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.     

Differences in sounds made by courting males of three closely related Lake Malawi cichlid species

Courtship sounds made by three sympatric cichlid species, Pseudotropheus zebra , P. callainos and an undescribed species known as P . 'zebra gold' were recorded and compared to investigate the potential role of acoustic signals in mate choice. Sounds were emitted during 'quiver' and 'circle' components of the male courtship display and consisted of rapidly repeated pulse units. Some sound variables differed significantly among species with P. callainos generally being separated from the other two species. This species produced sounds with higher peak frequency (for a given length) and lower number of pulses than P . 'zebra gold' and higher pulse durations than P. zebra . In addition, standard length was inversely related to peak frequency in both P . 'zebra gold' and P. callainos (this relation was not tested in P. zebra due to the small sample size). These differences might indicate different regimes of intraspecific sexual selection among the three species.     

Does the time spent near a male predict female mate choice in a Malawian cichlid?

Gravid female Malawian Pseudotropheus cichlids spent significantly more time with males that they subsequently chose as mates, indicating that time spent near a male is a valid and accurate method of measuring female preference. Furthermore, females preferred to mate with males that had longer pelvic fins and a larger number of eggspots on their anal fins. In some instances, females chose to mate with both of the males with which they were presented, possibly because they were unable to discriminate between them.     

Extreme microallopatric divergence in a cichlid species from Lake Malawi

We demonstrate significant population structuring on an extremely small spatial scale between adjacent demes of a Lake Malawi haplochromine cichlid species of the mbuna group, Pseudotropheus callainos, separated by only 35 m of habitat discontinuity. This substantiates further the notion that intralacustrine allopatric divergence may help to explain the high level of species richness of the mbuna in comparison to other Malawian cichlids, as well as of the Malawian haplochromines as a whole.     

Assortative mating among rock-dwelling cichlid fishes supports high estimates of species richness from Lake Malawi

It has been estimated that Lake Malawi, Africa, contains 500–650 endemic species of cichlid fishes, the largest number of vertebrate species endemic to any comparable sized area on the planet. As many of these putative species cannot be distinguished anatomically, these estimates of species richness depend to a great extent on the assumption that sympatrically occurring male colour morphs represent biological species. We have tested this assumption using a combination of behavioural observations of courtship and microsatellite DNA analysis for six putative species of the Pseudotropheus ( Tropheops ) complex and three of the Pseudotropheus ( Maylandia ) complex occurring sympatrically at Nkhata Bay. We were unable to demonstrate assortative courtship for the species pairs Pseudotropheus ( Maylandia ) zebra / P . 'gold zebra' or P. ( Tropheops ) 'band'/ P. ( T .) 'rust' because we were unable to distinguish between the females of these taxa. All other taxa showed clear assortative courtship, except for P. ( T .) 'deep', a deep-water species which was rarely observed. Fixation indices (θ_ST for the infinite allele model, and R _ST for the stepwise mutation model) calculated from six microsatellite DNA loci demonstrated significant deviations from panmixia in all pairwise comparisons of putative species, indicating little or no gene flow between populations. All taxa showed high levels of allelic diversity providing evidence that genetic bottlenecking may have been of limited importance in the speciation process. Assortative mating among taxa differing only in male colouration is supportive of theories that speciation in these fishes has been driven by sexual selection by female choice.     

Indirect mate choice, direct mate choice and species recognition in a bower-building cichlid fish lek

Sexual selection arising through female mate choice typically favours males with larger, brighter and louder signals. A critical challenge in sexual selection research is to determine the degree to which this pattern results from direct mate choice, where females select individual males based on variation in signalling traits, or indirect mate choice, where male competition governs access to reproductively active females. We investigated female mate choice in a lekking Lake Malawi cichlid fish, Hemitilapia oxyrhynchus, in which males build and aggressively defend sand 'bowers'. Similar to previous studies, we found that male reproductive success was positively associated with bower height and centrality on the lek. However, this pattern resulted from males holding these territories encountering more females, and thus their greater success was due to indirect mate choice. Following initial male courtship, an increase in the relative mating success of some males was observed, but this relative increase was unrelated to bower size or position. Crucially, experimentally manipulating bowers to resemble those of a co-occurring species had no appreciable effect on direct choice by females or male spawning success. Together, these results suggest indirect mate choice is the dominant force determining male-mating success in this species, and that bowers are not signals used in direct mate choice by females. We propose that, in this species, bowers have a primary function in intraspecific male competition, with the most competitive males maintaining larger and more central bowers that are favoured by sexual selection due to higher female encounter rates.     

