Mitogenomic insights into the grunts
Despite the differences in gene intergenic, the mitochondrial genome of two Pomadasys species comprises 37 genes, with 28 genes located on the heavy strand and nine on the light strand, which is consistent with other teleost mitogenomes (Miya et al. 2005; Kundu et al. 2023a). Since mitochondrial genomes are generally conserved, gene arrangement is the key area of study in organismal systematics (Gong et al. 2020; Zhang et al. 2020). The arrangement of genes in the mitochondria can impact various facets of fish physiology, molecular mechanisms, life cycles, and the processes involved in genomic evolution (Montaña-Lozano et al. 2022). Therefore, our results indicate that the mitochondrial genome of two Pomadasys species are conserved compared to other species in the Haemulidae family. Furthermore, the mitogenome's nucleotide composition of two Pomadasys species also shows an A-T preference, which are typical of vertebrate and haemulid mitogenomes (He et al. 2022). It is also aligning with the hydrophobic characteristics of mitochondrial proteins (Naylor et al. 1995).
In PCGs, the majority of genes of two Pomadasys species employ ATG as the start codon, except for COI (GTG), and the stop codon of both species also exhibits similarity with terminated either with complete or incomplete codons. The configuration of initiation and termination codon usage in PCGs are consistent with that examined in other haemulids species (He et al. 2022; Liang et al. 2019). Our findings showed various codon combinations can be contributed to the translating of specific amino acids. Codon usage was further assessed using RSCU, where different RSCU values represent different codon usage scenarios. Overall, the RSCU value was determined to be unequaled to ‘1’, suggesting that there were different degrees of bias in each codon utilization. The codon usage bias is correlated to the intensity of gene expression, where the codons used by the genes with high efficiency expression have significantly different gene use frequencies compared with the genes with low expression (Liao et al. 2024; Parvathy et al. 2022). Compared to non-efficient genes, genes exhibiting efficient expression display a greater bias in their codon usage, and they usually utilize a set of preferential synonymous codons. The results of this study suggest that PCGs in the Haemulidae family may conserve and have comparable functions. In several organisms, codon use bias has developed as a result of mutation, natural selection, and genetic drift. Codon bias is influenced by several parameters, including tRNA quantity and interactions, recombination rates, mRNA folding, codon position and context in the gene, GC content, expression level and length of the gene, and genome makeup (Parvathy et al. 2022).
The Ka/Ks ratio is a well-established measure of selective pressure according to Darwinian theory and reflects evolutionary kinships at the genetic level, pertinent to both uniform and diverse species (Yang and Nielsen 2000; Zhao et al. 2022). The selection pressure is crucial for explaining the evolutionary dynamics of any genes under selection pressure and is key to understanding species divergence (Foote et al. 2011). A Ka/Ks ratio greater than ‘1’ indicates positive selection, a ratio of ‘1’ signifies neutrality, and a ratio less than ‘1’ reflects negative selection (Nei and Kumar 2000). Our results showed that all PCGs had Ka/Ks ratios less than ‘1’, signifying strong negative selection between the two Pomadasys species and other haemulids, with mutations largely replaced by synonymous substitutions. This remark mirrors the effect of natural selection in mitigating deleterious nucleotide mutations with negative selective factors, aligning with universal patterns detected in other vertebrates, including teleost’s (Kundu et al. 2024). Thus, the analysis of Ka/Ks ratios from the mitogenomes of Pomadasys and related haemulid species provides a platform for earning new perspectives on the distinctions of natural selection. This incorporates the evolutionary path and distribution among the species, aiding in explaining the complex interaction between mutations and selective pressures and illuminating their collective role in navigation protein evolution, which was represented by the abundance and composition of amino acids.
The 12S and 16S rRNA were located on heavy strand and separated by tRNA-Val as similar with another species under Pomadasys group (Chen et al. 2019). These ribosomal genes, which are generally conserved to ribonucleoproteins, play a critical role in translating genetic information from mRNAs into proteins, offering worthful insights into the fundamental catalytic actions of protein synthesis (Satoh et al. 2016). In addition, the secondary structure of tRNAs in P. perotaei exhibited a traditional canonical cloverleaf pattern, except the tRNA-Ser1 (GCT) displayed a simplified loop due to lack of base pairing in the dihydrouridine arm (DHU arm). This characteristic is commonly observed in the mitogenomes of many teleosts, specifically haemulids fish (He et al. 2022; Liang et al. 2019; Chen et al. 2022). For bony fish, the missing DHU arm could be a typical characteristic and potential to be recognized. The tRNA molecules act as adaptors, facilitating the translation of genetic evidence into proteins by rendering amino acids throughout the translation process. The high-level arrangement of tRNA and heteroplasmic genes in the WANCY region is crucial for mitochondrial gene expression (Cantatore et al. 1987; Ponce et al. 2008).
