Academic literature on the topic 'Sex-determining mechanisms'

Sex determination refers to the developmental decision that directs the bipotential genital ridge to develop as a testis or an ovary. Genetic studies on mice and humans have led to crucial advances in understanding the molecular fundamentals of sex determination and the mutually antagonistic signaling pathway. In this review, we summarize the current molecular mechanisms of sex determination by focusing on the known critical sex determining genes and their related signaling pathways in mammalian vertebrates from mice to humans. We also discuss the underlying delicate balance between testis and ovary sex determination pathways, concentrating on the antagonisms between major sex determining genes.

Reptiles exhibit marked diversity in sex-determining mechanisms. Many species exhibit genotypic sex determination (GSD) with male heterogamety (XX females/XY males), others have GSD with female heterogamety (ZW females/ZZ males), and still others exhibit temperature-dependent sex determination (TSD). The distribution of these mechanisms throughout the reptile phylogeny implies evolutionary lability in sex determination, and in some lineages there has been a number of transitions between GSD and TSD. Despite this diversity, GSD and TSD have traditionally been viewed as mutually-exclusive mechanisms of sex determination in reptiles, since there is little evidence for their co-occurrence. Considerable empirical and theoretical effort has been directed towards understanding the adaptive significance of TSD in reptiles. In comparison, there has been little focus on understanding how evolutionary transitions between GSD and TSD occur at a genetic and mechanistic level. I addressed this question by applying both empirical and theoretical approaches to investigate interaction of genotypic and temperature influences in the sex determination of two endemic species of Australian lizards. The three-lined skink, Bassiana duperreyi, has XX/XY chromosomal sex determination, yet a previous investigation reported a significant male bias in the sex ratio of eggs incubated at low temperatures. To enable an explicit test for temperature induced sex reversal in this species, a 185 bp Y chromosome marker was isolated by Amplified Fragment Length Polymorphism (AFLP) analysis. The marker was subsequently converted into a duplex PCR assay that co-amplified a 185 bp (or 92 bp) Y chromosome fragment and a 356 bp fragment of the single-copy nuclear gene C-mos (from both sexes) as a positive control. The accuracy of the PCR sex assay was tested on 78 individuals for which sex reversal was not expected. PCR genotype and sex phenotype were concordant for 96% of the animals. This is one of the very few sex tests developed for a reptile, and the first report of Y chromosome sequence from a reptile. The PCR assay was subsequently applied to genotype hatchlings from both cool (16-7.5C) and warm (22-7.5C) cyclical incubation temperature treatments, and identified sex reversal in 15% of genotypically female (XX) embryos (n=26) from the cool treatment, but no sex reversal in eggs from the warmer treatment (n=35). Thus, low incubation temperatures can over-ride genotypic sex determination in B. duperreyi, indicating that GSD and TSD co-occur in this species. The Central bearded dragon, Pogona vitticeps (Agamidae), has ZZ/ZW chromosomal sex determination, and is a member of a lizard family in which GSD and TSD are both widespread, indicating evolutionary lability in sex determination. AFLP analysis was applied to isolate homologous Z and W chromosome-linked markers (71 bp and 72 bp, respectively) from this species. The AFLP sequences were subsequently extended into larger genomic fragments by a reiterated genome walking procedure, producing three non-overlapping contigs of 1.7 kb, 2.2 kb and 4.5 kb. The latter two fragments were verified as distinct, homologous Z/W chromosome fragments by PCR analyses. An amplified 3 kb fragment of the 4.5 kb contig was physically mapped to metaphase spreads, identifying the W microchromosome, and for the first time in this species, the Z microchromosome. PCR analyses indicated the presence of homologous sequences in other Australian agamid species, including both GSD and TSD species. The isolated sequences should therefore prove useful as a comparative genomic tool for investigating the genomic changes that have occurred in evolutionary transitions between sexdetermining mechanisms in agamids, by enabling the identification of chromosomes in TSD species that are homologous to the sex chromosomes of P. vitticeps. The isolated sequences were further converted into a duplex DNA sex assay that co-amplified a 224 bp W chromosome fragment and a 963 bp positive control fragment in both sexes. This PCR assay diagnosed chromosomal sex in three Pogona species, but was not effective outside the genus. Incubation treatment of P. vitticeps eggs revealed a strong and increasing female bias at high constant temperatures (34-36C), but an unbiased sex ratio between 22-32C. Hatchlings from three clutches split between 28C and 34 or 36C incubation treatments were genotyped with the W chromosome AFLP marker. At 28C, the sex ratio was 1:1 but the high temperature treatments produced 2 males and 33 females. All but one of the 30 lizards (97%) incubated at 28C had concordant sex phenotype and genotype, but only 18 of 35 animals (51%) from the high temperature treatment were concordant. All discordant animals were genotypic males (ZZ) that developed as females. Thus, temperature and genotypic influences can interact to determine sex in P. vitticeps. These empirical findings for B. duperreyi and P. vitticeps were extended into a novel theory for the evolution of sex-determining mechanisms in reptiles, working within the framework that species with temperature-induced reversal of chromosomal sex determination are a window to transitional stages of evolution between GSD and TSD. A model was derived from the observation that in both lizards, an extreme of incubation temperature causes sex reversal of the homogametic genotype. In this model, the strength of a genetic regulatory signal for sex determination must exceed a threshold for development of the homogametic sex to occur (male in Pogona, female in Bassiana). The strength of this signal is also temperature-sensitive, so diminishes at extremes of temperature. Simulation modelling demonstrated that increasing the relative magnitude of the threshold for sexual development can cause evolutionary transitions between GSD and TSD. Even more remarkably, decreasing the relative magnitude of the threshold value causes an evolutionary transition between female and male heterogametic GSD. Quantitative adjustment of a single model parameter (the threshold value) thus charts a continuous evolutionary pathway between the three principal mechanisms of sex determination in reptiles (XX/XY-ZZ/ZW-TSD), which were previously considered to be qualitatively distinct mechanisms. The experimental demonstration of temperature-induced reversal of chromosomal sex determination in both B. duperreyi and P. vitticeps presents a challenge to the traditional view that reptilian sex determination is strictly dichotomous (GSD or TSD), and suggests instead that sex determination in reptiles consists of a continuum of systems of interaction between genotypic and temperature influences. Simulation modelling provided solid theoretical support for this proposition, demonstrating that transitions along this continuum are effected simply through shifts in the mean population value for the sex-determining threshold, without requiring substantial genotypic innovation. An important implication of this theory is that transitions between XX/XY and ZZ/ZW modes of GSD may retain the same sex chromosome pair, and the same primary sexdetermining gene, in contrast to previous models for heterogametic transitions. A more immediate implication of these findings is that many reptile species believed to have strict TSD (in particular, lizards and crocodilians), may in fact have a sex-determining system of GSD-TSD interaction, where there is an equilibrium between GSD and TSD individuals within the population.

