Genetic Mechanism Of Sex Determination Research Articles

The Molecular and Genetic Mechanisms of Sex Determination in Poplar

—The study of molecular and genetic mechanisms of sex determination in the poplar is of interest not only in the fundamental science, but also in the applied research. In landscaping of large settlements, it is advisable to use male individuals of the Populus genus due to their hypoallergenicity and increased resistance to environmental pollution, stress conditions, and pathogens. However, sex determination in poplars is complicated by the complex genetic structure of the sex-determining region of the genome (SDR). In this review, the emergence, evolution, structure, and function of the SDR in the genus Populus are discussed. Current insights into the structure and function of the key regulator of sex selection in poplars, orthologue of the ARR16/ARR17 gene and the possible roles of other genes that are differentially expressed between male and female plants, including microRNAs, in this process are discussed in detail. The great diversity of species and the high complexity of SDR organization justify the need for further study of the molecular mechanisms of sex determination in poplars.

Divergent evolution of genetic sex determination mechanisms along environmental gradients.

Sex determination (SD) is a crucial developmental process, but its molecular underpinnings are very diverse, both between and within species. SD mechanisms have traditionally been categorized as either genetic (GSD) or environmental (ESD), depending on the type of cue that triggers sexual differentiation. However, mixed systems, with both genetic and environmental components, are more prevalent than previously thought. Here, we show theoretically that environmental effects on expression levels of genes within SD regulatory mechanisms can easily trigger within-species evolutionary divergence of SD mechanisms. This may lead to the stable coexistence of multiple SD mechanisms and to spatial variation in the occurrence of different SD mechanisms along environmental gradients. We applied the model to the SD system of the housefly, a global species with world-wide latitudinal clines in the frequencies of different SD systems, and found that it correctly predicted these clines if specific genes in the housefly SD system were assumed to have temperature-dependent expression levels. We conclude that environmental sensitivity of gene regulatory networks may play an important role in diversification of SD mechanisms.

Natural and Engineered Sex Ratio Distortion in Insects.

Insects have evolved highly diverse genetic sex-determination mechanisms and a relatively balanced male to female sex ratio is generally expected. However, selection may shift the optimal sex ratio while meiotic drive and endosymbiont manipulation can result in sex ratio distortion (SRD). Recent advances in sex chromosome genomics and CRISPR/Cas9-mediated genome editing brought significant insights into the molecular regulators of sex determination in an increasing number of insects and provided new ways to engineer SRD. We review these advances and discuss both naturally occurring and engineered SRD in the context of the Anthropocene. We emphasize SRD-mediated biological control of insects to help improve One Health, sustain agriculture, and conserve endangered species.

Sex biased expression of hormone related genes at early stage of sex differentiation in papaya flowers

Sex types of papaya are controlled by a pair of nascent sex chromosomes, but molecular genetic mechanisms of sex determination and sex differentiation in papaya are still unclear. We performed comparative analysis of transcriptomic profiles of male and female floral buds at the early development stage before the initiation of reproductive organ primordia at which there is no morphological difference between male and female flowers. A total of 1734 differentially expressed genes (DEGs) were identified, of which 923 showed female-biased expression and 811 showed male-biased expression. Functional annotation revealed that genes related to plant hormone biosynthesis and signaling pathways, especially in abscisic acid and auxin pathways, were overrepresented in the DEGs. Transcription factor binding motifs, such as MYB2, GAMYB, and AP2/EREBP, were enriched in the promoters of the hormone-related DEGs, and transcription factors with those motifs also exhibited differential expression between sex types. Among these DEGs, we also identified 11 genes in the non-recombining region of the papaya sex chromosomes and 9 genes involved in stamen and carpel development. Our results suggested that sex differentiation in papaya may be regulated by multiple layers of regulation and coordination and involved transcriptional, epigenetic, and phytohormone regulation. Hormones, especially ABA and auxin, transcription factors, and genes in the non-recombination region of the sex chromosome could be involved in this process. Our findings may facilitate the elucidation of signal transduction and gene interaction in sex differentiation of unisexual flowers in papaya.

