influence melanistic color patterns, 2) female mate choice may be controlled by a minimum of 1–2 non-additive genetic factors, and 3) F₂ female mate choice is not influenced by male courting effort. Furthermore, a joint analysis of color pattern and female mate choice indicates that the genes underlying these two traits are unlikely to be physically linked. These results suggest that reproductive isolation may evolve rapidly owing to the few genetic factors underlying female mate choice. Hence, female mate choice likely played an important role in the unparalleled speciation of East African cichlid fish.     

Heritability and adaptive significance of the number of egg-dummies in the cichlid fish Astatotilapia burtoni

Cichlid fishes are a textbook example of rapid speciation and exuberant diversity--this applies especially to haplochromines, a lineage with approximately 1800 species. Haplochromine males uniquely possess oval, bright spots on their anal fin, called 'egg-spots' or 'egg-dummies'. These are presumed to be an evolutionary key innovation that contributed to the tribe's evolutionary success. Egg-spots have been proposed to mimic the ova of the mouthbrooding females of the corresponding species, contribute to fertilization success and even facilitate species recognition. Interestingly, egg-spot number varies extensively not only between species, but also within some populations. This high degree of intraspecific variation may appear to be counterintuitive since selection might be expected to act to stabilize traits that are correlated with fitness measures. We addressed this 'paradox' experimentally, and found that in the haplochromine cichlid Astatotilapia burtoni, the number of egg-spots was related to male age, body condition and dominance status. Intriguingly, the egg-spot number also had a high heritable component (narrow sense heritability of 0.5). These results suggest that the function of egg-spots might have less to do with fertilization success or species recognition, but rather relate to mate choice and/or male-male competition, helping to explain the high variability in this important trait.     

Female mate choice and species recognition between two closely related cichlid fish of Lake Malawi,Metriaclima estherae and M. callainos

Cichlids are one of the most diverse and colourful groups of freshwater fishes in the world. Despite much investigation, the factors that promote speciation in these fishes are still uncertain. However, previous studies suggest that sexual selection on male colour is one of the main drivers of speciation among these fishes. Metriaclima estherae is a polymorphic cichlid species from Lake Malawi, and thus provides an ideal model for the investigation of the importance of colour as a species recognition cue. M. callainos is a closely related and morphologically similar species, with male colour pattern very similar to that of M. estherae. We tested female choice by giving females of the two species a choice between conspecific and heterospecific males in the presence and absence of visual (colour) and chemical cues. The results show that females of M. callainos were able to reliably recognize conspecific males, even when colour was eliminated as a cue. However, females of M. estherae did not prefer conspecific males, although they were able to discriminate between red and blue conspecific colour morphs by using chemical cues. These results suggest that species recognition cues may differ even among closely related species of cichlid fish, and that female preferences for male coloration may be weak in certain species.     

Behavioural isolation may facilitate homoploid hybrid speciation in cichlid fish

Hybrid speciation is constrained by the homogenizing effects of gene flow from the parental species. In the absence of post‐mating isolation due to structural changes in the genome, or temporal or spatial premating isolation, another form of reproductive isolation would be needed for homoploid hybrid speciation to occur. Here, we investigate the potential of behavioural mate choice to generate assortative mating among hybrids and parental species. We made three‐first‐generation hybrid crosses between different species of African cichlid fish. In three‐way mate‐choice experiments, we allowed hybrid and nonhybrid females to mate with either hybrid or nonhybrid males. We found that hybrids generally mated nonrandomly and that hybridization can lead to the expression of new combinations of traits and preferences that behaviourally isolate hybrids from both parental species. Specifically, we find that the phenotypic distinctiveness of hybrids predicts the symmetry and extent of their reproductive isolation. Our data suggest that behavioural mate choice among hybrids may facilitate the establishment of isolated hybrid populations, even in proximity to one or both parental species.     

