INTRODUCTION

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Controlling the sex ratio is essential in finfish determjnation. A balanced sex ratio is usually good for broodstock management, determination it enables to develop appropriate breeding schemes. However, in some species the production of monosex populations is desirable because the existence of sexual dimorphism, primarily in growth or first time sex sexual maturation, but also in color or shape, can render one sex more valuable.

The knowledge of the genetic architecture of sex determination SD is convenient for controlling sex ratio and for the implementation of breeding programs. Unlike mammals and birds, which show highly conserved master genes sex control a conserved genetic network responsible for determinnation differentiation GDa huge diversity of SD mechanisms has been reported in fish.

Despite theory predictions, more than one gene is in many cases involved in fish SD and genetic differences have been observed in the GD network. Environmental factors also play a relevant role and epigenetic mechanisms are becoming increasingly recognized for the establishment and maintenance of the GD pathways.

Fish major genetic factors are frequently involved in fish SD, these observations strongly suggest degermination SD in this group resembles a complex trait.

Accordingly, the application of quantitative genetics combined with genomic tools is desirable to address its study and in fact, when applied, it has frequently demonstrated a multigene trait interacting with environmental factors in model and cultured fish species. This scenario has determinatjon implications for aquaculture and, depending upon the species, from chromosome manipulation or environmental control techniques up to classical selection or marker assisted selection programs, are being applied.

In this review, we pdff four relevant species or fish ses to ppdf this diversity and hence the technologies that can be used by the industry for fish control of sex ratio: turbot and European sea bass, two reference species of the European aquaculture, and salmonids and tilapia, representing the fish for which there are well established breeding programs.

Fish represent the most diverse group of vertebrates pdf fksh than 28, species Nelson, This diversity is a reflection of their high capacity for adaptation to a broad spectrum of environmental conditions. As a result, fish show amazing morphological, physiological, and behavioral adaptations to live in pdf highly diverse aquatic environment.

Fish also show all types of determinatoon strategies, including gonochorism, proterandrous, protogynous, and seex hermaphroditism, and unisexuality Sex and Nagahama, These reproductive strategies emerged independently in different lineages during evolution demonstrating a polyphyletic origin Avise and Mank, Domestication of fish for production is an ancient practice and again shows the high adaptation capacity of this group, especially considering that more than fish species are cultivated determination over the world Food and Agriculture Organization [FAO], Production of domestic determonation largely relies on reproduction, and sex vast amount of information has been gathered for its control.

Reproduction techniques include production of monosex populations because the existence of sexual growth dimorphism, either in favor of males or females depending on the species, determiantion also because sometimes the most valuable trait is associated with one sex e. Here, we review the available data on the genetic architecture of sex determination SD in fish and how sex ratio is controlled in aquaculture production. We contrast this information with models emerging from the classical studies in Drosophilamammals, or birds with highly conserved mechanisms associated to marked sex chromosome heteromorphisms.

Determinatoon show the huge intra and interspecific diversity of SD systems in fish associated to a high evolutionary turnover.

So, its genetic architecture, although commonly supported by major genes, is also influenced by fetermination genes, and environmental factors approaching it to a complex trait.

We illustrate this diversity and its influence on the strategies aimed at the production of the most desired sex in the last section of the paper by analyzing the genetic basis of SD in two important species of the European aquaculture, turbot Scophthalmus maximusand European sea bass Dicentrarchus labrax.

We also consider two zex the main fish groups with established breeding programs, salmonids, and tilapias, all of them with remarkable differences in sex determination. The genetic architecture of a complex trait refers to the genes involved in that trait, their influence and interactions to establish the phenotype. It takes also into account the influence of environmental factors and their interactions with the genotype on the final phenotype McKay, Although getting all this information is a very ambitious enterprise, the more we approximate to this cetermination the better we shall understand key questions related to the genetic variation for its exploitation in genetic breeding programs.

Pdf development includes all developmental processes aimed to transform an undifferentiated primordium into a mature gonad, either determinztion of testis. Once determintaion future of the gonad has been established, morphogenetic GD processes work until maturation is completed.

Major events leading to ovarian vs. The first event, sex determination — the establishment of determination — can be triggered by the action of a major sex determining master gene, several sex-associated loci, an environmental factor e. During this period, the germ cell-somatic cell crosstalk fish very important, but still largely uncharacterized. Also, during these early events, biotic and abiotic factors e. Finally, also at the beginning of gonad differentiation — the transformation of an undifferentiated gonad into a testis or ovary — the accidental i.

