Prostaglandin F2α drives female pheromone signaling in cichlids, revealing a basis for evolutionary divergence in olfactory signaling.

Pheromones play essential roles in reproduction in many species. Prostaglandin F2α (PGF2α) acts as a female reproductive hormone and as a sex pheromone in some species. An olfactory receptor (OR) for PGF2α was recently discovered in zebrafish, but this signaling pathway is evolutionarily labile. To understand the evolution of signals that attract males to fertile females, we used the African cichlid Astatotilapia burtoni and found that adult males strongly prefer fertile female odors. Injection of a prostaglandin synthesis inhibitor abolishes this attractivity of fertile females, indicating these hormones are necessary for pheromonal signaling. Unlike zebrafish, A. burtoni males are insensitive to PGF2α, but they do exhibit strong preference for females injected with PGF2α. This attractiveness is independent of the PGF2α hormonal receptor Ptgfr, indicating that this pheromone signaling derives from PGF2α metabolization into a yet-undiscovered pheromone. We further discovered that fish that are insensitive to PGF2α lack an ortholog for the OR Or114 that zebrafish use to detect PGF2α. These results indicate that PGF2α itself does not directly induce male preference in cichlids. Rather, it plays a vital role that primes females to become attractive via an alternative male OR.

Ancient and Recent Hybridization in the Oreochromis Cichlid Fishes.

Cichlid fishes of the genus Oreochromis (tilapia) are among the most important fish for inland capture fisheries and global aquaculture. Deliberate introductions of non-native species for fisheries improvement and accidental escapees from farms have resulted in admixture with indigenous species. Such hybridization may be detrimental to native biodiversity, potentially leading to genomic homogenization of populations and the loss of important genetic material associated with local adaptation. By contrast, introgression may fuel diversification when combined with ecological opportunity, by supplying novel genetic combinations. To date, the role of introgression in the evolutionary history of tilapia has not been explored. Here we studied both ancient and recent hybridization in tilapia, using whole genome resequencing of 575 individuals from 23 species. We focused on Tanzania, a natural hotspot of tilapia diversity, and a country where hybridization between exotic and native species in the natural environment has been previously reported. We reconstruct the first genome-scale phylogeny of the genus and reveal prevalent ancient gene flow across the Oreochromis phylogeny. This has likely resulted in the hybrid speciation of one species, O. chungruruensis. We identify multiple cases of recent hybridization between native and introduced species in the wild, linked to the use of non-native species in both capture fisheries improvement and aquaculture. This has potential implications for both conservation of wild populations and the development of the global tilapia aquaculture industry.

Severe Bottleneck Impacted the Genomic Structure of Egg-Eating Cichlids in Lake Victoria.

Within 15,000 years, the explosive adaptive radiation of haplochromine cichlids in Lake Victoria, East Africa, generated 500 endemic species. In the 1980s, the upsurge of Nile perch, a carnivorous fish artificially introduced to the lake, drove the extinction of more than 200 endemic cichlids. The Nile perch predation particularly harmed piscivorous cichlids, including paedophages, cichlids eat eggs and fries, which is an example of the unique trophic adaptation seen in African cichlids. Here, aiming to investigate past demographic events possibly triggered by the invasion of Nile perch and the subsequent impacts on the genetic structure of cichlids, we conducted large-scale comparative genomics. We discovered evidence of recent bottleneck events in 4 species, including 2 paedophages, which began during the 1970s to 1980s, and population size rebounded during the 1990s to 2000s. The timing of the bottleneck corresponded to the historical records of endemic haplochromines" disappearance and later resurgence, which is likely associated with the introduction of Nile perch by commercial demand to Lake Victoria in the 1950s. Interestingly, among the 4 species that likely experienced bottleneck, Haplochromis sp. "matumbi hunter," a paedophagous cichlid, showed the most severe bottleneck signatures. The components of shared ancestry inferred by ADMIXTURE suggested a high genetic differentiation between matumbi hunter and other species. In contrast, our phylogenetic analyses highly supported the monophyly of the 5 paedophages, consistent with the results of previous studies. We conclude that high genetic differentiation of matumbi hunter occurred due to the loss of shared genetic components among haplochromines in Lake Victoria caused by the recent severe bottleneck.