Direct mate choice for simultaneous acoustic and visual courtship displays in the damselfish, Dascyllus albisella (Pomacentridae)

Acoustic signals are well established as key components of mate selection in terrestrial species, but not in aquatic species. It has long been known that damselfish (Pomacentridae) use a combined visual and acoustic display in their courtship.. This study examined several male qualities including individual size, courtship vigor, territory size and complexity, as well as components of the acoustic call including dominant frequency, pulse characteristics and repetition rate. The objective was to determine which male traits were correlated with mating success. Observations made over ten reproductive cycles revealed that female mate choices were not random and that male mating success was correlated with courtship rate (a simultaneous visual and acoustic cue) and the number of neighboring females, but not with male morphological traits, territory quality, or acoustic call structure. These results suggest that females choose mates based on a condition-dependent trait (courtship rate) that advertises quality of paternal care, which supports good parent models of sexual selection, thereby demonstrating the importance of the combined acoustic/visual display for sexual selection in fishes     

Starting over: experimental effects of breeding delay on reproductive success in early‐arriving male American redstarts

Birds that arrive and breed early often have higher reproductive success than late individuals, either as a consequence of timing‐specific advantages (the timing hypothesis) or because these individuals and/or their resources are of higher quality (the quality hypothesis). In this study, we examined the potential influence of several factors affecting reproductive success by experimentally delaying breeding of early‐arriving male American redstarts Setophaga ruticilla, a species for which early male arrival is strongly related to increased reproductive success. Our manipulation involved the capture, holding, and release of males following pairing and territory establishment, resulting in the majority of subjects (67%) losing their initial mate (47%) or mate and territory (20%) and forcing them to start over approximately 12 d after their initial arrival. Males forced to start over (i.e. those losing their first territory and/or mate) did not experience any decrease in body condition, nor did their reproductive behaviour differ from that of early‐arriving control males. We found that naturally early‐arriving but experimentally manipulated males suffered reduced fledging success in comparison to early‐arriving males that bred early or late, but equivalent success in comparison to males that arrived and bred late. Based on our results, we propose that the relationship between early arrival and higher reproductive success in this species is mediated not simply by individual male quality or absolute arrival timing alone, but rather some other aspect of resource quality is likely important. We discuss and present evidence for two alternative explanations under the quality hypothesis: female quality and territory quality. To our knowledge, our study is the first to investigate the effects of experimentally delaying male breeding time, strengthening previous correlational evidence for resource quality as a potentially important selective agent driving early arrival in migratory birds.     

Sexual selection,reproductive behavior,and speciation in the mbuna species flock of Lake Malawi (Pisces: Cichlidae)

Synopsis The cichlid species flocks of the African Great Lakes represent the most extreme case of adaptive radiation among vertebrates. Recently, attention has focused on the potential for sexual selection to drive or accelerate speciation in these fishes. Cichlids as a whole are social in nature and display complex behavior, particularly during courtship and spawning; however, the extent to which changes in species recognition cues may account for species diversity among haplochromine lineages has remained speculative. Our investigations have indicated that oral incubating haplochromines show a reduction in diversity and extent of courtship relative to substrate brooding cichlids, and apparently retain aspects of a primitive specific mate recognition system. Laboratory observations of courtship in the Malawian endemicPseudotropheus zebra suggest that organization of the spawning bout is loose, and lacking in any well defined stimulus-response chain. Interspecific comparisons of in situ courtship behavior among male mbuna, lithophilous haplochromines of Lake Malawi, revealed only one potential example of species-specific behavior, and indicated that mate choice occurred prior to the onset of intense courtship. Courtship display by male mbuna does not appear critical to species recognition and may represent an evolutionary relict. Alternatively, male courtship display may have an indirect effect on successful reproduction or may be maintained through pleiotropic effects. The mbuna display no evidence of behavioral innovation and show limited interspecific differentiation in behavioral expression. More generally, there is no evidence at present to suggest that epigamic sexual selection, acting on courtship behavior, has been a major mechanism in the diversification of the haplochromine species flocks.     

Colour pattern as a single trait driving speciation in Hypoplectrus coral reef fishes?

Theory shows that speciation in the presence of gene flow occurs only under narrow conditions. One of the most favourable scenarios for speciation with gene flow is established when a single trait is both under disruptive natural selection and used to cue assortative mating. Here, we demonstrate the potential for a single trait, colour pattern, to drive incipient speciation in the genus Hypoplectrus (Serranidae), coral reef fishes known for their striking colour polymorphism. We provide data demonstrating that sympatric Hypoplectrus colour morphs mate assortatively and are genetically distinct. Furthermore, we identify ecological conditions conducive to disruptive selection on colour pattern by presenting behavioural evidence of aggressive mimicry, whereby predatory Hypoplectrus colour morphs mimic the colour patterns of non-predatory reef fish species to increase their success approaching and attacking prey. We propose that colour-based assortative mating, combined with disruptive selection on colour pattern, is driving speciation in Hypoplectrus coral reef fishes.     