The biasness towards AT-rich in the control region (CR) of P. perotaei was consistent with findings in many other teleost fish, including P. kaakan and haemulids species (Chen et al., 2022; Miya et al. 2003). Moreover, the CR of Pomadasys species consists of four conserved domains (CSB-D, CSB-1, CSB-2, and CSB-3), as observed in other fish mitogenomes, including other Haemulidae species such as Diagramma picta (Accession No. AP009167) (Satoh et al. 2016). The CR is particularly substantial due to its typical in dynamic character, as it is the most variable region in the mitochondrial genome. Notably, the repeat-rich within ETAS region of the CR is characterized by its high variability and specific motifs that likely form stable hairpin loops, which are thought to serve as sequence-based indicators for the end of mitochondrial DNA replication. The complex mechanisms affecting the CR, such as gene rearrangements through dual replications, dimer-mitogenomes, non-random and random loss, play a crucial role in explaining the structural assortment of mitogenomes and the intricate processes involved in mitochondrial genome evolution (Satoh et al. 2016; Kundu et al. 2024). Thus, generating more mitogenomes of Pomadasys species and screening these variable sequences, along with polymorphic nucleotides, can be utilized for species-level identification and elucidating population structures within this group.
The mitochondrial genome-based phylogenetic analysis in this study has proven effective in explaining higher teleostean phylogenies based on maternal lineage, particularly for grunts and sweetlips fishes belonging to the family Haemulidae. The cladistic pattern derived from mitogenome data of the two Pomadasys species in this study is consistent with previous results on their evolutionary hypotheses (Tavera et al. 2012, 2018). However, to better understand their non-monophyletic clustering pattern, more mitogenomes data alongside with morphometrical information from diverse geographical locations are needed for Pomadasys species in the further studies. Nevertheless, phylogenetic analyses often help elucidate the evolutionary and diversification patterns of taxa through cladistic patterns (Tavera et al. 2012, 2018). Understanding the diversification pattern of these two grunt species based on recent matrilineal phylogeny is crucial, particularly in relation to their ecology, as a mix of biotic (such as distribution range in tropical and temperate environments) and abiotic factors likely affect the speciation of teleost fish. The parrot grunt, P. perotaei, inhabits the eastern Atlantic Ocean off West Africa, while P. kaakan is distributed across the Indo-West Pacific Ocean, indicating that allopatric speciation might have occurred between these two species in ancient times. The separation of their native distributions is potentially maintained by the cold-water barrier formed by the Benguela and Agulhas currents off the southern coast of South Africa, which separates the Atlantic and Indian Oceans. This barrier has also facilitated allopatric speciation in other marine teleosts such as flatfishes (Pleuronectiformes: Psettodidae) and trevallies (Carangiformes: Carangidae) (Glass et al. 2023; Kundu et al. 2023b). Generating molecular data from other Pomadasys species and estimating divergence times will help identify the ancestral and descendant lineages of Pomadasys and their evolutionary patterns. Additionally, gaining in-depth knowledge of oceanographic factors and their correlation with evolutionary patterns will be essential in understanding the triggers for speciation, diversification, and adaptation of Pomadasys species in diverse marine environments.
Genetic insights into haemulid conservation in the Eastern Atlantic
The sustainable management of natural resources, including fisheries, is increasingly critical to mitigating adverse effects on ecosystems, such as biodiversity loss, genetic erosion, and ecological disruption, all of which are crucial for economic stability and food security (FAO 2020a; Takyi et al. 2023). Consequently, adhering to international guidelines and agreements, such as the FAO’s Code of Conduct for Responsible Fisheries (CCRF) and the Sustainable Development Goals (SDGs), underscores the significance of these efforts (Durussel et al. 2018). Integrating multidimensional research is essential to promote the conservation and sustainable use of fisheries at local, national, and regional levels (FAO 2020b). In the East Atlantic Ocean, illegal, unreported, and unregulated (IUU) fishing results in an annual economic loss exceeding USD 2.3 billion for West African nations (Doumbouya et al. 2017). These countries rely heavily on marine fisheries, including haemulids, for livelihood, revenue generation, and nutritional needs. This marine region is also a crucial spawning ground and migratory route for various fish species, which are co-managed regionally by the International Commission for the Conservation of Atlantic Tunas (ICCAT) (FAO 2024). However, numerous marine fish species endemic to this ecosystem faces significant threats and declining stock status, necessitating enhanced monitoring and surveillance through integrated management approaches. Reef-associated species, such as haemulids and lutjanids, are frequently captured by multinational artisanal and industrial fishing fleets, often without coordinated management among West African countries (Polidoro et al. 2016).
Globally, marine teleosts and their genetic diversity are increasingly threatened by overexploitation and habitat loss (Kenchington et al. 2003; Blasiak et al. 2020). This threat is further exacerbated by rising sea temperatures and other environmental changes, which affect species responses, adaptation, reproduction, and physiology, potentially influencing species-specific genetic information (Hoban et al. 2020). Consequently, enhancing the availability of genetic data, such as mitogenome sequences of indigenous fish species in global databases, is crucial for genomic characterization and understanding evolutionary significance and adaptive capabilities (Setiamarga et al. 2008; Alvarenga et al. 2024). Molecular characterization can also identify current exploitation levels and potential local overfishing through genetic diversity and population genetic analyses (Nielsen et al. 2009). Marine ecosystems are traditionally viewed as highly interconnected due to gene flow and population dynamics that transcend national boundaries, sharing biodiversity resources among neighboring countries (Conover et al. 2006; Cano et al. 2008). Thus, data on population genetics and genetic diversity serve as critical baselines for establishing co-management strategies and assessing the impacts of fishing pressure in distant maritime regions (Binashikhbubkr et al. 2024). By generating the mitogenome for the endemic P. perotaei from the eastern Atlantic Ocean, this study recommends for the development of additional mitogenomes for haemulid species to support sustainable fisheries management in West African countries.