Evolution of brain–body allometry in Lake Tanganyika cichlids

Brain size is strongly associated with body size in all vertebrates. This relationship has been hypothesized to be an important constraint on adaptive brain size evolution. The essential assumption behind this idea is that static (i.e., within species) brain–body allometry has low ability to evolve. However, recent studies have reported mixed support for this view. Here, we examine brain–body static allometry in Lake Tanganyika cichlids using a phylogenetic comparative framework. We found considerable variation in the static allometric intercept, which explained the majority of variation in absolute and relative brain size. In contrast, the slope of the brain–body static allometry had relatively low variation, which explained less variation in absolute and relative brain size compared to the intercept and body size. Further examination of the tempo and mode of evolution of static allometric parameters confirmed these observations. Moreover, the estimated evolutionary parameters indicate that the limited observed variation in the static allometric slope could be a result of strong stabilizing selection. Overall, our findings suggest that the brain–body static allometric slope may represent an evolutionary constraint in Lake Tanganyika cichlids.     

Foraging environment determines the genetic architecture and evolutionary potential of trophic morphology in cichlid fishes

Phenotypic plasticity allows organisms to change their phenotype in response to shifts in the environment. While a central topic in current discussions of evolutionary potential, a comprehensive understanding of the genetic underpinnings of plasticity is lacking in systems undergoing adaptive diversification. Here, we investigate the genetic basis of phenotypic plasticity in a textbook adaptive radiation, Lake Malawi cichlid fishes. Specifically, we crossed two divergent species to generate an F₃ hybrid mapping population. At early juvenile stages, hybrid families were split and reared in alternate foraging environments that mimicked benthic/scraping or limnetic/sucking modes of feeding. These alternate treatments produced a variation in morphology that was broadly similar to the major axis of divergence among Malawi cichlids, providing support for the flexible stem theory of adaptive radiation. Next, we found that the genetic architecture of several morphological traits was highly sensitive to the environment. In particular, of 22 significant quantitative trait loci (QTL), only one was shared between the environments. In addition, we identified QTL acting across environments with alternate alleles being differentially sensitive to the environment. Thus, our data suggest that while plasticity is largely determined by loci specific to a given environment, it may also be influenced by loci operating across environments. Finally, our mapping data provide evidence for the evolution of plasticity via genetic assimilation at an important regulatory locus, ptch1. In all, our data address long‐standing discussions about the genetic basis and evolution of plasticity. They also underscore the importance of the environment in affecting developmental outcomes, genetic architectures, morphological diversity and evolutionary potential.     

Physical factors determining spawning site selection in a Central American hole nester,Cichlasoma nigrofasciatum

Synopsis Most Central American cichlids spawn in caves. There has been considerable work documenting where cichlids lay their eggs, and how this may be beneficial to the offspring. However, it has yet to be determined which physical factors of a spawning site influence choice. This study examines the importance of light intensity, potential position of eggs, and number of entrances on spawning site selection in the convict cichlid, Cichlasoma nigrofasciatum. The results suggest that cave light intensity and entrance number determine spawning site selection in this species. Both factors are important in concealing the brood and offering it maximum protection from predators.     

Post‐embryonic development of canal and superficial neuromasts and the generation of two cranial lateral line phenotypes

The relatively simple structural organization of the cranial lateral line system of bony fishes provides a valuable context in which to explore the ways in which variation in post‐embryonic development results in functionally distinct phenotypes, thus providing a link between development, evolution, and behavior. Vital fluorescent staining, histology, and scanning electron microscopy were used to describe the distribution, morphology, and ontogeny of the canal and superficial neuromasts on the head of two Lake Malawi cichlids with contrasting lateral line canal phenotypes (Tramitichromis sp. [narrow‐simple, well‐ossified canals with small pores] and Aulonocara stuartgranti [widened, more weakly ossified canals with large pores]). This work showed that: 1) the patterning (number, distribution) of canal neuromasts, and the process of canal morphogenesis typical of bony fishes was the same in the two species, 2) two sub‐populations of neuromasts (presumptive canal neuromasts and superficial neuromasts) are already distinguishable in small larvae and demonstrate distinctive ontogenetic trajectories in both species, 3) canal neuromasts differ with respect to ontogenetic trends in size and proportions between canals and between species, 4) the size, shape, configuration, physiological orientation, and overall rate of proliferation varies among the nine series of superficial neuromasts, which are found in both species, and 5) in Aulonocara, in particular, a consistent number of canal neuromasts accompanied by variability in the formation of canal pores during canal morphogenesis demonstrates independence of early and late phases of lateral line development. This work provides a new perspective on the contributions of post‐embryonic phases of lateral line development and to the generation of distinct phenotypes in the lateral line system of bony fishes. J. Morphol. 277:1273–1291, 2016. © 2016 Wiley Periodicals, Inc.     

An insular record of the Mayan cichlid Mayaheros urophthalmus (Günther, 1862) (Cichliformes: Cichlidae) on an island 70 km offshore in the Honduran Caribbean

Here we report the first record of the Mayan cichlid, Mayaheros urophthalmus, occurring on an offshore island 70 km from the mainland, the most distant record from the mainland to date. Specimens were collected during several ichthyological surveys on the island of Guanaja, Honduras in November 2019. Mayan cichlids are euryhaline fishes that perform very well in fresh, brackish and marine waters. It has been hypothesized that their high tolerance to salinity and potential of using costal waterways has facilitated the colonization and expansion of M. urophthalmus.     