Colour forms of Amazonian cichlid fish represent reproductively isolated species

Laboratory mate choice experiments have confirmed species status for cichlid fish in the African Great Lakes that differ in colour and little else. Colour differences between allopatric populations of the South American cichlid genus Apistogramma are known for many species, yet the status of such populations has not been previously tested. Analysis of the genetic relationships and mate choice characteristics of populations previously described as Apistogramma caetei from eastern Amazonia indicates genetic differentiation into at least three allopatric lineages, which also show strong prezygotic isolation through female mate choice, confirming them as Biological species. If future studies confirm that this result is indicative of a general trend, the species richness of the South American cichlid fishes may presently be seriously underestimated.     

Disruptive sexual selection on male nuptial coloration in an experimental hybrid population of cichlid fish

Theory suggests that genetic polymorphisms in female mating preferences may cause disruptive selection on male traits, facilitating phenotypic differentiation despite gene flow, as in reinforcement or other models of speciation with gene flow. Very little experimental data have been published to test the assumptions regarding the genetics of mate choice that such theory relies on. We generated a population segregating for female mating preferences and male colour dissociated from other species differences by breeding hybrids between species of the cichlid fish genus Pundamilia. We measured male mating success as a function of male colour. First, we demonstrate that non-hybrid females of both species use male nuptial coloration for choosing mates, but with inversed preferences. Second, we show that variation in female mating preferences in an F2 hybrid population generates a quadratic fitness function for male coloration suggestive of disruptive selection: intermediate males obtained fewer matings than males at either extreme of the colour range. If the genetics of female mate choice in Pundamilia are representative for those in other species of Lake Victoria cichlid fish, it may help explain the origin and maintenance of phenotypic diversity despite some gene flow.     

Ecological and evolutionary consequences of the trophic polymorphism in Cichlasoma citrinellum (Pisces: Cichlidae)

The neotropical cichlid fish Cichlasoma citrinellum is polymorphic in the structure of its pharyngeal jaw apparatus and external morphology. The pharyngeal jaws are either gracile and bear slender, pointed teeth (papilliform) or robust with strong, rounded teeth (molariform). Molariform morphs have a ‘benthic’, and papilliform morphs a ‘limnetic’ body form. Furthermore, this species is also polychromatic, with yellow and black morphs. The molariform morphology of the pharyngeal jaw apparatus adapts the fish for cracking and feeding on snails. Based on analysis of stomach contents, 94% of the molariform morph ate snails whereas only 19%, of the papilliform morph did so. This result suggests that the morphs occupy different ecological niches. The morphology of the pharyngeal jaw apparatus does not correlate significantly with sex, but it does with body colouration (P<0.005). Cichlasoma citrinellum mate assortatively with their own colour; therefore a mating preference for colour may lead to genetic isolation of trophic morphs. The frequency of the molariform morph differs strikingly among populations of five Nicaraguan lakes and its abundance is correlated with the abundance of snails, the fishes' principal prey item. Among populations the frequency of molariform morphs decreases in the dry season. Morphology possibly changes reversibly within particular individuals between seasons. These results suggest that phenotypic plasticity and polymorphisms may be an adaptive characteristic of cichlid fishes. Patterns of intraspecific morphological variation match patterns of interspecific morphological diversification which suggests that universal developmental mechanisms canalize the possible expressions of morphology. The ability to respond morphologically to environmental shifts, in conjunction with genetically determined trophic polymorphisms and sexual selection via mate choice, could be the basis for speciation through intermediate stages of polymorphism of the impressive adaptive radiation of cichlid fishes.     

Female preference for conspecific males based on olfactory cues in a Lake Malawi cichlid fish

Research on reproductive isolation in African cichlid fishes has largely focused on the role of nuptial colours, but other sensory modes may play an important role in mate choice. Here, we compare the relative importance of visual and olfactory cues in mate recognition by females of a Lake Malawi cichlid species. Female Pseudotropheus emmiltos were given a choice of spawning next to a conspecific male or a male of the closely-related sympatric Pseudotropheus fainzilberi. Significant preference for conspecific males only occurred when olfactory cues were present. This suggests that divergence of olfactory signals may have been an important influence on the explosive radiation of the East African species flock.     

