Showing papers by "Martine E. Maan published in 2008"

Speciation through sensory drive in cichlid fish

Abstract: Theoretically, divergent selection on sensory systems can cause speciation through sensory drive. However, empirical evidence is rare and incomplete. Here we demonstrate sensory drive speciation within island populations of cichlid fish. We identify the ecological and molecular basis of divergent evolution in the cichlid visual system, demonstrate associated divergence in male colouration and female preferences, and show subsequent differentiation at neutral loci, indicating reproductive isolation. Evidence is replicated in several pairs of sympatric populations and species. Variation in the slope of the environmental gradients explains variation in the progress towards speciation: speciation occurs on all but the steepest gradients. This is the most complete demonstration so far of speciation through sensory drive without geographical isolation. Our results also provide a mechanistic explanation for the collapse of cichlid fish species diversity during the anthropogenic eutrophication of Lake Victoria.

Female preferences for aposematic signal components in a polymorphic poison frog

Abstract: Aposematic signals may be subject to conflicting selective pressures from predators and conspecifics. We studied female preferences for different components of aposematic coloration in the polymorphic poison frog Oophaga pumilio across several phenotypically distinct populations. This frog shows striking diversity in color and pattern between geographically isolated populations in western Panama. Results indicate that male dorsal color is the most important determiner of female preferences. We did not find consistent evidence for effects of other signal components, such as spotting pattern or ventral color. Females in two populations showed assortative preferences mediated by male dorsal coloration. In a third population we found incomplete color-assortative preference behavior, with females exhibiting strong discrimination toward one novel color but not another. These results hint at a possible interaction between sexual and natural selection: female tolerance of unfamiliar coloration patterns could facilitate the establishment of novel phenotypes that are favored by other selective pressures (e.g., predator biases). Furthermore, our study suggests that specific components of the aposematic signal (i.e., dorsal color, ventral color, and spotting pattern) are affected differently by natural and sexual selection.

Color polymorphism and predation in a Lake Victoria cichlid fish

Abstract: Haplochromine cichlid fish have radiated into hundreds of species in East-African lakes, possibly driven by divergent sexual selection on body coloration. We studied the color polymorphic Lake Victoria cichlid Neochromis omnicaeruleus, in which a presumably ancestral phenotype with blue males and brown females co-occurs with two distinct classes of blotched phenotypes in both sexes. Similar blotch polymorphisms occur in other haplochromine species, and in all studied cases blotched females are much more common than blotched males. In N. omnicaeruleus, the near absence of blotched males seems to be partly due to genetic linkage to a dominant female determiner that turns blotched males into females. However, laboratory breeding suggests that blotched males should be much more common than observed. Here we studied whether differential predation on blotched males contributes to their scarcity. First, in a predation experiment with wild birds, blotched fish indeed incurred more predator attacks. Second, underwater observations revealed behavioral differences between the sexes, consistent with an additional predation risk for males. These data suggest that differential predation with regard to color pattern and sex may be an important selective force in the evolution and maintenance of this color polymorphism. However, we also carried out a population census which revealed that blotched males were rare already as juveniles. To explain the scarcity of blotched males in nature, we therefore have to invoke either selection against blotched males early in life, or a more complex genetic model. These results emphasize the need for further research on the ecology and genetics of this widespread color polymorphism in cichlid fish.

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

Abstract: 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.

Sexual conflict over breeding substrate causes female expulsion and offspring loss in a cichlid fish

Abstract: Females of the Lake Tanganyika cichlid Lamprologus callipterus exclusively breed in empty snail shells that males collect in their territories. Male-male competition for shells is severe, leading to frequent shell stealing and territory takeover. As a consequence, males have breeding females in their shells that spawned with competitors. In this field study, we investigated both naturally occurring and experimentally induced encounters of territorial males with females that had spawned with other males. We found that the breeding success of females that were taken over by a different male was significantly reduced. Behavioral observations after experimental shell relocation further showed that males recognized females that they had not spawned with: males directed more exploration and manipulation behavior toward such shells compared with controls. Reoccupation rate of emptied experimental shells was significantly higher than that of unmanipulated empty shells. This indicates that shell stealing and nest takeover, followed by female expulsion, contribute to the reproductive success of L. callipterus males. We also found that female mate choice reduces expulsion risk: females preferred to mate with large males, and male size correlated with dominance. We conclude that the limited availability of breeding substrate is a key determinant of both intrasexual competition and intersexual conflict in this species.