As a consequence of the hierarchical nature of gonad development, a single determination or environmental cue operating at the beginning of development can drive the gonad pathway towards one direction or another. Thus, in contrast to other characters influenced by many genes operating in different routes with important additive effects, here gene interactions likely represent an important genetic component. Particularly, epistatic effects may be relevant because a single gene acting upstream or even downstream of a preexisting SD gene SDg may take the control of gonad fate, thus, masking extant genetic variation at other involved loci.

Epistatic interactions have been reported between major SD loci in different fish groups Cnaani et al. Gonad development of fish is unusual in the sense that the sexually undifferentiated period can last from weeks until years Saito and Tanaka, ; Berbejillo et al.

In such a long period, it is tempting to speculate that the brain may be involved through the hypothalamic—pituitary—gonadal axis Baroiller et al.

However, despite the fact that the brain certainly integrates environmental stimuli and, in particular, social interactions, which have been shown to be implicated in the process of sex-change in hermaphrodites, currently there is no convincing proof that the brain plays any role in the SD process in gonochoristic fishes.

Homoeothermic vertebrates show a conserved morphogenetic development supported by a strongly canalized genetic sex determining system Charlesworth et al. Fiah, fish show inter-specific differences in the morphogenetic events occurring along gonad development. Variation exists both pdf the general pattern of differentiation, the interaction between somatic and germ cells, and in the time of occurrence and relative weight of the different steps.

The amount of primordial germ cells have been reported to be the first development difference between males and females in species such as medaka O. Also, gonad development has been classified as undifferentiated determination differentiated type, fish, pdc on the existence of an initial transitory female stage that can subsequently revert to testis like in zebrafish Danio rerio ; Orban et sfx.

Interaction between somatic and germ cells is also recognized as an important feature for gonad development. In fact, several key genes related to the initial steps of differentiation like niamhor sox9 in males Lee et al. In this communication there are species-specific differences and, for example, the ablation of the female germ cells determines the reversal of development towards a testis in zebrafish D.

Pdf consensus determinatjon until recently on the high conservation of the gene network controlling gonad development among vertebrates, differences being mainly related to changes in the switching mechanism. Theories on the evolution and genetic architecture of SD in animals have been largely influenced by fish on Drosophilamammals, and birds, all of them showing convergent patterns, with a heteromorphic sex chromosome pair and, as a consequence, a particular sex-linked determination model Bachtrog et al.

To maintain the beneficial association between the antagonistic allele and the SD locus, recombination would be restricted, giving rise to the permanent heterozygous state of that portion of the sexual pair Bergero and Charlesworth, That determination would promote the accumulation ib repetitive elements and deleterious variants in the SDg-bearing chromosome, contributing to its progressive degeneration and the pdf heteromorphic shape of the sexual pair Charlesworth et al.

Mathematical models based on this fish suggested that sex one gene should underlie the SD system, and if more than one gene were segregating, this should represent cetermination unstable equilibrium towards a new SD aex Rice, Model on the origin and evolution of the SD region-bearing chromosome from studies in mammals, birds and Drosophila.

This model, although has been demonstrated in some fish, shows a large variation on its progression, which is reflected on the degree of differentiation between the chromosomes of the sexual pair.

The origin of a new gish pair is related to the origin of genes A,B with antagonistic alleles favorable to females Af, Bf or fsih males Am, Bm associated with a determinatjon SDg. Subsequent steps involve accumulation of repetitive elements rep and the dstermination of the Y chromosome because of its permanent heterozygotic state at the differential region d: recessive non functional variant of a sex-linked gene. Initial data in ectothermic vertebrates, particularly fish, demonstrated a sharply different picture Devlin and Nagahama, More than one sex-associated gene has been reported in many fish species Cnaani fisj al.

Although antagonistic alleles demonstrated to be associated with the SD region in fish Roberts determinayion al. This has been related to the huge evolutionary turnover of SD mechanism which limits chromosome differentiation and, as a consequence, eex SD systems have been reported between closely related species and even between fish of the same species.

However, degeneration and differentiation of sex chromosomes can evolve very quickly Charlesworth et al. Thus, sex heteromorphism can involve the whole chromosome and be detectable with the usual cytogenetic techniques, as reported in Neotropical fish Sex et al.

To understand the origin of SD regions both external pressures i. Several genes with different functions have been recruited along evolution as SDg in fish, which shows the opportunistic nature of selection to face new evolutionary pressures. In this regard, the specific genome duplication occurred within teleosts may have provided a suitable raw material for new sex determinants Mank and Avise, A remarkable case which exemplifies the many options available to change SD mechanisms is the association of sex to B chromosomes, usually considered as junk DNA, in species of the determinatiion Astyanax and in two species of pufferfish Vicente et al.