Patterns in fish radiation are compatible with Pleistocene desiccation of Lake Victoria and 14,600 year history for its cichlid species flock

Geophysical data are currently being interpreted as evidence for a late Pleistocene desiccation of Lake Victoria and its refilling 14,600 years ago. This implies that between 500 and 1000 endemic cichlid fish species must have evolved in 14,600 years, the fastest large-scale species radiation known. A recent review concludes that biological evidence clearly rejects the postulated Pleistocene desiccation of the lake: a 14,600 year history would imply exceptionally high speciation rates across a range of unrelated fish taxa. To test this suggestion, I calculated speciation rates for all 41 phylogenetic lineages of fish in the lake. Except for one cichlid lineage, accepting a 14 600 year history does not require any speciation rates that fall outside the range observed in fishes in other young lakes around the world. The exceptional taxon is a lineage of haplochromine cichlids that is also known for its rapid speciation elsewhere. Moreover, since it is unknown how many founding species it has, it is not certain that its speciation rates are really outside the range observed in fishes in other young lakes. Fish speciation rates are generally faster in younger than in older lakes, and those in Lake Victoria, by far the largest of the young lakes of the world, are no exception. From the speciation rates and from biogeographical observations that Lake Victoria endemics, which lack close relatives within the lake basin, have such relatives in adjacent drainage systems that may have had Holocene connections to Lake Victoria, I conclude that the composition of the fish assemblage does not provide biological evidence against Pleistocene desiccation. It supports a hypothesis of recent colonization from outside the lake basin rather than survival of a diverse assemblage within the basin.     

Construction of a BAC library for Haplochromis chilotes,a cichlid fish from Lake Victoria

Cichlid fishes in Lake Victoria are model organisms for studying rapid radiation and speciation. On the way to examine the molecular basis of how these cichlid fishes achieved such a remarkable morphological diversification, we constructed a bacterial artificial chromosome (BAC) library derived from a cichlid species, Haplochromis chilotes, from Lake Victoria. The library includes 157,056 clones with the average insert size of 128 kb, corresponding to a 10-fold coverage of the H. chilotes genome. Given that the cichlid fishes endemic to Lake Victoria are closely related to one another phylogenetically and their genomes are nearly identical, this BAC library can be utilized to isolate genes from the more than 200 Haplochromine cichlid species in Lake Victoria.     

Mate Choice and Hybridization in Lake Malawi Cichlids, Sciaenochromis fryeri and Cynotilapia afra

Lake Malawi is home to hundreds of cichlid species, most of which are endemic to the lake. Several of these species exhibit a remarkably similar behavioural repertoire including territorial and reproductive behaviours. Despite their diverse anatomy and body colouration, hybridization among Malawi cichlids has been observed in captivity and has also been reported to occur in nature. Here, possible factors underlying hybridization between two species, Sciaenochromis fryeri, a large piscivore, and Cynotilapia afra (Jalo reef), a small planktivore biocover grazer, were investigated in captivity. In a series of experiments the size of the territory, the size of the males and the availability of conspecific mates were manipulated. It was found that mate selection in these two species under semi‐natural conditions is dependent not only upon species‐specific cues but also upon the status of the male. Larger male body size and larger territory size was generally preferred by females and when these attributes characterized a heterospecific male they also increased the probability of heterospecific courtship. Given that spawning between the above‐mentioned species resulted in fertile offspring and that their geographical location and habitat overlap in nature, the effect of their mate‐choice strategies on hybridization and speciation is considered.     

Unusually fine-scale genetic structuring found in rapidly speciating Malawi cichlid fishes

Mechanisms behind the explosive radiation of over 500 cichlid fish species from a single founding population in Lake Malawi during the last 700 000 years are poorly understood. Recent studies have suggested that the degree of population subdivision among the habitat patches within the lake may be responsible, but the evidence has been circumstantial: lack of a dispersal stage in haplochromine cichlids; genetic and colour variation among populations separated by large-scale geographical barriers; and fluctuating lake levels. One reason for the rapidity of speciation in these fishes may be that population subdivision is on a much finer scale than previously thought. Here we quantify the level of population subdivision and estimate migration at a scale of 700-1400 m, in order to investigate whether cichlid populations are sufficiently isolated from each other for allopatric divergence and perhaps speciation to take place. Using six microsatellite loci, we demonstrate the existence of highly significant genetic differentiation between subpopulations on adjacent headlands in each of four rock-dwelling haplochromine cichlid species. Our results suggest that these fish populations are divided into thousands of subunits among which genetic divergence is currently occurring, and that this may provide unprecedented opportunities for allopatric speciation. <br>     

