Semvua Isa Mzighani

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.

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.

Multiple Episodic Evolution Events in V1R Receptor Genes of East-African Cichlids

Fish use olfaction to detect a variety of nonvolatile chemical signals, and thus, this sense is key to survival and communication. However, the contribution of the olfactory sense to social—especially reproductive—interactions in cichlids is still controversial. To obtain insights into this issue, we investigated the genes encoding V1Rs—possible candidates for reproductive pheromone receptors—among East-African cichlids. Interestingly, we found an excess of nonsynonymous over synonymous substitutions in four of six V1R genes in multiple cichlid lineages. First, we found that highly dimorphic V1R2 allele groups were shared among the cichlids inhabiting all East-African Great Lakes emerged through the episodic accumulation of the nonsynonymous substitutions prior to the radiation of the Lake Tanganyika species flock. We further detected such episodic events in V1R1 of the tribe Tropheini, and in V1R3 and V1R6 of the tribe Trematocarini. The excess of nonsynonymous substitutions in these examples were indicated as dN/dS > 1, which were all statistically significant by Fisher’s exact test. Furthermore, we speculate that the amino acid changes in these episodic events are likely functional switch because they occurred in the putative ligand-binding pocket. Our finding of the occurrence of multiple episodic events and the unexpected gene diversity in one unique gene family is suggestive of the contribution of the V1R to the species diversification and the social interaction in cichlids.

Genetically distinct coelacanth population off the northern Tanzanian coast.

Since the sensational discovery of a living coelacanth off the east coast of South Africa, the geographic distribution of viable coelacanth populations has been a subject of debate. In the past, the coelacanths off the African mainland were thought to be strays from the Comoros because most coelacanths captured were caught in the waters surrounding the Comoros archipelagos. However, in recent years, a large number of coelacanths were captured off the coast of Tanzania, including nine living specimens observed in a remotely operated vehicles survey. Thus, it is possible that there is a reproducing population inhabiting waters off the Tanzania coast. We have sequenced the complete mitochondrial genomes of 21 Tanzanian and 2 Comoran coelacanths and analyzed these sequences together with two additional full mitochondrial genomes and 47 d-loop sequences from the literature. We found that the coelacanth population off the northern Tanzanian coast is genetically differentiated from those of the southern Tanzania coast and the Comoros, whereas no significant genetic differentiation occurs between the latter two localities. The differentiation between the northern and southern Tanzanian coast populations is consistent with the hypothesis that the existence of northward-flowing ocean current along the Tanzanian coast may reduce or prevent gene flow from the northern to the southern population. Finally, we estimated that the population localized to the southern Tanzanian coast and the Comoros diverged from other coelacanths at least 200,000 y ago. These results indicate that the coelacanths off the northern Tanzania coast are not strays but a genetically distinct group. Our study provides important information for the conservation of this threatened “living fossil.”

Geographical ancestry of Lake Malawi's cichlid fish diversity

The Lake Malawi haplochromine cichlid flock is one of the largest vertebrate adaptive radiations. The geographical source of the radiation has been assumed to be rivers to the south and east of Lake Malawi, where extant representatives of the flock are now present. Here, we provide mitochondrial DNA evidence suggesting the sister taxon to the Lake Malawi radiation is within the Great Ruaha river in Tanzania, north of Lake Malawi. Estimates of the time of divergence between the Lake Malawi flock and this riverine sister taxon range from 2.13 to 6.76 Ma, prior to origins of the current radiation 1.20–4.06 Ma. These results are congruent with evaluations of 2–3.75 Ma fossil material that suggest past faunal connections between Lake Malawi and the Ruaha. We propose that ancestors of the Malawi radiation became isolated within the catchment during Pliocene rifting that formed both Lake Malawi and the Kipengere/Livingstone mountain range, before colonizing rivers to the south and east of the lake region and radiating within the lake basin. Identification of this sister taxon allows tests of whether standing genetic diversity has predisposed Lake Malawi cichlids to rapid speciation and adaptive radiation.

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.

Nile tilapia invades the Lake Malawi catchment

The Lake Malawi/Nyasa catchment contains over 835 endemic cichlid fish species. This unique biodiversity has made it widely recognised as one of the worlds most significant freshwater ecosystems. Here we report the first occurrence records of two invasive tilapiines, Oreochromis niloticus and Oreochromis leucostictus, inside the Lake Malawi catchment. The introductions took place during initiatives to develop aquaculture and new capture fisheries. Oreochromis niloticus is an important competitor and predator of native species, has potential to hybridise with indigenous Oreochromis species, and has been widely implicated in biodiversity loss globally. It was a key contributor to the destruction of the Lake Victoria indigenous Oreochromis fishery. In light of apparent risks to unique biodiversity, and in the absence of robust evidence that introductions will bring enhanced socio-economic benefits over indigenous species, it is advisable that efforts be made to eradicate invasive species. The precautionary principle holds that future fisheries and aquaculture development in the region should be based exclusively on non-invasive indigenous species.

Visual adaptation in Lake Victoria cichlid fishes: depth-related variation of color and scotopic opsins in species from sand/mud bottoms

BackgroundFor Lake Victoria cichlid species inhabiting rocky substrates with differing light regimes, it has been proposed that adaptation of the long-wavelength-sensitive (LWS) opsin gene triggered speciation by sensory drive through color signal divergence. The extensive and continuous sand/mud substrates are also species-rich, and a correlation between male nuptial coloration and the absorption of LWS pigments has been reported. However, the factors driving genetic and functional diversity of LWS pigments in sand/mud habitats are still unresolved.ResultsTo address this issue, nucleotide sequences of eight opsin genes were compared in ten Lake Victoria cichlid species collected from sand/mud bottoms. Among eight opsins, the LWS and rod-opsin (RH1) alleles were diversified and one particular allele was dominant or fixed in each species. Natural selection has acted on and fixed LWS alleles in each species. The functions of LWS and RH1 alleles were measured by absorption of reconstituted A1- and A2-derived visual pigments. The absorption of pigments from RH1 alleles most common in deep water were largely shifted toward red, whereas those of LWS alleles were largely shifted toward blue in both A1 and A2 pigments. In both RH1 and LWS pigments, A2-derived pigments were closer to the dominant light in deep water, suggesting the possibility of the adaptation of A2-derived pigments to depth-dependent light regimes.ConclusionsThe RH1 and LWS sequences may be diversified for adaptation of A2-derived pigments to different light environments in sand/mud substrates. Diversification of the LWS alleles may have originally taken place in riverine environments, with a new mutation occurring subsequently in Lake Victoria.

Presence of Acapoeta tanganicae (Actinopterygii: Cypriniformes: Cyprinidae ) within the Lake Rukwa catchment supports historic riverine connectivity with Lake Tanganyika

Freshwater fish biogeography can provide insight into past connectivity of river systems. Here we report the first discovery of the large-bodied cyprinid Acapoeta tanganicae (Boulenger, 1900) inside the Lake Rukwa catchment. Previously this species was thought to be endemic to Lake Tanganyika and immediately proximate sections of connected rivers. This shared distribution supports a scenario of direct connectivity of the two catchments during Holocene high stands of Lake Rukwa.