Epigenetics and the evolution of form: Experimental manipulation of a chromatin modification causes species-specific changes to the craniofacial skeleton.

A central question in biology is the molecular origins of phenotypic diversity. While genetic changes are key to the genotype-phenotype relationship, alterations to chromatin structure and the physical packaging of histone proteins may also be important drivers of vertebrate divergence. We investigate the impact of such an epigenetic mechanism, histone acetylation, within a textbook example of an adaptive radiation. Cichlids of Lake Malawi have adapted diverse craniofacial structures, and here we investigate how histone acetylation influences morphological variation in these fishes. Specifically, we assessed the effect of inhibiting histone deacetylation using the drug trichostatin A (TSA) on developing facial structures. We examined this during three critical developmental windows in two cichlid species with alternate adult morphologies. Exposure to TSA during neural crest cell (NCC) migration and as postmigratory NCCs proliferate in the pharyngeal arches resulted in significant changes in lateral and ventral shape in Maylandia, but not in Tropheops. This included an overall shortening of the head, widening of the lower jaw, and steeper craniofacial profile, all of which are paedomorphic morphologies. In contrast, treatment with TSA during early chondrogenesis did not result in significant morphological changes in either species. Together, these data suggest a sensitivity to epigenetic alterations that are both time- and species-dependent. We find that morphologies are due to nonautonomous or potentially indirect effects on NCC development, including in part a global developmental delay. Our research bolsters the understanding that proper histone acetylation is essential for early craniofacial development and identifies a species-specific robustness to developmental change. Overall, this study demonstrates how epigenetic regulation may play an important role in both generating and buffering morphological variation.

Foraging-induced craniofacial plasticity is associated with an early, robust and dynamic transcriptional response.

Phenotypic plasticity is the ability of a single genotype to vary its phenotype in response to the environment. Plasticity of the skeletal system in response to mechanical input is widely studied, but the timing of its transcriptional regulation is not well understood. Here, we used the cichlid feeding apparatus to examine the transcriptional dynamics of skeletal plasticity over time. Using three closely related species that vary in their ability to remodel bone and a panel of 11 genes, including well-studied skeletal differentiation markers and newly characterized environmentally sensitive genes, we examined plasticity at one, two, four and eight weeks following the onset of alternate foraging challenges. We found that the plastic species exhibited environment-specific bursts in gene expression beginning at one week, followed by a sharp decline in levels, while the species with more limited plasticity exhibited consistently low levels of gene expression. This trend held across nearly all genes, suggesting that it is a hallmark of the larger plasticity regulatory network. We conclude that plasticity of the cichlid feeding apparatus is not the result of slowly accumulating gene expression difference over time, but rather is stimulated by early bursts of environment-specific gene expression followed by a return to homeostatic levels.

Sexually divergent selection, allometric constraints, and the evolution of sexual dimorphism in cichlids from Lake Tanganyika.

The evolution of sexual dimorphism is widely acknowledged as manifestation of sex-specific genetic architecture. Although empirical studies suggested that sexual dimorphism evolves as a joint consequence of constraints arising from the genetic architecture and sexually divergent selection, it remains unclear whether and how these established microevolutionary processes scale up to the macroevolutionary patterns of sexual dimorphism among taxa. Here, we studied how sexual selection and parental care drive sexual dimorphism in cichlid fishes from Lake Tanganyika. We found that male-male competition, female choice, and maternal mouthbrooding are associated with sexual dimorphism in body length, body color, and head length, respectively, despite strong allometric relationships between body length and head length. Within-species (static) allometry of head length on body length evolved as sex-specific responses to mouthbrooding where females evolved higher intercepts while males evolved steeper slopes. Thus, selection to increase mouth size in mouthbrooders may have broken down and reorganized the pattern of allometric constraints that are inherently strong and concordant between sexes. Furthermore, sex-specific responses to mouthbrooding left a remarkably clear signature on the macroevolutionary pattern, resulting in a decoupling of co-evolution in parameters of static allometries observed exclusively within maternal mouthbrooders. Our study provides multiple lines of evidence that are consistent with the idea that macroevolutionary patterns of sexual dimorphism in Lake Tanganyika cichlids result from sexually divergent selection. Our approach illustrates that an examination of within-population phenotypic variance in the phylogenetic comparative framework may facilitate nuanced understandings of how macroevolutionary patterns are generated by underlying microevolutionary processes.