Microsatellite paternity analysis on captive Lake Malawi cichlids supports reproductive isolation by direct mate choice

Three sympatrically occurring members of a cichlid fish species complex from Lake Malawi have previously been reported to show assortative mating in the field. Offspring from three experimental tanks each housing both sexes of all three members of this complex were screened at one to four microsatellite loci to determine paternity. Reproductive isolation was found to persist without exception under laboratory conditions, providing strong evidence for behavioural barriers to mating. Direct mate choice is suggested as the process preventing interbreeding. The results are interpreted with reference to the rapid and explosive evolution of the cichlid group.     

Hybrid origin of a cichlid population in Lake Malawi: implications for genetic variation and species diversity

The importance of species recognition to taxonomic diversity among Lake Malawi cichlids has been frequently discussed. Hybridization - the apparent breakdown of species recognition - has been observed sporadically among cichlids and has been viewed as both a constructive and a destructive force with respect to species diversity. Here we provide genetic evidence of a natural hybrid cichlid population with a unique colour phenotype and elevated levels of genetic variation. We discuss the potential evolutionary consequences of interspecific hybridization in Lake Malawi cichlids and propose that the role of hybridization in generating both genetic variability and species diversity of Lake Malawi cichlids warrants further consideration.     

Aggressive biases towards similarly coloured males in Lake Malawi cichlid fishes

In haplochromine cichlids, female mate choice based on male nuptial coloration has played an important role in speciation. Recent studies suggest that male coloration strongly influences the distribution of these fishes based on male-male aggression; males direct more aggression towards similarly coloured opponents while tolerating differently coloured individuals. We explored the role of male nuptial colour in aggression among the mbuna of Lake Malawi, examining aggression by male Metriaclima mbenjii, the red top cobalt zebra, towards conspecific opponents, similarly coloured heterospecific opponents and differently coloured heterospecifics. In trials in which focal males were offered a single opponent, while the total number of aggressive behaviours did not vary among opponent species, the types of behaviours did; focal males directed more lateral displays towards conspecifics than towards the other opponent species. When focal males were offered two opponents simultaneously, M. mbenjii directed more aggressive behaviours and more lateral displays towards similarly coloured opponents, regardless of species. Furthermore, when offered a conspecific and a similarly coloured opponent simultaneously, there were no differences in behaviour towards either opponent. Thus, nuptial coloration is used by males to identify competitors, and it suggests that male-male aggression may have also been an important diversifying force in speciation in rock-dwelling Lake Malawi cichlids.     

Parasite-mediated sexual selection and species divergence in Lake Victoria cichlid fish

We investigate the role of parasite-mediated sexual selection in the divergence of two species of Lake Victoria cichlids. Pundamilia pundamilia and Pundamilia nyererei represent a common pattern of male nuptial colour divergence between haplochromine sister species: metallic grey–blue in P. pundamilia and bright yellow and red in P. nyererei . Female mating preferences for different male colours maintain the genetic and phenotypic differentiation of the two species in clear water. Previous work indicated that the red coloration of P. nyererei males, which is subject to directional sexual selection, may be a carotenoid-dependent signal of parasite infestation rate. In the present study, we find a parallel result for P. pundamilia : bright blue males are infected with fewer species of parasites. We also find that parasite infestation rates differ quantitatively between the two species in a way that is consistent with species differences in diet and microhabitat. We conclude that parasite-mediated sexual selection may have contributed to the divergence of female mating preferences between P. pundamilia and P. nyererei , and may currently strengthen reproductive isolation between these species.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94 , 53–60.     

Early learning influences species assortative mating preferences in Lake Victoria cichlid fish

The Lake Victoria 'species flock' of cichlids is puzzling because reproductive isolation often occurs in the absence of substantial ecological differences among species. Theory predicts that this cannot evolve with most genetic mechanisms for mate choice. We provide the first evidence that learning, in the form of sexual imprinting, helps maintain reproductive isolation among closely related cichlid species. Using a cross-fostering experiment, we show that young females develop a sexual preference for males of their foster mothers' species, even reversing species assortative mating preferences. We suggest that learning creates favourable conditions for reproductive isolation to evolve.     

