Full-Length Gonad Transcriptome Analysis of Amur Sturgeon Dmrt Family Genes: Identification, Characterization and Expression Patterns During Gonadal Differentiation

The regulatory mechanisms that govern sex differentiation in sturgeon are still poorly understood. The doublesex and Mab-3-related transcription factor (Dmrt) gene family is known for its extensive roles in sex determination and differentiation across vertebrates. This study aimed to identify new members of sturgeon Dmrt family genes and core actors in the gonadal differentiation of Amur sturgeon. A full-length gonad transcriptome database was exploited to identify Dmrt gene orthologs. Analyses of phylogenetic relationships and selection pressure were performed, and tissue expression profiles and spatiotemporal expression patterns in gonads were then analyzed using real-time PCR. In total, five Dmrt family genes were identified from the full-length gonad transcriptome, including Dmrt2, DmrtA1, DmrtA2, DmrtB1a and DmrtB1b. Phylogenetic analysis showed that these genes were clustered into clades corresponding to the doublesex/Mav-3 (DM) genes of vertebrates. Furthermore, the analysis of evolutionary selective pressure indicated that DmrtB1a and DmrtB1b were subject to positive selection, suggesting the existence of adaptive evolution in sturgeon. The extensive tissues expression profiling of each Dmrt family gene revealed typical characteristics. Remarkably, according to a spatiotemporal expression pattern analysis, DmrtA1 and DmrtA2 were predominantly distributed in undifferentiated gonads (UGs) and developing ovaries, whereas DmrtB1b showed the lowest expression in UGs. In later stages, DmrtB1b expression continuously increased in testes and was significantly higher in testes than in ovaries at 24 months after hatching (M) and 36M, which strongly suggests that DmrtB1b is a core regulatory factor involved in sexual differentiation in sturgeon. This study provides a genetic resource of full-length Dmrt family genes and increases the understanding of Dmrt functions in sex differentiation in sturgeon.

Interaction between sex-determining genes from two species: clues from Xenopus hybrids

Hybrids provide an interesting model to study the evolution of sex-determining genes and sex chromosome systems as they offer the opportunity to see how independently evolving sex-determining pathways interact in vivo . In this context, the genus Xenopus represents a stimulating model, since species with non-homologous sex chromosomes and different sex-determining genes have been identified. In addition, the possibility of interspecies breeding is favoured in this group, which arose by alloploidization events, with species ploidy ranging from 2 n = 2 x = 20 in X. tropicalis (the only diploid representative of the genus) to 2 n = 12 x = 108 in X. ruwenzoriensis . To study how two sex-determining genes interact in vivo , X. laevis × X. tropicali s hybrids were produced. Gonadal differentiation in these hybrids revealed that the dm-w gene is dominant over X. tropicalis male-determining sex chromosomes (Y or Z), even though the Y chromosome is dominant in X. tropicalis (Y > W>Z). In the absence of the dm-w gene (the Z chromosome from X. laevis is present), the W chromosome from X. tropicalis is able to trigger ovarian development. Testicular differentiation will take place in the absence of W chromosomes from any of the parental species. The dominance/recessivity relationships between these sex-determining loci in the context of either parental genome remains unknown. This article is part of the theme issue ‘Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)’.

Expanding the classical paradigm: what we have learnt from vertebrates about sex chromosome evolution

Until recently, the field of sex chromosome evolution has been dominated by the canonical unidirectional scenario, first developed by Muller in 1918. This model postulates that sex chromosomes emerge from autosomes by acquiring a sex-determining locus. Recombination reduction then expands outwards from this locus, to maintain its linkage with sexually antagonistic/advantageous alleles, resulting in Y or W degeneration and potentially culminating in their disappearance. Based mostly on empirical vertebrate research, we challenge and expand each conceptual step of this canonical model and present observations by numerous experts in two parts of a theme issue of Phil. Trans. R. Soc. B. We suggest that greater theoretical and empirical insights into the events at the origins of sex-determining genes (rewiring of the gonadal differentiation networks), and a better understanding of the evolutionary forces responsible for recombination suppression are required. Among others, crucial questions are: Why do sex chromosome differentiation rates and the evolution of gene dose regulatory mechanisms between male versus female heterogametic systems not follow earlier theory? Why do several lineages not have sex chromosomes? And: What are the consequences of the presence of (differentiated) sex chromosomes for individual fitness, evolvability, hybridization and diversification? We conclude that the classical scenario appears too reductionistic. Instead of being unidirectional, we show that sex chromosome evolution is more complex than previously anticipated and principally forms networks, interconnected to potentially endless outcomes with restarts, deletions and additions of new genomic material. This article is part of the theme issue ‘Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)’.

Effects of Diethylstilbestrol on Zebrafish Gonad Development and Endocrine Disruption Mechanism

Environmental estrogen is a substance that functions as an endocrine hormone in organisms and can cause endocrine system disruption. A typical environmental estrogen, diethylstilbestrol (DES), can affect normal sexual function and organism development. However, even though the effects of different exposure stages of DES on the endocrine system and gonadal development of zebrafish juveniles are unknown, sex determination is strongly influenced by endocrine-disrupting chemicals (EDCs). From 10–90 days post fertilization (dpf), juvenile zebrafish were exposed to DES (100 and 1000 ng/L) in three different stages (initial development stage (IDS), 10–25 dpf; gonadal differentiation stage (GDS), 25–45 dpf and gonadal maturity stage (GMS), 45–60 dpf). Compared with that of IDS and GMS, the growth indicators (body length, body weight, and others) decreased significantly at GDS, and the proportion of zebrafish females exposed to 100 ng/L DES was significantly higher (by 59.65%) than that of the control; in addition, the zebrafish were biased towards female differentiation. The GDS is a critical period for sex differentiation. Our results show that exposure to environmental estrogen during the critical gonadal differentiation period not only affects the development of zebrafish, but also affects the population development.

Is Thermal Responsiveness Affected by Maternal Estrogens in Species with Temperature-Dependent Sex Determination?

In species with temperature-dependent sex determination (TSD), incubation temperatures regulate the expression of genes involved in gonadal differentiation and determine whether the gonads develop into ovaries or testes. For most species, natural incubation conditions result in transient exposure to thermal cues for both ovarian and testis development, but how individuals respond to this transient exposure varies and can drive variation in the resulting sex ratios. Here, we argue that variation in the timing to respond to temperature cues, or thermal responsiveness, is a trait needing further study. Recent work in the red-eared slider turtle (<i>Trachemys scripta</i>) has found that when embryos experience transient exposure to warm conditions (i.e., heatwaves), some embryos show high responsiveness, requiring only short exposures to commit to ovarian development, while others show low responsiveness, developing testes even after more extended exposures to warm conditions. We discuss how maternal estrogens might influence thermal responsiveness for organisms that develop under thermal fluctuations. Examining the interplay of molecular responses to more subtle thermal and endocrine environments may reveal significant insights into the process of sex determination in species with TSD.