Study on artificial induction, growth and gamete quality of mitogynogenetic turbot Scophthalmus maximus

Artificial gynogenesis induction is one of the important techniques of chromosome manipulation. The eggs are fertilized with inactivated sperm to prevent the inheritance from the paternal genome, and genome information of the offspring is totally contributed by the maternal genome. Mitogynogenesis, in which diploidization of chromosome set is performed by blocking the cleavage, is theoretically homozygous and has got more attention in breeding. In this study, we performed the mitogynogenetic diploid induction in turbot Scophthalmus maximus, one of the most important maricultural fish in China and Europe, by applying hydrostatic pressure to the eggs activated with UV-irradiated homologous sperm. The optimal inducing conditions were 36,000 erg mm-2 for UV-irradiated and pressure of 65 MPa for 6 min at 15 min before the appearance of the cleavage furrow. Two mitogynogenesis stocks were obtained in 2016 and 2017. The results showed that there was no distinct difference in incubation time and morphological traits between mitogynogenetic diploid and control diploid. The survival rates of mitogynogenetic diploid sharply decreased in the first month post hatching (mph). Their total weight (tW) and total length (tL) increased rapidly from 12 to 18 mph. The mitogynogenetic diploids induced in 2016 and 2017 showed very different growth. The mean tW and tL of 10 fast-growing individuals at 18 mph were 2.78 and 1.32 times of the others in 2016 stock, while 4.50 and 1.74 times in 2017 stock, respectively. The higher male ratio and rising trend of male ratio indicated the female heterogametic ZW/ZZ genetic mechanism of sex determination in turbot. Twenty-four adults in 2017 stock survived until 24 mph. While twenty-nine adults in 2016 stock survived until 36 mph, in which 11 adults could be promoted mature. Then the qualities of gametes were evaluated and showed that both the egg and the sperm qualities of mitogynogenetic diploid were lower than those of control diploid. However, the embryo and larvae development showed no difference between mitogynogenetic diploid progeny and control diploid.

The first case of induced gynogenesis in Neotropical fishes using the yellowtail tetra (Astyanax altiparanae) as a model organism

Gynogenesis is a chromosome manipulation technique applicable on the production of all-female populations, generation of inbred or clonal lines and identification of genetic sex determination mechanisms in fish. Therefore, the aim of this study was induce meiotic gynogenesis in Astyanax altiparanae and determine the genetic basis of sex determination. Firstly, we evaluated the best UV-irradiation dosage in order to inactivate sperm genome, preserving the capacity of egg activation without the genetic material contribution to the offspring. After the fertilization using the inactivated sperm, diplodization was achieved by heat-shock (40 °C; 2 min) by suppression of the 2nd polar body extrusion. Diploid control and gynogenetic individuals were raised until 240 days. The gynogenetic offspring were confirmed by genotyping using microsatellite markers and the sex ratio of gynogenetic progenies suggests that most data fit to a XX/XY model of sex determination. However, unexpected sex ratios were also observed and possible reasons are discussed. This was the first protocol of induced gynogenesis in a Neotropical fish and such a procedure may be used to produce clonal linage of fish and for future mass production of monosex populations in A. altiparanae and other related species.

Meiotic gynogenesis with heterologous sperm in the mandarin fish Siniperca chuatsi and evidence for female homogamety

The mandarin fish Siniperca chuatsi is a historically important aquaculture species in China and exhibits sexually dimorphic growth. However, sex determination of this fish remains unclear so far. In this study, we induced meiotic gynogenesis in S. chuatsi using irradiated heterologous sperm from spotted mandarin fish (Siniperca scherzeri) to uncover its mechanism of sex determination. Up to 7.52% diploid progeny were obtained among three gynogenetic families in this study. Molecular analysis of female and male donors and sampled young gynogens by seven microsatellite loci further confirmed no genetic contributions from the ‘father’ S. scherzeri. After 8 months of culture, external morphology of adult fish showed that all gynogens were cloned from their mothers. Gonads of the gynogenetic progeny were examined by histological observations and the sexing results showed that they were almost 100% females, strongly supporting an assumption of female homogamety in mandarin fish. By this study, we obtained pure lines of S. chuatsi and elucidated its genetic mechanism of sex determination, providing a basis for possible sex control breeding in this species.