Social regulation of arginine vasopressin and oxytocin systems in a wild group-living fish.

The neuropeptides arginine vasopressin (AVP) and oxytocin (OXT) are key regulators of social behaviour across vertebrates. However, much of our understanding of how these neuropeptide systems interact with social behaviour is centred around laboratory studies which fail to capture the social and physiological challenges of living in the wild. To evaluate relationships between these neuropeptide systems and social behaviour in the wild, we studied social groups of the cichlid fish Neolamprologus pulcher in Lake Tanganyika, Africa. We first used SCUBA to observe the behaviour of focal group members and then measured transcript abundance of key components of the AVP and OXT systems across different brain regions. While AVP is often associated with male-typical behaviours, we found that dominant females had higher expression of avp and its receptor (avpr1a2) in the preoptic area of the brain compared to either dominant males or subordinates of either sex. Dominant females also generally had the highest levels of leucyl-cystinyl aminopeptidase (lnpep)-which inactivates AVP and OXT-throughout the brain, potentially indicating greater overall activity (i.e., production, release, and turnover) of the AVP system in dominant females. Expression of OXT and its receptors did not differ across social ranks. However, dominant males that visited the brood chamber more often had lower preoptic expression of OXT receptor a (oxtra) suggesting a negative relationship between OXT signalling and parental care in males of this species. Overall, these results advance our understanding of the relationships between complex social behaviours and neuroendocrine systems under natural settings.

Female cichlids mate with novel androgen receptor mutant males that lack coloration.

A key challenge in animal behavior is disentangling the social stimuli that drive conspecific behaviors. For some species, like teleost fish, putative sexual signaling cues are inextricably linked to others, making it difficult to parse the precise roles distinct signals play in driving conspecific behaviors. In the African cichlid Astatotilapia burtoni, males are either dominant or subordinate, wherein bright coloration, territoriality, and courtship behavior inextricably correlate positively with rank. Here, we leveraged androgen receptor (AR) mutant male A. burtoni that lack dominance-typical coloration but not behavior to isolate the role of male coloration in driving female mating behaviors in this species. We found in independent behavioral assays that females behave aggressively towards AR mutant but not WT males, yet still mated with both types of males. Females showed enhanced activation of esr2b + cells in the hypothalamus when housed with either mutant or WT males and this activation scaled with spawning activities. Therefore, there is not a simple relationship between male coloration and female mating behaviors in A. burtoni, suggesting independent sensory mechanisms converge on hypothalamic esr2b cells to coordinate behavioral output.

Disruption in cortisol synchrony and pair-dissolution in the serially monogamous convict cichlid (Amatitlania nigrofasciata).

Endocrine synchronization is a biological process often associated with social bonding. The mechanisms that mediate this process have been well studied in many vertebrate clades with evolved complex social behaviors. However, studies focusing on such processes in the less neurologically complex teleost clade are surprisingly lacking. In this study, we investigated the hypothesis that mated pairs of convict cichlids (Amatitlania nigrofasciata) perform cortisol synchronization and that the disruption of this might accompany pair-bond instability. Mated pairs were subjected to both behavioral and non-invasive waterborne hormonal assays to better understand the biological complexity of endocrine synchrony and its role in pair-bonding. Baseline cortisol assays indicated a positive correlation between male and female cortisol levels. Individuals that were subjected to a prolonged separation from their mate exhibited a negative correlation in cortisol synchrony after being reunited with their mate. Cortisol synchrony was disrupted, but pairs did not show a significant variance of intrapair aggression after initial pair reunion. However, more than half of the pairs that received the stressor exhibited significantly higher levels of intrapair aggression than their time matched controls approximately 1-7 days following this reunion, indicating pair-dissolution. Concurrently, pairs who underwent the stressor but maintained their bonds did not display an increase in intrapair aggression and also re-synchronized their cortisol levels. Not only does this study provide crucial insights in regard to the role of cortisol synchrony in serially monogamous systems, but it also suggests that the mechanisms that mediate the synchronization of endocrine through the formation of social bonds are more evolutionarily conserved than originally thought.