Recent speciation between sympatric Tanganyikan cichlid colour morphs

Lake Tanganyika, Africa's oldest lake, harbours an impressive diversity of cichlid fishes. Although diversification in its radiating groups is thought to have been initially rapid, cichlids from Lake Tanganyika show little evidence for ongoing speciation. In contrast, examples of recent divergence among sympatric colour morphs are well known in haplochromine cichlids from Lakes Malawi and Victoria. Here, we report genetic evidence for recent divergence between two sympatric Tanganyikan cichlid colour morphs. These Petrochromis morphs share mitochondrial haplotypes, yet microsatellite loci reveal that their sympatric populations form distinct genetic groups. Nuclear divergence between the two morphs is equivalent to that which arises geographically within one of the morphs over short distances and is substantially smaller than that among other sympatric species in this genus. These patterns suggest that these morphs diverged only recently, yet that barriers to gene flow exist which prevent extensive admixture despite their sympatric distribution. The morphs studied here provide an unusual example of active diversification in Lake Tanganyika's generally ancient cichlid fauna and enable comparisons of speciation processes between Lake Tanganyika and other African lakes.     

Asymmetric evolution in two fish-specifically duplicated receptor tyrosine kinase paralogons involved in teleost coloration

The occurrence of a fish-specific genome duplication (FSGD) in the lineage leading to teleost fishes is widely accepted, but the consequences of this event remain elusive. Teleosts, and the cichlid fishes from the species flocks in the East African Great Lakes in particular, evolved a unique complexity and diversity of body coloration and color patterning. Several genes involved in pigment cell development have been retained in duplicate copies in the teleost genome after the FSGD. Here we investigate the evolutionary fate of one of these genes, the type III receptor tyrosine kinase (RTK) colony-stimulating factor 1 receptor (csf1r). We isolated and shotgun sequenced two paralogous csf1r genes from a bacterial artificial chromosome library of the cichlid fish Astatotilapia burtoni that are both linked to paralogs of the pdgfr beta gene, another type III RTK. Two pdgfr beta-csf1r paralogons were also identified in the genomes of pufferfishes and medaka, and our phylogenetic analyses suggest that the pdgfr beta-csf1r locus was duplicated during the course of the FSGD. Comparisons of teleosts and tetrapods suggest asymmetrical divergence at different levels of genomic organization between the teleost-specific pdgfr beta-csf1r paralogons, which seem to have evolved as coevolutionary units. The high-evolutionary rate in the teleost B-paralogon, consisting of csf1rb and pdgfr betab, further suggests neofunctionalization by functional divergence of the extracellular, ligand-binding region of these cell-surface receptors. Finally, we hypothesize that genome duplications and the associated expansion of the RTK family might be causally linked to the evolution of coloration in vertebrates and teleost fishes in particular.     

COLOR ASSORTATIVE MATING CONTRIBUTES TO SYMPATRIC DIVERGENCE OF NEOTROPICAL CICHLID FISH

It is still debated vigorously whether sexual selection can result in speciation without physical barriers to gene flow. In this study, we used field data and molecular methods to investigate the gold–normal color polymorphism in two endemic cichlid fish species of crater lake Xiloá, Nicaragua. We found significant assortative mating by color in both Amphilophus xiloaensis and A. sagittae . Focusing on A. xiloaensis , microsatellite allele frequencies, an assignment test, and model-based cluster analysis demonstrates significant and clear genetic differentiation ( F _ST= 0.03) between gold and normal individuals in sympatry. In addition, we find genetic differentiation between all three sympatric and ecologically distinct Midas cichlid species of Lake Xiloá, A. amarillo , A. sagittae , and A. xiloaensis ( F _ST= 0.03 – 0.19), and clear genetic isolation of these species from their closest relative ( A. citrinellus ) in the neighboring great lake Managua. The A. xiloaensis gold morph is genetically more distinct from the lake's other two Midas cichlid species than is A. xiloaensis -normal. Thus, we have identified sexual isolation based on color that is evident in population genetics and mate choice. Our results suggest that sexual selection through color assortative mating may play an important role in incipient sympatric speciation in Midas cichlids of Nicaragua.     

Differences in stable isotope composition within and among zooplanktivorous Utaka cichlid populations from Lake Malawi

Stable isotope analysis was used to determine whether five sympatric zooplanktivorous cichlids of the Utaka assemblage from Lake Malawi vary in isotopic signature as an indication of possible differences in food resource composition. The isotopic composition of the Utaka in combination with literature data about diet composition suggests that these species exploit a narrow range of zooplanktonic prey types. At five sampling locations, significant differences in δ¹³C and δ¹⁵N were detected among species but a consistent pattern across locations was absent. Significant intraspecific differences between locations were found. These differences were relatively low in view of the large geographic distances between the populations and there was no consistent spatial pattern among the species. The observed differences may be indicative of local variations in diet composition, which may help in reducing niche overlap among these zooplanktivores.