Rethinking sex determination of non-gonadal tissues.

Evolution of genetic mechanisms of sex determination led to two processes causing sex differences in somatic phenotypes: gonadal differentiation and sex chromosome dosage inequality. In species with heteromorphic sex chromosomes, the sex of the individual is established at the time of formation of the zygote, leading to inherent sex differences in expression of sex chromosome genes beginning as soon as the embryonic transcriptome is activated. The inequality of sex chromosome gene expression causes sexual differentiation of the gonads and of non-gonadal tissues. The difference in gonad type in turn causes lifelong differences in gonadal hormones, which interact with unequal effects of X and Y genes acting within cells. Separating the effects of gonadal hormones and sex chromosomes has been possible using mouse models in which gonadal determination is separated from the sex chromosomes, allowing comparison of XX and XY mice with the same type of gonad. Sex differences caused by gonadal hormones and sex chromosomes affect basic physiology and disease mechanisms in most or all tissues.

Induction of mitotic gynogenesis in turbot Scophthalmus maximus

All-female production will maximize the profit potential for the aquaculture of the turbot Scophthalmus maximus, which exhibits significant sexually dimorphic growth. However, the basis for the technology of all-female production, i.e. the genetic mechanism of sex determination in turbot has been obscure so far, and might be ascertained by analysis of the sex ratio of its diploid mitotic gynogens. In this study, artificial mitotic gynogenesis in turbot was induced by hydrostatic pressure shock of its eggs activated by ultraviolet (UV)-irradiated sperm of red sea bream Pagrus major. Before the activation of turbot eggs, cryopreserved sperm of red sea bream were thawed, diluted 1:20 with Hank's solution, and UV-irradiated at a dosage of 6480–7200erg/mm2. Under water temperature of 14.5±0.5°C, treatments were tested by shocking the activated eggs at designed protocols: timings of 70–95maf (min after fertilization) with an intensity of 70MPa for 6min; intensities of 60–85MPa with a timing of 90maf for 6min; durations of 2–10min with an intensity of 75MPa initiated at 90maf. The optimal protocol of the induction was determined to be pressure-shocking activated eggs initiated at 85–90maf with 75MPa for 6min. A mitotic gynogen of 124 juveniles survived by 150days after hatching. The ploidy of these juveniles was confirmed to be diploid by flow cytometry and their sex ratio was 1:1 female to male. The sex ratio of these juveniles supported the assumption of a genetic mechanism of sex determination of female heterogametes (ZW/ZZ) in turbot. This protocol can be used to create homozygous clones of double haploids that are very useful for researches on both the sex determination mechanism and selective breeding of this important farmed fish species.

Validation of clonal line females for sex determination studies in nile tilapia Oreochromis niloticus L.

Sex determination in the Nile tilapia Oreochromis niloticus is more complex than a simple XX-XY sex determining mechanism, as evidenced from fairly frequent unexpected sex ratios in progeny. The production of uniform, homozygous experimental material is particularly advantageous for studying sex determining mechanism as well as for the genetic mapping and genome sequencing studies in which interpretations are facilitated by homozygosity. To better understand the genetic mechanism of sex determination, a fully inbred line of clonal females (XX) was verified in controlled environmental conditions using test crosses and microsatellite DNA markers from the tilapia linkage map. A total of successfully amplified 87 microsatellite DNA markers covering all 24 linkage groups were selected for screening sexually mature females from this line. 67 markers were found polymorphic in outbred individuals screened. Markers from LG1, LG3 and LG23 were given more emphasis because sex determining genes have been mapped on these LGs in different species of tilapia. The verification and validation of this clonal line of females made them an important resource to use as a ‘standard reference line’ in genomics, sex determination studies and other studies in Nile tilapia. DOI: http://dx.doi.org/10.3329/ralf.v1i1.22378 Res. Agric., Livest. Fish.1(1): 147-158, Dec 2014

Identification of a Sex-Linked SNP Marker in the Salmon Louse (Lepeophtheirus salmonis) Using RAD Sequencing