Species differences in courtship acoustic signals among five Lake Malawi cichlid species (Pseudotropheus spp.)

Male courtship acoustic signals from five Lake Malawi cichlid fish species of the Pseudotropheus zebra complex were recorded and compared. Sounds made by males of P. zebra , Pseudotropheus callainos and the undescribed species known as Pseudotropheus 'zebra gold' from Nkhata Bay, and Pseudotropheus emmiltos and Pseudotropheus faizilberi from Mphanga Rocks, differed significantly in the number of pulses and in pulse period. The largest differences in acoustic variables were found among the sympatric Mphanga Rocks species that, in contrast to the other three species, show relatively minor differences in male colour and pattern. These findings suggest that interspecific mate recognition is mediated by multimodal signals and that the mass of different sensory channels varies among sympatric species groups. This study also showed that sound peak frequency was significantly negatively correlated with male size and that sound production rate increased significantly with courtship rate.     

The effect of territory quality on female preference in Metriaclima zebra

The cichlid fishes of Lake Malawi are an excellent model group with which to study adaptive radiation and speciation; due to the vast number of individual species, large variation in feeding adaptations and behaviours, and relatively recent divergence are found. Males of many cichlid species will obtain and defend lek‐like territories, where they court potential mates in addition to foraging for food. Aspects of territory quality warrant investigation because of the relationship between reproductive success and territory defence success. In this study, we tested whether the presence of a small or large food resource had any effect on the preference of a gravid female for a potential mate. The results of this experiment suggest that the presence of a large food resource increases the attractiveness of a male to a potential mate. This may suggest that the presence of food alone may not be the most important factor, or there may be a threshold amount over which females are attracted. These results indicate that certain aspects of territory quality such as habitat type or proximity to a food resource may play a role in both intersexual and intrasexual selection mechanisms: perhaps even driving and possibly accelerating the speciation of certain lineages of cichlid fishes.     

A population genetic assessment of taxonomic species: The case of Lake Malawi cichlid fishes

Organisms sampled for population‐level research are typically assigned to species by morphological criteria. However, if those criteria are limited to one sex or life stage, or the organisms come from a complex of closely related forms, the species assignments may misdirect analyses. The impact of such sampling can be assessed from the correspondence of genetic clusters, identified only from patterns of genetic variation, to the species identified using only phenotypic criteria. We undertook this protocol with the rock‐dwelling mbuna cichlids of Lake Malawi, for which species within genera are usually identified using adult male coloration patterns. Given high local endemism of male colour patterns, and considerable allele sharing among species, there persists considerable taxonomic uncertainty in these fishes. Over 700 individuals from a single transect were photographed, genotyped and separately assigned: (a) to morphospecies using photographs; and (b) to genetic clusters using five widely used methods. Overall, the correspondence between clustering methods was strong for larger clusters, but methods varied widely in estimated number of clusters. The correspondence between morphospecies and genetic clusters was also strong for larger clusters, as well as some smaller clusters for some methods. These analyses generally affirm (a) adult male‐limited sampling and (b) the taxonomic status of Lake Malawi mbuna, as the species in our study largely appear to be well‐demarcated genetic entities. More generally, our analyses highlight the challenges for clustering methods when the number of populations is unknown, especially in cases of highly uneven sample sizes.     

Biogeography and population genetics of the Lake Malawi cichlid Melanochromis auratus: habitat transience, philopatry and speciation

Migration rates among nine populations of the endemic Lake Malawi cichlid Melanochromis auratus were estimated along a 42-km stretch of habitat in the southern end of the lake. Allele frequencies were surveyed at four simple sequence repeat (SSR) loci. The data suggest migration rates among populations are quite low. Exact tests indicate that statistically detectable allele frequency differences exist between many adjacent populations in the study. The F_ST value among all populations was estimated to be 0.151 (P < 0.0002). A biogeographic survey suggests that the highest levels of genetic differentiation exist between populations separated by stretches of deep water. Migration is more common between populations separated by shallower water or with shoreline dispersal routes. Reduced allelic diversity was observed at more recently created habitat patches, suggesting that either bottlenecks are associated with the colonization of new habitat patches or that these shallower sites were all founded by genetically depauperate ancestral populations. The extreme philopatry of M. auratus, coupled with the patchy distribution and transient nature of its preferred habitat, provides opportunities for both selection and genetic drift to produce genetic differentiation among populations. Both processes may be important to the evolution of taxonomic diversity in the East African cichlid species flocks.     

