Dimorphism of sex and gonad-development-related genes in male and female lined seahorse, Hippocampus erectus, based on transcriptome analyses

Navigating sex and sex roles: deciphering sex-biased gene expression in a species with sex-role reversal (Syngnathus typhle)

Sexual dimorphism, the divergence in morphological traits between males and females of the same species, is often accompanied by sex-biased gene expression. However, the majority of research has focused on species with conventional sex roles, where females have the highest energy burden with both egg production and parental care, neglecting the diversity of reproductive roles found in nature. We investigated sex-biased gene expression in Syngnathus typhle, a sex-role reversed species with male pregnancy, allowing us to separate two female traits: egg production and parental care. Using RNA sequencing, we examined gene expression across organs (brain, head kidney and gonads) at various life stages, encompassing differences in age, sex and reproductive status. While some gene groups were more strongly associated with sex roles, such as stress resistance and immune defence, others were driven by biological sex, such as energy and lipid storage regulation in an organ- and age-specific manner. By investigating how genes regulate and are regulated by changing reproductive roles and resource allocation in a model system with an unconventional life-history strategy, we aim to better understand the importance of sex and sex role in regulating gene expression patterns, broadening the scope of this discussion to encompass a wide range of organisms.

Semen promotes oocyte development in Sebastesschlegelii elucidating ovarian development dynamics in live-bearing fish

In some vertebrates and invertebrates, semen release factors affecting female physiology and behavior. Here, we report that semen delivered to females is potentially beneficial for promoting oocyte development in a viviparous teleost, Sebastes schlegelii. 88% of mated ovaries develop normally and give birth to larval fish, whereas 61% of non-mated ovaries are arrested in the previtellogenic stage. Semen's significant role (p < 0.0001) in promoting oocyte development may involve remodeling follicular cells and regulating the expression of the extracellular matrix, which facilitates cell communication. Furthermore, the ovarian response to semen may influence the brain, affecting hormone release, follicular cell development and steroid production, and crucial for oocyte growth. This mechanism, which could potentially delay maternal investment in offspring until male genetic input occurs to avoid energy wastage, has not been previously described in teleosts. These findings enhance our understanding of ovarian development in viviparous fish, with broader implications for reproductive biology.

Divergence and gene flow history at two large chromosomal inversions underlying ecotype differentiation in the long-snouted seahorse

Chromosomal inversions can play an important role in divergence and reproductive isolation by building and maintaining distinct allelic combinations between evolutionary lineages. Alternatively, they can take the form of balanced polymorphisms that segregate within populations until one arrangement becomes fixed. Many questions remain about how inversion polymorphisms arise, how they are maintained over the long term, and ultimately, whether and how they contribute to speciation. The long-snouted seahorse (Hippocampus guttulatus) is genetically subdivided into geographic lineages and marine-lagoon ecotypes, with shared structural variation underlying lineage and ecotype divergence. Here, we aim to characterize structural variants and to reconstruct their history and suspected role in ecotype formation. We generated a near chromosome-level genome assembly and described genome-wide patterns of diversity and divergence through the analysis of 112 whole-genome sequences from Atlantic, Mediterranean, and Black Sea populations. By also analysing linked-read sequencing data, we found evidence for two chromosomal inversions that were several megabases in length and showed contrasting allele frequency patterns between lineages and ecotypes across the species range. We reveal that these inversions represent ancient intraspecific polymorphisms, one likely being maintained by divergent selection and the other by pseudo-overdominance. A possible selective coupling between the two inversions was further supported by the absence of specific haplotype combinations and a putative functional interaction between the two inversions in reproduction. Lastly, we detected gene flux eroding divergence between inverted alleles at varying levels for the two inversions, with a likely impact on their dynamics and contribution to divergence and speciation.

Influence of EE2 exposure, age and sex on telomere length in European long-snouted seahorse (Hippocampus guttulatus)