The salmon louse (Lepeophtheirus salmonis (Krøyer, 1837)) is a parasitic copepod that can, if untreated, cause considerable damage to Atlantic salmon (Salmo salar Linnaeus, 1758) and incurs significant costs to the Atlantic salmon mariculture industry. Salmon lice are gonochoristic and normally show sex ratios close to 1:1. While this observation suggests that sex determination in salmon lice is genetic, with only minor environmental influences, the mechanism of sex determination in the salmon louse is unknown. This paper describes the identification of a sex-linked Single Nucleotide Polymorphism (SNP) marker, providing the first evidence for a genetic mechanism of sex determination in the salmon louse. Restriction site-associated DNA sequencing (RAD-seq) was used to isolate SNP markers in a laboratory-maintained salmon louse strain. A total of 85 million raw Illumina 100 base paired-end reads produced 281,838 unique RAD-tags across 24 unrelated individuals. RAD marker Lsa101901 showed complete association with phenotypic sex for all individuals analysed, being heterozygous in females and homozygous in males. Using an allele-specific PCR assay for genotyping, this SNP association pattern was further confirmed for three unrelated salmon louse strains, displaying complete association with phenotypic sex in a total of 96 genotyped individuals. The marker Lsa101901 was located in the coding region of the prohibitin-2 gene, which showed a sex-dependent differential expression, with mRNA levels determined by RT-qPCR about 1.8-fold higher in adult female than adult male salmon lice. This study’s observations of a novel sex-linked SNP marker are consistent with sex determination in the salmon louse being genetic and following a female heterozygous system. Marker Lsa101901 provides a tool to determine the genetic sex of salmon lice, and could be useful in the development of control strategies.

Sex reversal in zebrafish fancl mutants is caused by Tp53-mediated germ cell apoptosis.

The molecular genetic mechanisms of sex determination are not known for most vertebrates, including zebrafish. We identified a mutation in the zebrafish fancl gene that causes homozygous mutants to develop as fertile males due to female-to-male sex reversal. Fancl is a member of the Fanconi Anemia/BRCA DNA repair pathway. Experiments showed that zebrafish fancl was expressed in developing germ cells in bipotential gonads at the critical time of sexual fate determination. Caspase-3 immunoassays revealed increased germ cell apoptosis in fancl mutants that compromised oocyte survival. In the absence of oocytes surviving through meiosis, somatic cells of mutant gonads did not maintain expression of the ovary gene cyp19a1a and did not down-regulate expression of the early testis gene amh; consequently, gonads masculinized and became testes. Remarkably, results showed that the introduction of a tp53 (p53) mutation into fancl mutants rescued the sex-reversal phenotype by reducing germ cell apoptosis and, thus, allowed fancl mutants to become fertile females. Our results show that Fancl function is not essential for spermatogonia and oogonia to become sperm or mature oocytes, but instead suggest that Fancl function is involved in the survival of developing oocytes through meiosis. This work reveals that Tp53-mediated germ cell apoptosis induces sex reversal after the mutation of a DNA–repair pathway gene by compromising the survival of oocytes and suggests the existence of an oocyte-derived signal that biases gonad fate towards the female developmental pathway and thereby controls zebrafish sex determination.

Molecular analysis of the sex chromosomes of the platyfish Xiphophorus maculatus: Towards the identification of a new type of master sexual regulator in vertebrates

In contrast to mammals and birds, fish display an amazing diversity of genetic sex determination systems, with frequent changes during evolution possibly associated with the emergence of new sex chromosomes and sex-determining genes. To better understand the molecular and evolutionary mechanisms driving this diversity, several fish models are studied in parallel. Besides the medaka (Oryzias latipes Temminck and Schlegel, 1846) for which the master sex-determination gene has been identified, one of the most advanced models for studying sex determination is the Southern platyfish (Xiphophorus maculatus, Günther 1966). Xiphophorus maculatus belongs to the Poeciliids, a family of live-bearing freshwater fish, including platyfish, swordtails and guppies that perfectly illustrates the diversity of genetic sex-determination mechanisms observed in teleosts. For X. maculatus, bacterial artificial chromosome contigs covering the sex-determination region of the X and Y sex chromosomes have been constructed. Initial molecular analysis demonstrated that the sex-determination region is very unstable and frequently undergoes duplications, deletions, inversions and other rearrangements. Eleven gene candidates linked to the master sex-determining gene have been identified, some of them corresponding to pseudogenes. All putative genes are present on both the X and the Y chromosomes, suggesting a poor degree of differentiation and a young evolutionary age for platyfish sex chromosomes. When compared with other fish and tetrapod genomes, syntenies were detected only with autosomes. This observation supports an independent origin of sex chromosomes, not only in different vertebrate lineages but also between different fish species.