Complex histories of repeated gene flow in Cameroon crater lake cichlids cast doubt on one of the clearest examples of sympatric speciation

One of the most celebrated examples of sympatric speciation in nature are monophyletic radiations of cichlid fishes endemic to Cameroon crater lakes. However, phylogenetic inference of monophyly may not detect complex colonization histories involving some allopatric isolation, such as double invasions obscured by genome‐wide gene flow. Population genomic approaches are better suited to test hypotheses of sympatric speciation in these cases. Here, we use comprehensive sampling from all four sympatric crater lake cichlid radiations in Cameroon and outgroups across Africa combined with next‐generation sequencing to genotype tens of thousands of SNPs. We find considerable evidence of gene flow between all four radiations and neighboring riverine populations after initial colonization. In a few cases, some sympatric species are more closely related to outgroups than others, consistent with secondary gene flow facilitating their speciation. Our results do not rule out sympatric speciation in Cameroon cichlids, but rather reveal a complex history of speciation with gene flow, including allopatric and sympatric phases, resulting in both reproductively isolated species and incipient species complexes. The best remaining non‐cichlid examples of sympatric speciation all involve assortative mating within microhabitats. We speculate that this feature may be necessary to complete the process of sympatric speciation in nature.     

Genetic architecture sets limits on transgressive segregation in hybrid cichlid fishes

The role of hybridization in the evolution of animal species is poorly understood. Transgressive segregation is a mechanism through which hybridization can generate diversity and ultimately lead to speciation. In this report we investigated the capacity of hybridization to generate novel (transgressive) phenotypes in the taxonomically diverse cichlid fishes. We generated a large F2 hybrid population by crossing two closely related cichlid species from Lake Malawi in Africa with differently shaped heads. Our morphometric analysis focused on two traits with different selective histories. The cichlid lower jaw (mandible) has evolved in response to strong directional selection, and does not segregate beyond the parental phenotype. The cichlid neurocranium (skull) has likely diverged in response to forces other than consistent directional selection (e.g., stabilizing selection), and exhibits marked transgressive segregation in our F2 population. We show that the genetic architecture of the cichlid jaw limits transgression, whereas the genetic basis of skull shape is permissive of transgressive segregation. These data suggest that natural selection, acting through the genome, will limit the degree of diversity that may be achieved via hybridization. Results are discussed in the context of the broader question of how phenotypic diversity may be achieved in rapidly evolving systems.     

Size-dependent use of territorial space by a rock-dwelling cichlid fish

Territoriality fundamentally influences animal mating systems and patterns of population structure. Although territory ownership is already known to contribute importantly to male reproductive success and the ecological coexistence of African rock-dwelling cichlids, the significance of variation in territory features has received little attention in these fishes. In Lake Malawi, males of Pseudotropheus tropheops "orange chest" defend territories on either of two substrate classes at Harbour Island: flat rock slabs lacking crevices and caves, or structurally complex boulder fields containing cave shelters. Focal watches of this species demonstrated that both territory size and occupancy on either substrate type depend on the size of male residents. Males larger than a threshold size exclusively held the largest and most structurally complex territories. After removal of conspecific residents, more vacant territorial areas on cave-containing substrate were reoccupied by "orange chest" males in full breeding coloration compared to vacant areas on flat substrate. These findings suggest competition among "orange chest" males for complex rocky substrate. Defense of caves was associated with enhanced male courtship rates: the number of caves within a male's territory was a better predictor of courtship activity than was male size or territory area. In addition to territories being crucial for male reproductive success and therefore likely playing a role in sexual selection, male-male competition for caves in rock-dwelling cichlids may be promoted by the ecological advantage of enemy-free space. Smaller "orange chest" males lacking caves tended to move into adjacent boulder fields in the presence of predators, particularly at night. In contrast, males defending caves were more likely to remain on their territories when nocturnal predators were present. The territorial behaviors of P. tropheops "orange chest" that we observed in situ provide an instructive natural framework for testing the roles of substrate and ecology in the mating systems of rock-dwelling cichlid fishes.