Correlation between nuptial colors and visual sensitivities tuned by opsins leads to species richness in sympatric Lake Victoria cichlid fishes.

Reproductive isolation that prevents interspecific hybridization between closely related coexisting species maintains sympatric species diversity. One of the reproductive isolations is mate choice based on color signals (breeding color perceived by color vision). This is well known in several animal taxa, yet little is known about its genetic and molecular mechanism. Lake Victoria cichlid fishes are thought to be an example of sympatric species diversity. In the species inhabiting different light environments in rocky shore, speciation by sensory drive through color signals has been proposed by analyses of the long wavelength-sensitive (LWS) opsin gene and the male nuptial coloration. However, the genetic and molecular mechanism of how diversity of sympatric species occurring in the same habitat is maintained remains unknown. To address this issue, we determined nucleotide sequences of eight opsins of six sympatric species collected from a sandy-muddy shore--an ideal model system for studying sympatric species. Among eight opsins, the LWS and RH1 alleles were diversified and one particular allele is dominant or fixed in each species, and we propose that this is due to natural selection. The functions of their LWS alleles were also diversified as shown by absorption measurements of reconstituted visual pigments. To analyze the relationship between nuptial coloration and the absorption of LWS pigments, we systematically evaluated and defined nuptial coloration. We showed that the coloration was species specific with respect to hue and significantly differentiated by the index values of hue (dominant wavelength: λ(d)). The λ(d) value of the male nuptial coloration correlated with the absorption of LWS pigments from all the species, suggesting that reproductive isolation through mate choice using color signals may prevent sympatric interspecific hybridization, thereby maintaining the species diversity in sympatric species in Lake Victoria.

Genetic structure of pelagic and littoral cichlid fishes from Lake Victoria.

The approximately 700 species of cichlids found in Lake Victoria in East Africa are thought to have evolved over a short period of time, and they represent one of the largest known examples of adaptive radiation. To understand the processes that are driving this spectacular radiation, we must determine the present genetic structure of these species and elucidate how this structure relates to the ecological conditions that caused their adaptation. We analyzed the genetic structure of two pelagic and seven littoral species sampled from the southeast area of Lake Victoria using sequences from the mtDNA control region and 12 microsatellite loci as markers. Using a Bayesian model-based clustering method to analyze the microsatellite data, we separated these nine species into four groups: one group composed of pelagic species and another three groups composed mainly of rocky-shore species. Furthermore, we found significant levels of genetic variation between species within each group at both marker loci using analysis of molecular variance (AMOVA), although the nine species often shared mtDNA haplotypes. We also found significant levels of genetic variation between populations within species. These results suggest that initial groupings, some of which appear to have been related to habitat differences, as well as divergence between species within groups took place among the cichlid species of Lake Victoria.

Population structure of two closely related pelagic cichlids in Lake Victoria, Haplochromis pyrrhocephalus and H. laparogramma.

Cichlid fishes in Lake Victoria show spectacular diversification that is thought to be recent. Therefore, by investigating those fishes, we may be able to elucidate recently completed or ongoing speciation processes. We studied the population structures of two closely related pelagic cichlid species, Haplochromis pyrrhocephalus and H. laparogramma, using a mitochondrial DNA locus and 12 nuclear microsatellite loci as putative neutral markers. Ten and two populations of H. pyrrhocephalus and H. laparogramma, respectively, were sampled from the southern part of Lake Victoria. We grouped those 12 populations into four mutually differentiated regional populations, one of which consisted of the two H. laparogramma populations. The levels of differentiation were substantial at the mitochondrial locus (F(ST) = 0.03-0.54), but very low at microsatellite loci (R(ST) = 0.008-0.116). The data from both types of loci indicated that the regional population of H. laparogramma was first separated from those of H. pyrrhocephalus if we set aside one erratic population of H. pyrrhocephalus. The data also suggested recent population expansions of the two species, the time scales for which were estimated to be on the order of 10(4)-10(5) years. These data suggested that dynamic speciation processes accompanied occasional spawning of new species and population size changes in this lake.