Problems of sex determination in birds exemplified by Gallus gallus domesticus

The current views of sex determination in birds are considered mostly with the example of Gallus gallus domesticus, the species best studied in this respect. Data on the appearance of primordial germ cells, their migration to the primordial gonads, the role of hormonal factors in the regulation of sex differentiation, the sex chromosomes, putative genetic mechanisms of sex determination, and a possible contribution of dosage compensation are described. The review discusses the two best-grounded hypotheses on the roles of the Z and W chromosomes in sex determination.

Mapping of five candidate sex-determining loci in rainbow trout (Oncorhynchus mykiss).

BackgroundRainbow trout have an XX/XY genetic mechanism of sex determination where males are the heterogametic sex. The homology of the sex-determining gene (SDG) in medaka to Dmrt1 suggested that SDGs evolve from downstream genes by gene duplication. Orthologous sequences of the major genes of the mammalian sex determination pathway have been reported in the rainbow trout but the map position for the majority of these genes has not been assigned.ResultsFive loci of four candidate genes (Amh, Dax1, Dmrt1 and Sox6) were tested for linkage to the Y chromosome of rainbow trout. We exclude the role of all these loci as candidates for the primary SDG in this species. Sox6i and Sox6ii, duplicated copies of Sox6, mapped to homeologous linkage groups 10 and 18 respectively. Genotyping fishes of the OSU × Arlee mapping family for Sox6i and Sox6ii alleles indicated that Sox6i locus might be deleted in the Arlee lineage.ConclusionAdditional candidate genes should be tested for their linkage to the Y chromosome. Mapping data of duplicated Sox6 loci supports previously suggested homeology between linkage groups 10 and 18. Enrichment of the rainbow trout genomic map with known gene markers allows map comparisons with other salmonids. Mapping of candidate sex-determining loci is important for analyses of potential autosomal modifiers of sex-determination in rainbow trout.

Genetic mapping of sex determination in a wild strawberry, Fragaria virginiana, reveals earliest form of sex chromosome

The evolution of separate sexes (dioecy) from hermaphroditism is one of the major evolutionary transitions in plants, and this transition can be accompanied by the development of sex chromosomes. Studies in species with intermediate sexual systems are providing unprecedented insight into the initial stages of sex chromosome evolution. Here, we describe the genetic mechanism of sex determination in the octoploid, subdioecious wild strawberry, Fragaria virginiana Mill., based on a whole-genome simple sequence repeat (SSR)-based genetic map and on mapping sex determination as two qualitative traits, male and female function. The resultant total map length is 2373 cM and includes 212 markers on 42 linkage groups (mean marker spacing: 14 cM). We estimated that approximately 70 and 90% of the total F. virginiana genetic map resides within 10 and 20 cM of a marker on this map, respectively. Both sex expression traits mapped to the same linkage group, separated by approximately 6 cM, along with two SSR markers. Together, our phenotypic and genetic mapping results support a model of gender determination in subdioecious F. virginiana with at least two linked loci (or gene regions) with major effects. Reconstruction of parental genotypes at these loci reveals that both female and hermaphrodite heterogamety exist in this species. Evidence of recombination between the sex-determining loci, an important hallmark of incipient sex chromosomes, suggest that F. virginiana is an example of the youngest sex chromosome in plants and thus a novel model system for the study of sex chromosome evolution.

