Differences among reciprocal hybrids of Labeotropheus.

Current evidence suggests that hybridization played a crucial role in the early evolution and diversification of the species flocks of cichlid fishes in the African Great Lakes. Nonetheless, evidence for hybridization in the extant cichlid fauna is scant, suggesting that hybridization is rare in the modern era, perhaps enforced by natural or sexual selection acting against F1 hybrids. Additionally, most experimental studies of hybridization perform a hybrid cross in one direction, ignoring the reciprocal hybrid. In this study, we perform reciprocal crosses between sympatric congeners from Lake Malawi, Labeotropheus fuelleborni and L. trewavasae, in order to compare the body shape and coloration of males of both of these hybrids, as well as to examine how these hybrids fare during both inter- and intrasexual interactions. We found that L. trewavasae-sired hybrid males are intermediate to the parental species both morphologically and chromatically, while the reciprocal L. fuelleborni-sired hybrids are likely transgressive hybrids. Males of these transgressive hybrids also fare poorly during our mate choice experiments. While female L. trewavasae reject them as possible mates, male L. trewavasae do not make a distinction between them and conspecific males. Selection against transgressive F1 hybrids as observed in our crossing experiments may help explain why contemporary hybridization in Lake Malawi cichlids appears to be rare. Supplementary Information: The online version contains supplementary material available at 10.1007/s10750-022-05092-4.

Assortative aggression among males in a sympatric pair of Labeotropheus from Lake Malawi, Africa.

Female mate choice has strong experimental support as a diversifying force in the speciation of the haplochromine cichlid fishes of Lake Malawi, Africa. Somewhat less understood is the role that male-male aggression might have played in the evolution of new species of these fishes. In the rock-dwelling haplochromines of Lake Malawi, primarily territory-holding males successfully court females; by determining which males gain territories, male-male aggression could support speciation by excluding less-fit males from the breeding population. To test the hypothesis that males should direct more aggression towards conspecific rivals, the aggression directed towards conspecific and heterospecific opponents was compared in a sympatric pair of cichlids of the genus Labeotropheus Ahl 1927 (Labeotropheus fuelleborni Ahl 1927 and Labeotropheus trewavasae Fryer 1956). It was found that when presented with a pair of rivals, males of both species did direct more aggression towards the conspecific opponent, and the amount of aggression increased when the conspecific opponent was larger than the heterospecific opponent. In addition, this study found a difference in the behavioural repertoire of the species: L. fuelleborni tends to rely on displays to intimidate opponents, whereas L. trewavasae employs more physical attacks to drive away opponents. Males of both species can thus recognize conspecifics and assess an opponent's relative threat to their ability to successfully reproduce, and use species-specific strategies to intimidate opponents.

Selection, hybridization, and the evolution of morphology in the Lake Malawi endemic cichlids of the genus Labeotropheus.

The cichlid fishes of Lake Malawi are the paramount example of adaptive radiation in vertebrates. Evidence of their astounding diversity is perhaps most visible in their adaptations for obtaining food; the genus Labeotropheus, due to their prominent snouts, are an interesting example of an extreme adaptation for feeding. Two different body types are found in this genus: a deep-bodied form (e.g., L. fuelleborni) found most often in turbulent shallow water; and a slender bodied form (e.g., L. trewavasae) found in structurally-complex deep water habitats. Here we test the hypothesis that L. trewavasae should suffer a loss in fitness, measured as growth rate, if raised in turbulence; additionally, we examined growth and morphology of L. fuelleborni and L. fuelleborni x L. trewavasae hybrids under these conditions. We did find the predicted loss of fitness in turbulent-raised L. trewavasae, but found no loss of fitness for L. fuelleborni in either condition; hybrids, due to an unusual morphology, performed better in turbulent as opposed to control conditions. Fitness in turbulent conditions was dependent upon morphology, with deeper bodies and upturned neurocrania allowing a greater growth rate under these conditions. Directional selection on morphology was crucial in the evolution of morphology in the Labeotropheus.

Changes in the colour of light cue circadian activity.

The discovery of melanopsin, the non-visual opsin present in intrinsically photosensitive retinal ganglion cells (ipRGCs), has created great excitement in the field of circadian biology. Now, researchers have emphasized melanopsin as the main photopigment governing circadian activity in vertebrates. Circadian biologists have tested this idea under standard laboratory, 12h Light: 12h Dark, lighting conditions that lack the dramatic daily colour changes of natural skylight. Here we used a stimulus paradigm in which the colour of the illumination changed throughout the day, thus mimicking natural skylight, but luminance, sensed intrinsically by melanopsin containing ganglion cells, was kept constant. We show in two species of cichlid, Aequidens pulcher and Labeotropheus fuelleborni, that changes in light colour, not intensity, are the primary determinants of natural circadian activity. Moreover, opponent-cone photoreceptor inputs to ipRGCs mediate the sensation of wavelength change, and not the intrinsic photopigment, melanopsin. These results have implications for understanding the evolutionary biology of non-visual photosensory pathways and answer long-standing questions about the nature and distribution of photopigments in organisms, including providing a solution to the mystery of why nocturnal animals routinely have mutations that interrupt the function of their short wavelength sensitive photopigment gene.

One fish, two fish, red fish, blue fish: geography, ecology, sympatry, and male coloration in the lake Malawi cichlid genus labeotropheus (perciformes: cichlidae).

While sexual selection on male coloration has been important in haplochromine cichlid speciation, few studies to date have examined potential environmental influences on color pattern evolution. Data from multiple sources on male nuptial coloration of the Lake Malawi endemic genus Labeotropheus were used to examine the relationship between color patterns and the environments in which these patterns were found. Red- or carotenoid-pigmented males were concentrated in the northwestern portion of Lake Malawi and were also associated with increasing depth. Further, the presence or absence of L. fuelleborni influenced the coloration of L. trewavasae populations; when L. fuelleborni was present, L. trewavasae males were more likely to exhibit some degree of red coloration. While these results support the idea that sexual selection on male coloration is an important factor in the haplochromine speciation, they also underscore the importance of environmental influences on the evolution of color patterns.

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.

Directional sexual selection on chroma and within-pattern colour contrast in Labeotropheus fuelleborni.

Speciation via intersexual selection on male nuptial colour pattern is thought to have been a major force in promoting the explosive speciation of African haplochromine cichlids, yet there is very little direct empirical evidence of directional preferences within populations. In this study, we used objective spectrophotometry and analyses based on visual physiology to determine whether females of the Katale population of Labeotropheus fuelleborni, a Lake Malawi haplochromine, prefer males that have higher chroma and more within-pattern colour contrast. In paired male preference tests, female Katale L. fuelleborni showed increasing preferences for males with more relatively saturated colours on their flanks. They also showed increasing preferences for males with relatively higher contrast levels among flank elements. This is the first empirical evidence, to our knowledge, for male colour as a directionally sexually selected trait within a haplochromine cichlid population.