After a Telomere Lengthening in juvenile stage, a progressive telomere shortening occurs with age despite higher telomerase level. Telomere Length (TL) may also reflect past physiological state such as a chronic chemical stress. Several studies have revealed a correlation between TL, ageing and/or sex in vertebrates, including teleosts; however, the patterns of telomere dynamics with telomerase mRNA expression, sex, lifespan or chemical stress in teleosts are unclear. The first aim of this study is to verify if telomere length is age and sex-dependent. The second aim is to consider if TL is a useful indicator of stress response in European long-snouted seahorse, Hippocampus guttulatus, an ectothermic and non-model system. We showed that after telomere lengthening during the juvenile stage, a telomeric attrition became significant in sexually mature individuals (p = 0.042). TL decreased in older seahorses despite the presence of somatic telomerase mRNA expression at all life stages studied. There was no difference in TL between males and females, but telomerase mRNA expression was consistently higher in females than males. Exposure to EE2 had no effect on TL in young seahorses, but was correlated with a significant increase in telomerase mRNA expression and various physiological disruptions. Here, a growth retardation of -10 % for body length (p = 0.016) and approximately -45 % for mass (p = 0.001) compared to healthy juvenile seahorses was observed. Our data suggest that telomere dynamics alone should not be used as a marker of EE2 exposure in juvenile seahorses.

Transcriptomic response in thermally challenged seahorses Hippocampus erectus: The effect of magnitude and rate of temperature change

Hippocampus erectus inhabiting the shallow coastal waters of the southern Gulf of Mexico are naturally exposed to marked temperature variations occurring in different temporal scales. Under such heterogeneous conditions, a series of physiological and biochemical adjustments take place to restore and maintain homeostasis. This study investigated the molecular mechanisms involved in the response of H. erectus to increased temperature using transcriptome analysis based on RNA-Seq technology. Data was obtained from seahorses after 0.5-h exposure to combinations of different target temperatures (26 °C: control, and increased to 30 and 33 °C) and rates of thermal increase (abrupt: < 5 min; gradual: 1-1.5 °C every 3 h). The transcriptome of seahorses was assembled de novo using Trinity software to obtain 29,211 genes and 30,479 transcripts comprising 27,520,965 assembled bases. Seahorse exposure to both 30 and 33 °C triggered characteristic processes of the cellular stress response, regardless of the rate of thermal change. The transcriptomic profiles of H. erectus suggest an arrest of muscle development processes, the activation of heat shock proteins, and a switch to anaerobic metabolism within the first 0.5 h of exposure to target temperatures to ensure energy supply. Interestingly, apoptotic processes involving caspase were activated principally in gradual treatments, suggesting that prolonged exposure to even sublethal temperatures results in the accumulation of deleterious effects that may eventually terminate in cellular death. Results herein validate 30 °C and 33 °C as potential upper limits of thermal tolerance for H. erectus at the southernmost boundary of its geographic distribution.

Genomic structure, expression, and functional characterization of the Fem-1 gene family in the redclaw crayfish, Cherax quadricarinatus

The Fem-1 (Feminization-1) gene, encoding an intracellular protein with conserved ankyrin repeat motifs, has been proven to play a key role in sex differentiation in Caenorhabditis elegans. In the present study, three members of the Fem-1 gene family (designating Fem-1A, Fem-1B, and Fem-1C, respectively) were cloned and characterized in the redclaw crayfish, Cherax quadricarinatus. Sequence analysis showed that all three Fem-1 genes contained the highly conserved ankyrin repeat motifs with variant repeat numbers, which shared similarity with other reported crustaceans. In addition, a phylogenetic tree revealed that the Fem-1 proteins from C. quadricarinatus were clustered with the crustacean Fem-1 homologs, and had the closest evolutionary relationship with Eriocheir sinensis. Quantitative real-time PCR (qRT-PCR) results demonstrated that Fem-1B exhibited a significant higher expression abundance in the ovary than in other tissues. In addition, a regular mRNA expression pattern of the Fem-1B gene appeared in the reproductive cycle of ovarian development. Furthermore, RNA interference experiments were employed to investigate the role of Fem-1B in ovarian development. Moreover, knockdown of Fem-1B by RNAi decreased the expression of VTG in the ovaries and hepatopancreas. In summary, this study pointed out that Fem-1B was involved in the sex differentiation process through regulating VTG expression in C. quadricarinatus, and provided new insights into the role of Fem-1B in ovary development.

Evidence of feminization in seahorses from a tropical estuary

Intersex has been reported regularly in wild populations of gonochoristic fish. The authors investigated the possible feminization of seahorses in a population of Hippocampus reidi in Brazil, as well as report a case of a brood pouch reduction after reproductive stimuli. The female seahorses of this population showed two kinds of morphological anomalies. The proportion of the animals affected in the study population was consistent and was uncorrelated with the sex ratio. The study indicates the changes in the sexual profile of the individual.