Just keep exploring: Genetics of fish niche adaptation.

Although the diversification of species has fascinated researchers for centuries, we know remarkably little about how behavior influences niche adaptation and the genetic mechanisms through which behavior evolves. In their recent study, Sommer-Trembo et al. (Science, 384, 470-475, 2024) demonstrate a critical role for the regulatory gene cacng5b in modulating phenotypic variation in exploratory behavior in one of the most exceptional adaptive radiations: the African cichlid fishes of Lake Tanganyika.

First report of Paracapillaria philippinensis infection in flowerhorn cichlid in India.

The flowerhorn cichlid is a popular ornamental fish in many Asian countries. The present study reports the occurrence of Paracapillaria philippinensis (Chitwood et al., 1968), a parasitic nematode in flowerhorn cichlid (Cichlasoma sp.) from south India. The infected fish demonstrated clinical symptoms viz. dark coloration, poor appetite, lethargy, erratic swimming, and white stringy faeces. Microscopic observation of the intestinal content and faeces revealed the presence of adult worms, larvae, and unembryonated eggs. PCR amplification of eukaryotic 18S rRNA, P. philippinensis-specific SSU rRNA gene, and the subsequent sequence analysis confirmed the species identity as P. philippinensis. The generated sequences were submitted in the GenBank, NCBI, under the accession numbers, MK895507.1, MK895446.1, MW144993.1, and OR685675.1. This is the first scientific report of P. philippinensis in fish from India, and it confirms that the flowerhorn cichlid can act as a definitive host for P. philippinensis. This report alerts fish handlers and enthusiasts to undertake suitable precautionary measures while handling live flowerhorn cichlids to prevent possible transmission of P. Philippinensis, which has the potential to infect humans causing intestinal capillariasis.

From hatching to juvenile: Larval development of Vieja fenestrata (Teleostei: Cichlidae).

This study delves into the early development of Vieja fenestrata (Cichlidae), with a specific focus on the description of external morphological and morphometric changes, and growth patterns from hatching to the loss of larval characters under controlled laboratory conditions at a temperature of 28°C. Asynchronous hatching was observed between 58 and 60 h postfertilization, with the posterior body emerging first. Over 14 days, significant morphological, physiological, and behavioral changes were observed, revealing a complex developmental trajectory. The initial developmental phases were characterized by rapid vascularization, fin differentiation, and heightened activity, and the subsequent days witnessed the flexion of the notochord, emergence of swim bladder functionality, and transition to exogenous feeding. Maturation progressed with the absorption of the yolk sac, regression of cement glands, and fin ray development, culminating in metamorphosis by 14 days post-hatching. Throughout this period, evolving pigmentation patterns and structural adaptations highlight the species' adaptive strategies. During the larval period of V. fenestrata, substantial changes in morphological proportions were observed. Before the inflection, tail length, trunk length, and body depth had negative allometric growth, and head length, eye diameter, and snout length had positive allometric growth. After the inflection, body depth and snout length showed positive allometric growth; head length and trunk length exhibited isometric growth, whereas tail length and eye diameter demonstrated negative allometric growth. These findings contribute insights into the intricate developmental dynamics of V. fenestrata. Moreover, further research may explore these developmental dynamics' ecological and evolutionary implications.

Evolution of miRNA-Binding Sites and Regulatory Networks in Cichlids.