Despite their huge diversity, abundance and ecological importance, very little is still known about sex determining mechanisms within Crustacea. Sex determination in crustaceans is known to be influenced by environmental factors, via parasitic infection and genetically, however, it is possible that all three mechanisms can be involved in a single species. The gonochoristic marine amphipod Echinogammarus marinus (Leach, 1815) is currently being used for the development of biomarkers to measure the influence of environmental contamination on crustacean sex determination and differentiation pathways. To truly understand whether anthropogenic disruption of sex determination is currently an issue, it is critical that all the mechanisms governing the process in E. marinus are fully evaluated. Therefore, the aim of this project was to fill gaps in our knowledge of the general population dynamics of E. marinus, with a particular focus on elucidating the mechanisms of sex determination in this ubiquitous amphipod. Sex determination in E. marinus has been linked with feminising parasites, however, to date, no such studies have linked this species with environmental sex determination (ESD) or genetic sex determination (GSD). This project investigated two E. marinus populations that differed in population structure. The Langstone Harbour E. marinus population (Southern England, UK) revealed no presence of parasitic sex determination (PSD). However, this study has shown that the population has a seasonal breeding pattern, with population growth and decline closely related to environmental parameters (temperature) and parasites (trematodes) respectively. The population data also revealed seasonally altered sex ratios, ranging from 36% to 71% males. ESD was recorded for the first time in an E. marinus population by revealing that photoperiod was the cue for sex determination. This finding was validated by a laboratory study that showed a male bias in broods that developed in long day light regimes (16h light: 8h dark) and a female bias in broods that developed in a short day light regime (8h light: 16h dark). The laboratory data and the seasonally altered sex ratios found in the field showed significant correlation with each other supporting these findings. A new species of trematode parasite belonging to the Microphallidae family has been identified that encysts in the amphipod brain and demonstrates clear capacity for behavioural changes in its host. Individuals infected with the trematode parasite displayed distinct positive phototaxic and negative geotaxic behavioural alterations that could potentially increase susceptilbility to predation. These behavioural alterations have been linked to changes at the level of gene expression suggesting modulation of neuronal genes in the infected individuals. Putative serotonin receptor 1A, inebratied neurotransmitter, tryptophan hydroxylase and amino acid decarboxylase like genes displayed the most dramatic change in their gene expression. This represents the first study to record such changes in the neuronal pathways of parasite infected amphipods. Another E. marinus population investigated from Invertkeithing (Scotland, UK) displayed a high female bias and high levels of intersexuality. The project has strengthened the evidence that PSD is present in this population with 40.4 % of the population being infected by either Paramyxea or microsporidia parasites. From the infected individuals 75% of that infection were female bias and 88.5% of intersexes, also presented an infection. The investigation explored the transmission pathways and efficiency of the parasites involved. Vertical transmission of a Paramyxean sp. was shown for the first time in an amphipod host and alsoshowed the highest transmission efficiency from the mother to the eggs (96.8%). This has lead to the question of whether the microsporidian D. duebenum is a feminiser and has highlighted another parasite candidate for E. marinus sex distortion. Despite the range of genomic techniques employed, the attempt to determine genomic sexual determination in E. marinus did not reveal any sex specific genomic regions. However, considering the preliminary nature of the work, this study has provided insight for future directions. Several key genes involved in sexual differentiation that presented sex exclusive expression were identified. In addition, crucial method development was performed that will allow future investigations of genetic variation in E. marinus. The transcriptome of the E. marinus has now been sequenced and along with population models enabling a greater understanding of the links between genome and population ecology. With such a large investment in E. marinus as an ecological model species, it is crucial that basic biological questions and gaps in the field are addressed. Consequently, the data presented within this thesis will aid in the study of E. marinus and other crustaceans from the level of genetics to population effects.