Variation in habitat preference and population structure among three species of the Lake Malawi cichlid genus Protomelas

Several studies have demonstrated strong population structuring over small distances in the rocky-shore mbuna cichlid fishes from Lake Malawi, suggesting the potential for allopatric speciation. However, many endemic Lake Malawi cichlids are neither mbuna, nor confined to rocky shores. Using microsatellites, we investigated the population structure in three species of the non-mbuna genus Protomelas. The rocky-shore P. taeniolatus showed high levels of population structure even over distances of less than 1 km, while the sandy-shore species P. similis showed no significant structure over distances up to 21 km. Protomelas fenestratus, which is generally found at the interface between rocks and sand, also showed low levels of population structure. Our results suggest that the model of allopatric speciation based on habitat fragmentation within the current lake basin may be equally applicable to rocky-shore non-mbuna as to mbuna, but that an alternative model is required to explain speciation among sandy-shore species as well as the deep-water and pelagic species.     

Diet alters species recognition in juvenile toads

Whether environmental effects during juvenile development can alter the ontogeny of adult mating behaviour remains largely unexplored. We evaluated the effect of diet on the early expression of conspecific recognition in spadefoot toads, Spea bombifrons. We found that juvenile toads display phonotaxis behaviour six weeks post-metamorphosis. However, preference for conspecifics versus heterospecifics emerged later and was diet dependent. Thus, the environment can affect the early development of species recognition in a way that might alter adult behaviour. Evaluating such effects is important for understanding variation in hybridization between species and the nature of species boundaries.     

Preference polymorphism for coloration but no speciation in a population of Lake Victoria cichlids

Female mating preference based on male nuptial coloration hasbeen suggested to be an important source of diversifying selectionin the radiation of Lake Victoria cichlid fish. Initial variationin female preference is a prerequisite for diversifying selection;however, it is rarely studied in natural populations. In clearwater areas of Lake Victoria, the sibling species Pundamiliapundamilia with blue males and Pundamilia nyererei with redmales coexist, intermediate phenotypes are rare, and most femaleshave species-assortative mating preferences. Here, we studya population of Pundamilia that inhabits turbid water wheremale coloration is variable from reddish to blue with most malesintermediate. We investigated male phenotype distribution andfemale mating preferences. Male phenotype was unimodally distributedwith a mode on intermediate color in 1 year and more blue-shiftedin 2 other years. In mate choice experiments with females ofthe turbid water population and males from a clearer water population,we found females with a significant and consistent preferencefor P. pundamilia (blue) males, females with such preferencesfor P. nyererei (red) males, and many females without a preference.Hence, female mating preferences in this population could causedisruptive selection on male coloration that is probably constrainedby the low signal transduction of the turbid water environment.We suggest that if environmental signal transduction was improvedand the preference/color polymorphism was stabilized by negativefrequency-dependent selection, divergent sexual selection mightseparate the 2 morphs into reproductively isolated species resemblingthe clear water species P. pundamilia and P. nyererei.     

No evidence for parallel sympatric speciation in cichlid species of the genus Pseudotropheus from north-western Lake Malawi

To test the hypothesis of parallel speciation by sexual selection, we examined length variation at six microsatellite loci of samples from four sites of four to six putative species belonging to two subgenera of rocky shore mbuna cichlids from Lake Malawi. Almost all fixation indices were significantly different from zero, suggesting that there is presently little or no gene flow among allopatric populations or sympatric species. Analysis of variance indicated that genetic distances among allopatric populations of putative conspecifics were significantly lower than among sympatric populations of heterospecifics. The topology of trees based on distance matrices was also largely consistent with the hypothesis that the putative species are monophyletic and have thus not evolved in parallel in their present locations. If parallel speciation does occur in Malawi cichlids, it may be on a larger spatial scale than investigated in our study.     

Lake flies and the deep-water demersal fish community of Lake Malawi

Larvae and pupae of Chaoborus edulis may be the most important food resource of some deep-water demersal fish species in southern Lake Malawi. C. edulis feed on crustacean zooplankton, which in turn feed on phytoplankton. Hence, demersal fish production is supported directly by planktonic primary production, rather than through a detrital food chain.