The divergence of regulatory regions and gene regulatory network (GRN) rewiring is a key driver of cichlid phenotypic diversity. However, the contribution of miRNA-binding site turnover has yet to be linked to GRN evolution across cichlids. Here, we extend our previous studies by analyzing the selective constraints driving evolution of miRNA and transcription factor (TF)-binding sites of target genes, to infer instances of cichlid GRN rewiring associated with regulatory binding site turnover. Comparative analyses identified increased species-specific networks that are functionally associated to traits of cichlid phenotypic diversity. The evolutionary rewiring is associated with differential models of miRNA- and TF-binding site turnover, driven by a high proportion of fast-evolving polymorphic sites in adaptive trait genes compared with subsets of random genes. Positive selection acting upon discrete mutations in these regulatory regions is likely to be an important mechanism in rewiring GRNs in rapidly radiating cichlids. Regulatory variants of functionally associated miRNA- and TF-binding sites of visual opsin genes differentially segregate according to phylogeny and ecology of Lake Malawi species, identifying both rewired, for example, clade-specific and conserved network motifs of adaptive trait associated GRNs. Our approach revealed several novel candidate regulators, regulatory regions, and three-node motifs across cichlid genomes with previously reported associations to known adaptive evolutionary traits.

Morphological and temporal variation in early embryogenesis contributes to species divergence in Malawi cichlid fishes.

The cichlid fishes comprise the largest extant vertebrate family and are the quintessential example of rapid "explosive" adaptive radiations and phenotypic diversification. Despite low genetic divergence, East African cichlids harbor a spectacular intra- and interspecific morphological diversity, including the hyper-variable, neural crest (NC)-derived traits such as coloration and craniofacial skeleton. Although the genetic and developmental basis of these phenotypes has been investigated, understanding of when, and specifically how early, in ontogeny species-specific differences emerge, remains limited. Since adult traits often originate during embryonic development, the processes of embryogenesis could serve as a potential source of species-specific variation. Consequently, we designed a staging system by which we compare the features of embryogenesis between three Malawi cichlid species-Astatotilapia calliptera, Tropheops sp. 'mauve' and Rhamphochromis sp. "chilingali"-representing a wide spectrum of variation in pigmentation and craniofacial morphologies. Our results showed fundamental differences in multiple aspects of embryogenesis that could underlie interspecific divergence in adult adaptive traits. First, we identified variation in the somite number and signatures of temporal variation, or heterochrony, in the rates of somite formation. The heterochrony was also evident within and between species throughout ontogeny, up to the juvenile stages. Finally, the identified interspecific differences in the development of pigmentation and craniofacial cartilages, present at the earliest stages of their overt formation, provide compelling evidence that the species-specific trajectories begin divergence during early embryogenesis, potentially during somitogenesis and NC development. Altogether, our results expand our understanding of fundamental cichlid biology and provide new insights into the developmental origins of vertebrate morphological diversity.

Ancient DNA is preserved in fish fossils from tropical lake sediments.

Tropical freshwater lakes are well known for their high biodiversity, and particularly the East African Great Lakes are renowned for their adaptive radiation of cichlid fishes. While comparative phylogenetic analyses of extant species flocks have revealed patterns and processes of their diversification, little is known about evolutionary trajectories within lineages, the impacts of environmental drivers, or the scope and nature of now-extinct diversity. Time-structured palaeodata from geologically young fossil records, such as fossil counts and particularly ancient DNA (aDNA) data, would help fill this large knowledge gap. High ambient temperatures can be detrimental to the preservation of DNA, but refined methodology now allows data generation even from very poorly preserved samples. Here, we show for the first time that fish fossils from tropical lake sediments yield endogenous aDNA. Despite generally low endogenous content and high sample dropout, the application of high-throughput sequencing and, in some cases, sequence capture allowed taxonomic assignment and phylogenetic placement of 17% of analysed fish fossils to family or tribe level, including remains which are up to 2700 years old or weigh less than 1 mg. The relationship between aDNA degradation and the thermal age of samples is similar to that described for terrestrial samples from cold environments when adjusted for elevated temperature. Success rates and aDNA preservation differed between the investigated lakes Chala, Kivu and Victoria, possibly caused by differences in bottom water oxygenation. Our study demonstrates that the sediment records of tropical lakes can preserve genetic information on rapidly diversifying fish taxa over time scales of millennia.