碩士
國立臺灣海洋大學
海洋生物研究所
92
The objectives of the present study were to investigate the molecular mechanism of sex determination and the role of sex development factors in sex differentiation, sex development and sex change of the protandrous black porgy (Acanthopagrus schlegeli). At 120 dah (days after hatching), the expression of sex differentiation genes increased significantly. Dmrt1 levels were generally higher in testis while Sox9 levels were similar in both testes and ovaries. Both Dax-1 and Sf-1 gene expression levels were highest in 0-year old fish. The expression levels of both Dax-1 and Cyp11B1 increased significantly during the pre-spawning season while estrogen receptor alpha levels showed a significant increase during the spawning season. At sex change, both estrogen receptor and androgen receptor showed a significant decrease in testes while in the ovaries, Sox9 expression levels decreased dramatically while Cyp19a levels increased significantly. Long-term oral administration of E2 resulted in a significant decrease in the expression of sex differentiation genes in the testes except for Sox9 which was maintained at high levels. Injection of sex steroids caused a significant increase in the Vasa expression levels in both testes and ovaries and also an increase in Sf-1 expression levels in testes. Injection of a high dose of testosterone resulted in an increase in Sf-1 expression in the ovaries. E2 treatment resulted in an increase in Cyp19a expression levels in ovaries, while Cyp11B1 levels decreased in testis. Thus, the results suggest that the fundamental cause of sex differentiation is regulated in the testis. Dmrt1, Dax-1 and Sf-1 induce testis development, while Sox9 and estrogen receptor play an important role in sex change and maturation.

This chapter argues that the timing of the initiation of a single breeding event, or the initiation of the first of multiple breeding events within the same breeding season, is completely dependent on the female-specific reproductive process of timing of egg production and egg-laying. It discusses how early-season events are critical in determining timing of breeding; fitness consequences of timing decisions; selection on timing of breeding; sex-specific response mechanisms for timing of breeding; physiological mechanisms associated with photoperiod (day length) as a proximate factor; physiological mechanisms associated with temperature as a proximate factor; and physiological mechanisms associated with food availability as a “proximate” factor.

Fetal antecedents such as maternal stress, infection or dietary challenges have long been associated with an increased disease risk, capable of affecting multiple generations. The mechanisms through which such determinants contribute to disease development likely involve complex and dynamic relationships between the maternal environment, the endocrine placenta, and the epigenetic programming of the developing embryo itself. While an appreciation for the importance of the epigenome in offspring disease predisposition had evolved, the incredible variability in critical factors such as gestational timing of insults, sex of the fetus, and maternal genetics make clear interpretations difficult. However, animal models have proven highly informative in providing the best knowledge yet as to just how dynamically responsive the epigenome is, and in determining important mechanisms that shape and reprogram the developing brain. This chapter will discuss the epidemiological and clinical evidence and supportive animal models related to environmental influences on neurodevelopmental and neuropsychiatric disease risk.