The high turnover of the SD region determimation fish has led to suggest that changes in the SD mechanism may be associated with speciation Ser et al. High determinwtion variation has also been described between fish species regarding the gene responsible for SD, the number of genes involved in such decision and the relationships between them. Currently, five different master genes have been documented in fish: dmYgsdfamhyamhr2and sdY.

It originated from gish segmental duplication of a small autosomal region containing the precursor dmrt1followed by an insertion of the duplicated region on detdrmination proto-Y chromosome Matsuda et al.

Determination gene has fisj inserted upstream pdf amh in the cascade of male development, becoming a male SDg Hattori et al. This gene contains a specific SNP variant in the kinase domain of amhr2 in the X chromosome which gish lower affinity for the amh hormone, thus fating the female pathway when homozygous XX; Kamiya et al.

Finally, sdY sexually dimorphic on the Y chromosome is linked to the SEX locus of salmonids and is necessary and sufficient to induce testicular differentiation. It has evolved through neofunctionalization from irf9 by losing fish role in ifn signaling pathway and acquiring a new role in SD Yano et al.

Fisu an evolutionary sex different SDgs have determinatoin reported either in closely related species like Determinarion. Also, the same SDg has been deyermination in closely related species like O. Very recently, dmrt1 has been suggested as a strong candidate in the half-smooth tongue cish Cynoglossus semilaevis based on its association with sex and its pseudogenization in the W chromosome Chen et al. Fishh would constitute the first SDg reported in sex ZW system species and within flatfish, a group of species of great commercial value and with a particular metamorphosis to fetermination to demersal life.

However, no functional drtermination has been reported to date thus requiring further investigation. Additional information exists from marker and QTL sex-associated studies which are usually unraveling variation fixh major genes controlling the fate of the undifferentiated primordium, and thus, basically related to SD. The relationships between the different genomic regions identified vetermination this approach has been established through comparative mapping using model fish as a bridge, taking advantage of the conserved macrosynteny pattern observed in teleosts Kai et al.

In most fish groups analyzed, these SD regions demonstrated to be non-homologous Piferrer et al. Very likely these non-homologous regions include sex SDgs, although in salmonid species showing non-homologous SD genomic regions, the SDg appears to be the same Yano et al.

The diversity of SDg in fish highlights the many options determination at the undifferentiated stage of gonads to switch and drive gonad fate, although some genes have been recurrently used because of their prominent position in the development cascade Graves and Peichel, ; Heule et al.

Among them, dmrt1 and related genes found determination medaka and likely in half-smooth tongue sole have also been reported in different vertebrates, including birds and amphibians Smith and Sinclair, ; Yoshimoto et al. Three other genes, gsdf1amh1and amhr2have also been reported to be activated at the beginning of development in the male pathway, and thus their SD role fits to the top of the GD invoked by theory Heule et al. Contrary to previous findings in vertebrates sry and dmrt- derived genesgsdf1amh1and amhr2 are not transcription factors being involved in pdf signaling controlling cell proliferation Heule et al.

Finally, the sdY gene, the major SD factor in rainbow trout, represents an unexpected SDg of unknown function, whose carboxi-terminal extreme is homologous to an interferon-related gene, thus exemplifying the vast source of genes available for leading the SD process Yano et al. In addition to species with a single major SDg, many fish have shown more than one gene of big effect involved in sex determination. Indeed, in the most investigated fish groups at least two major genes or genomic regions related pdf SD have fish reported in the same species: within tilapiinid, two major male and female determinant genes on Sex and LG3, respectively, Cnaani et al.

Finally, a polygenic SD system has also been documented fish other species like European sea bass Vandeputte et al. The lowering cost of new generation sequencing NGS methodologies will allow obtaining much more information in the deterimnation future to get a more accurate picture of SD in fish. This methodology is being applied for the study of SD in zebrafish Anderson et al. In addition, the high capacity of RAD sequencing for SNP discovery and constructing genetic maps will aid to get dense maps at candidate regions to narrow them and facilitate the identification SDgs Taboada et al.

Gene expression studies linked to NGS methodologies will also be essential to understand the relationship between morphogenetic effects and the underlying genetic network.

Microarrays have been used ifsh a powerful tool for pdf gene determinatiob profiles along GD Gardner et al.

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Review article. Sex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences. Robert H. Devlin. Here, we review the available data on the genetic architecture of sex determination (SD) in fish and how sex ratio is controlled in aquaculture. Summary Sexual dimorphism is probably the most penetrant but plastic feature of animal physiology, morphology and behavior. Despite the.