Diversity of Seahorse Species (Hippocampus spp.) in the International Aquarium Trade

Seahorses (Hippocampus spp.) are threatened as a result of habitat degradation and overfishing. They have commercial value as traditional medicine, curio objects, and pets in the aquarium industry. There are 48 valid species, 27 of which are represented in the international aquarium trade. Most species in the aquarium industry are relatively large and were described early in the history of seahorse taxonomy. In 2002, seahorses became the first marine fishes for which the international trade became regulated by CITES (Convention for the International Trade in Endangered Species of Wild Fauna and Flora), with implementation in 2004. Since then, aquaculture has been developed to improve the sustainability of the seahorse trade. This review provides analyses of the roles of wild-caught and cultured individuals in the international aquarium trade of various Hippocampus species for the period 1997–2018. For all species, trade numbers declined after 2011. The proportion of cultured seahorses in the aquarium trade increased rapidly after their listing in CITES, although the industry is still struggling to produce large numbers of young in a cost-effective way, and its economic viability is technically challenging in terms of diet and disease. Whether seahorse aquaculture can benefit wild populations will largely depend on its capacity to provide an alternative livelihood for subsistence fishers in the source countries. For most species, CITES trade records of live animals in the aquarium industry started a few years earlier than those of dead bodies in the traditional medicine trade, despite the latter being 15 times higher in number. The use of DNA analysis in the species identification of seahorses has predominantly been applied to animals in the traditional medicine market, but not to the aquarium trade. Genetic tools have already been used in the description of new species and will also help to discover new species and in various other kinds of applications.

Comparison of Gonadal Transcriptomes Uncovers Reproduction-Related Genes with Sexually Dimorphic Expression Patterns in Diodon hystrix

Diodon hystrix is a new and emerging aquaculture species in south China. However, due to the lack of understanding of reproductive regulation, the management of breeding and reproduction under captivity remains a barrier for the commercial aquaculture of D. hystrix. More genetic information is needed to identify genes critical for gonadal development. Here, the first gonadal transcriptomes of D. hystrix were analyzed and 151.89 million clean reads were generated. All reads were assembled into 57,077 unigenes, and 24,574 could be annotated. By comparing the gonad transcriptomes, 11,487 differentially expressed genes were obtained, of which 4599 were upregulated and 6888 were downregulated in the ovaries. Using enrichment analyses, many functional pathways were found to be associated with reproduction regulation. A set of sex-biased genes putatively involved in gonad development and gametogenesis were identified and their sexually dimorphic expression patterns were characterized. The detailed transcriptomic data provide a useful resource for further research on D. hystrix reproductive manipulation.

Dynamic Gene Expression and Alternative Splicing Events Demonstrate Co-Regulation of Testicular Differentiation and Maturation by the Brain and Gonad in Common Carp

The common carp (Cyprinus carpio) accounts for approximately 10% of the annual freshwater aquaculture production and is an ideal model to study cyprinidae reproduction. Female common carp grow faster than the males; therefore, related research presents an opportunity with high application value. Although we have a detailed understanding of common carp's early gonadal differentiation process, information about genome-wide gene expression, regulation, and underlying molecular mechanisms during this process remain limited. Here, time-course data comprising six key stages during testicular differentiation and maturation were investigated to further understand the molecular mechanisms underlying the testicular development in cyprinid species. After integrating these time-series data sets, common carp genome, including 98,345 novel transcripts and 3,071 novel genes were re-annotated and precisely updated. Gene co-expression network analysis revealed that the ubiquitin-mediated proteolysis pathway was essential for metabolism during testicular differentiation in the endocrine system of C. carpio. Functional enrichment analyses indicated that genes mainly related to amino acid metabolism and steroid hormone synthesis were relatively highly expressed at the testicular undifferentiation stages, whereas genes associated with cell cycle and meiosis were expressed from the beginning of testicular differentiation until maturation. The dynamics of alternative splicing events demonstrated that exon skipping accounted for majority of the alternative splicing events in the testis and the brain during gonad development. Notably, several potential male-specific genes (fanci and sox30) and brain-specific genes (oxt, gad2, and tac1, etc.) were identified. Importantly, we traversed beyond the level of transcription to test for stage- and gonad-specific alternative splicing patterns between the brain and testis. This study is the first to describe a comprehensive landscape of alternative splicing events and gene expression patterns during gonadogenesis in common carp. This work is extremely valuable to elucidate the mechanisms underlying gonadal differentiation in Cyprinidae as well as other fish species.