The transcriptional state and chromatin landscape of cichlid jaw shape variation across species and environments.

Adaptive phenotypes are shaped by a combination of genetic and environmental forces, but how they interact remains poorly understood. Here, we utilize the cichlid oral jaw apparatus to better understand these gene-by-environment effects. First, we employed RNA-seq in bony and ligamentous tissues important for jaw opening to identify differentially expressed genes between species and across foraging environments. We used two Lake Malawi species adapted to different foraging habitats along the pelagic-benthic ecomorphological axis. Our foraging treatments were designed to force animals to employ either suction or biting/scraping, which broadly mimic pelagic or benthic modes of feeding. We found a large number of differentially expressed genes between species, and while we identified relatively few differences between environments, species differences were far more pronounced when they were challenged with a pelagic versus benthic foraging mode. Expression data carried the signature of genetic assimilation, and implicated cell cycle regulation in shaping the jaw across species and environments. Next, we repeated the foraging experiment and performed ATAC-seq procedures on nuclei harvested from the same tissues. Cross-referencing results from both analyses revealed subsets of genes that were both differentially expressed and differentially accessible. This reduced dataset implicated notable candidate genes including the Hedgehog effector, KIAA0586 and the ETS transcription factor, etv4, which connects environmental stress and craniofacial morphogenesis. Taken together, these data provide novel insights into the epigenetic, genetic and cellular bases of species- and environment-specific bone shapes.

Impact of genetic relatedness on reproductive behavior in Pelvicachromis pulcher, a biparental cichlid fish with mutual mate choice and ornamentation.

Inbreeding can result in inbreeding depression. Therefore, many species seek to avoid inbreeding. However, theory predicts that inbreeding can be beneficial. Accordingly, some species tolerate inbreeding or even prefer mating with close relatives. Evidence for active inbreeding, i.e., kin-mating preference was reported in the biparental African cichlid fish Pelvicachromis taeniatus. Related mating partners revealed better parental cooperation due to kin selection, a potential benefit of inbreeding. In this study, we investigated kin-mating preference in a genetically diverse, outbred F2-lab population of Pelvicachromis pulcher, a closely related species to P. taeniatus. Like P. taeniatus, this species shows mutual ornamentation and mate choice as well as intense biparental brood care. The F1 P. pulcher generation had revealed signs of inbreeding depression but no inbreeding avoidance. We studied mating behavior and aggression in trios consisting of a male P. pulcher, an unfamiliar sister, and an unfamiliar, unrelated female. Because the study focused on kin-mating patterns, female pairs were matched for body size and coloration. The results provide no evidence for inbreeding avoidance but rather suggest inbreeding preference. We also found no significant impact of inbreeding on offspring survival. The results suggest no inbreeding avoidance in P. pulcher; however, the strength of inbreeding preference and inbreeding depression seems to be variable. We discuss possible causes for this variation like context-dependent inbreeding depression. The number of eggs positively correlated with female body size and coloration. Furthermore, the female aggressiveness was positively correlated with female coloration indicating that coloration signal female dominance and quality.

Characterization and phylogenetic analysis of the complete mitochondrial genome of Mango tilapia (Sarotherodon galilaeus: Cichlidae).