The genetics of sex determination in stinging nettle (Urtica dioica)

Urtica dioica (“stinging nettle”) includes both dioecious and monoecious forms. In most sexually dimorphic angiosperm species, the genetic mechanisms of sex determination are completely unknown. The few species that include both monoecious and dioecious forms provide an unusual opportunity to examine the genetic mechanisms that underlie the separation of sexual functions, through crossing experiments and analysis of progeny segregation. Our focus is on the genetic mechanisms distinguishing monoecious and dioecious forms of U. dioica. A complicated picture of sex determination in this species has resulted from crosses between dioecious and monoecious subspecies, as well as between dioecious and monoecious forms of the same subspecies. Most significant is evidence for a maternal influence on sex determination and for the possibility of gynodioecy as an intermediate stage in the evolutionary pathway to dioecy.

The role of genetic mechanisms of sex determination in the survival of small populations of Silene littorea: A reintroduction experiment

Loss of genetic variation in populations may have other effects than inbreeding depression and loss of adaptive potential. In the case of the numerous gynodioecious plant species having cytonuclear systems of sex determination with dominant nuclear restorers of the male function, reduced genetic variation and increased inbreeding would increase the proportion of non-restorer recessive genes found in homozygosity, and therefore that of female plants producing pistillate flowers. This could have opposite effects on the extinction risk of the population. Female plants produce no pollen and may cause pollen limitation, but at the same time they may reduce average homozygosity and inbreeding depression because they are forced outcrossers. We observed that inbreeding indeed increased the frequency of female plants and pistillate flowers in the population, and that this effect was accompanied by reductions in population recruitment independent of inbreeding depression and likely due to pollen limitation. These results suggest that dominant nuclear restorers of pollen production speed up the extinction of small populations of Silene littorea, and that sex determination mechanisms might be an important factor to consider in the conservation of many plant species.

MYSTERY OF MYSTERIES NO LONGER?

Speciation has always been one of the biggest challenges in evolutionary biology, and the nature of species has been one of the most contentious. Perhaps this is why so few scientists have attempted to write major texts on the topic. Since Mayr's (1963) landmark volume Animal Species and Evolution, there have only been three significant books (Grant 1971; White 1978; King 1993), none of them attempting a full coverage of the topic and all now dated. This stands in stark contrast to the enormous and growing research effort directed at the origin of species. No wonder, then, that Coyne and Orr's Speciation has been eagerly awaited. Of course, this is not strictly a single-author book but it certainly does provide a unified and authoritative voice. If its messages are heeded, it will bring much-needed order and rigor to the current burst of activity. Speciation has a standard approach in each section. Coyne and Orr outline a problem, analyze the available theory, present the evidence with a concentration on the best studies rather than any attempt to be comprehensive, and then sum up the state of the field. There is a risk, common to major synthetic volumes like this, that many authors will use the examples cited here and not make the effort to consider those that are not. Coyne and Orr recognize that much valuable work had to be omitted. Systematic and incisive analysis is what makes the book so powerful. Take, for example, the question of the frequency of occurrence of polyploid speciation in plants and animals. It is often said that the rarity of animal polyploids is a result of their genetic sex-determination mechanisms, but this argument is flawed since it is only the relatively rare X-autosome balance form of sex determination that is an obstacle to polyploidy. Speciation by polyploidy actually seems to be common in dioecious plants. Alternative arguments have been based on differences in development, rates of hybridization and rates of selling, but none of these is sufficient to explain observed patterns. Coyne and Orr discuss two additional consequences of heteromorphic sex chromosomes that may be important. Dosage compensation is a necessary consequence, at least where the sex chromosomes are not very small, and it is disrupted by polyploidy. Rapid evolution of genic incompatibilities follows from hemizygosity of sex-

Amphibian sex determination and sex reversal.

Amphibians employ a genetic mechanism of sex determination, according to all available information on sex chromosomes or breeding tests. Sex reversal allows breeding tests to establish which sex is heterogametic and provides an indication of the mechanism of sex determination. Cases of spontaneous and experimental sex reversal (by temperature, hormones or surgery) are reviewed and illustrated by previously unpublished studies on crested newts. These newts respond conventionally to temperature and hormone treatment but provide anomalous results from breeding tests. It is suggested that both the evolution from temperature dependency to a genetic switch and from ZZ/ZW to XX/XY are superimposed on a generally uniform mechanism of sex determination in all vertebrates.