BACKGROUND: Sarotherodon galilaeus (Linné, 1758) is a member of the family Cichlidae, which is considered the most important aquaculture freshwater species endemic to Africa and the Middle East. The genetics and molecular biology of this species are rare. This requires more comprehensive mitochondrial genomes-based phylogenetics to enhance understanding of the relationship and delineate this species. METHODS AND RESULTS: Here, we assembled the complete mitogenome of S. galilaeus using Illumina high-throughput sequencing technology. The mango tilapia mitogenome was 16,631 bp in length with an AT composition of 53.4% and 46.4% GC content. It encodes 37 genes comprising two ribosomal RNA genes (rRNAs), 22 transfer RNA genes (tRNAs), and 13 protein-coding genes (PCGs) as well as the D-loop known as the control region. The phylogenetic tree was conducted to provide a relationship within the haplotilapiine lineage based on the maximum likelihood method, and the newly sequenced S. galilaeus was clustered with other Sarotherodon species. CONCLUSION: Our results provide a new perception of the genetic basis of S. galilaeus species for further research on systematics, evolution, population genetics, and molecular ecology.

Developmental Process of a Pronounced Laterality in the Scale-eating Cichlid Fish Perissodus microlepis in Lake Tanganyika.

Lateral preference in behaviors has been widely documented in many vertebrates and invertebrates. Such preferences are strange, puzzling, and on the surface, not adaptive. However, behavioral laterality may increase an individual's fitness as well as foraging accuracy and speed. There is little experimental evidence regarding the developmental process of laterality, and unsolved questions have perplexed researchers for several decades. Related to these issues, here, I review that the scale-eating cichlid Perissodus microlepis found in Lake Tanganyika is a valuable model to address the developmental mechanism of animal laterality. The scale-eating cichlid has pronounced behavioral laterality and uses its asymmetric mouth during feeding events. Recent studies have shown that behavioral laterality in this fish depends on both genetic factors and past experience. The attack-side preference of scale eaters is an acquired trait in an early developmental stage. Juvenile fish empirically learn which side of the prey is more effective for tearing scales and gradually select the dominant side for attacking. However, the superior kinetics of body flexion during the dominant side attack has innate characteristics. Additionally, left-right differences in scale-eater mandibles also develop during ontogeny. Further progress toward understanding the comprehensive mechanisms of laterality should address the following persistent barriers: (1) the effects of phylogenetic constraints and ecological factors on the level of laterality; and (2) the neuronal and molecular mechanisms that produce left-right behavioral differences.

First characterization of PIWI-interacting RNA clusters in a cichlid fish with a B chromosome.

BACKGROUND: B chromosomes are extra elements found in several eukaryote species. Usually, they do not express a phenotype in the host. However, advances in bioinformatics over the last decades have allowed us to describe several genes and molecular functions related to B chromosomes. These advances enable investigations of the relationship between the B chromosome and the host to understand how this element has been preserved in genomes. However, considering that transposable elements (TEs) are highly abundant in this supernumerary chromosome, there is a lack of knowledge concerning the dynamics of TE control in B-carrying cells. Thus, the present study characterized PIWI-interacting RNA (piRNA) clusters and pathways responsible for silencing the mobilization of TEs in gonads of the cichlid fish Astatotilapia latifasciata carrying the B chromosome. RESULTS: Through small RNA-seq and genome assembly, we predicted and annotated piRNA clusters in the A. latifasciata genome for the first time. We observed that these clusters had biased expression related to sex and the presence of the B chromosome. Furthermore, three piRNA clusters, named curupira, were identified in the B chromosome. Two of them were expressed exclusively in gonads of samples with the B chromosome. The composition of these curupira sequences was derived from LTR, LINE, and DNA elements, representing old and recent transposition events in the A. latifasciata genome and the B chromosome. The presence of the B chromosome also affected the expression of piRNA pathway genes. The mitochondrial cardiolipin hydrolase-like (pld6) gene is present in the B chromosome, as previously reported, and an increase in its expression was detected in gonads with the B chromosome. CONCLUSIONS: Due to the high abundance of TEs in the B chromosome, it was possible to investigate the origin of piRNA from these jumping genes. We hypothesize that the B chromosome has evolved its own genomic guardians to prevent uncontrolled TE mobilization. Furthermore, we also detected an expression bias in the presence of the B chromosome over A. latifasciata piRNA clusters and pathway genes.