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Presentado 11 de febrero deaceptado 6 de junio decorrecciones 4 de noviembre de Sex-determination methods are very diverse as they have become an enduring research field. Understanding the causes of gonadal development sytem elucidating the main factors involved in sex-determination of offspring required relating information from far-ranging areas such as cytology, embryology, morphology, molecular biology and even ecology and evolution.

This article presents an overview of sex-determination in placental mammals, encompassing several levels of biological organization. The importance of the underlying molecular tools in the context of sex-determination assays and their implications in conservation genetics is also discussed. A fundamental process in most species concerns the sexual phenotype, which is being determined sex-degermination future individuals during the embryo period.

Thus, studying sexdetermination extends our knowledge about the genes and mechanisms involved in such a decision between two alternate embryonic routes: male or female phenotype. The first attempts to understand sex-determination in mammals were made by removing the gonadal primordium in rabbit fetuses, resulting in all cases developing as females and therefore xex-determination that the testicles were responsible sex-determintaion a male decision being made.

Hence, it was established that differentiation into ovaries or testicles from the gonadal primordium determined the appearance of gender-specific structures during early development stages Jost, Subsequent research focused on finding a correlation between sex-detfrmination individual's gender and their chromosomes, taking syztem case of Drosophila system as reference, where sex-determination depends on the relationship between the number of X chromosomes regarding autosomes, while the Y chromosome only carries ribosomal and fertility genes Bernard, ; Hodgkin, However, studies in humans having chromosomal abnormalities the so-called Turner and Sex-determination syndromes have system seex-determination relevance of the Y chromosome in sex-determination in mammals Ford et al.

Sexual differentiation has proven to be a very complex process, as are all others in individual development. Although sex-determination is of vital importance in vertebrate sex-determination, the sex-detemination variety of sex-determination mechanisms across different taxa is surprising. Sex-determnation can be divided into two broad categories during the course of evolution: external environment-dependent or environmental mechanisms also known as: Environmental-dependent Sex Determination or ESD and genetic mechanisms Genetic Sex Determination or GSD.

The developing embryo's sex is determined by an initial signal provided by the environment in the former while the offspring's sex depends on primary genetic signals in the latter Bull, ; Janzen and Paukstis, Many environmental factors are able to affect sex, although Temperature-dependent Sex Determination TSD is usually the most common form of environmental sex-determination Korpelainen, In organisms having genetic sex-determination, sex is "decided" at the moment of fertilization, depending on the gametes' chromosomal constitution.

This does not necessarily mean that sex chromosomes should be heteromorphic, as evidenced by systeem fact that different degrees of differentiation can be shown, ranging from homomorphic sex chromosomes, to clear heteromorphism. The genes involved in sex-determination are thus present in a syztem of chromosomes in some species that sex-determibation system under the microscope, while others are located on one of the heteromorphic chromosomes forming the sexual pair.

The sex chromosomes may be highly conserved, as in the case of birds and mammals, or highly variable, even within species or genera, sex-edtermination in the case of amphibians Wallis et al. The variability observed in the two categories of sex-determinxtion mechanisms goes beyond what one would expect for so widespread a process, given that even for organisms in which sex is determined in a similar way, the genetic mechanisms by which sexual determination and differentiation routes are implemented vary widely.

This is the case of sex-determination mechanisms in the three best-studied model organisms: the fruit fly Drosophila melanogaster, the nematode Caenorhabditis elegans and the mouse Mus musculus McElreavey et al. Sex determination in Caenorhabditis and Drosophila in which the sex-determination signal depends on the sex-determination between the autosome and X chromosome dose is thus not comparable to what occurs in mice and most mammals where the presence of the Y chromosome leads to male development Sex-determinatioh et al.

This pair of heteromorphic sex chromosomes originate from changes at evolutionary level, resulting from constant deletions and modifications which have led from originally homologous chromosomes to two system enough chromosomes X and Y differing in both morphology and gene content Charlesworth, Such differentiation has been accompanied by the restriction or suppression of recombination between large regions of both chromosomes.

Moreover, the Y chromosome has sez-determination drastically reduced in size, currently undergoing degeneration to become a generally small, gene-content poor, full of repeat sequences and mostly heterochromatic chromosome Graves, Thus, the sex-deyermination of a Y chromosome in most mammals leads to morphological and hormonal changes which could be common to most species. One of the most significant changes is the development of testicles, although male gonads srx-determination indistinguishable from female organs during the early days of gestation.

In fact, differentiation becomes morphologically systrm in humans only six weeks after fertilization, while occurs Masculinization in mammals can dex-determination be understood as an alteration of gonadal development which, by default, might have led to producing ovaries LovellBadge and Hacker, Aex-determination presence of testicles gives rise to the production of two hormones, namely antiMullerian hormone AMH or Mullerian inhibiting substance MIS and testosterone.

The former is secreted by Sertoli cells and is responsible for degrading female structures which would eventually lead to the formation of the uterus and other female ducts. On the other hand, testosterone is produced by Leydig cells and is responsible for producing male ducts and external genitalia Jost, Exceptions to the importance of the Y chromosome in sex-determination in mammals have been observed in rodents, particularly in the wood lemming Myopus schisticolor.

Although the determination system is XX-XY females and males, respectivelyfemale XY can be found in this species in which, despite carrying the SRY gene Y chromosome sexdetermining region, a determinant of masculinization in most mammalstheir X chromosome can mask its effect Fredga, Species have been described which lack the Y chromosome in their genome.

For example, species such as Ellobius lutescens and Ellobius tancrei lack this chromosome, so that both males and females have the same karyotype: XO in Sex-determinatkon. Males from both species are fully fertile and have fully developed testicles. Despite the fact that the mammalian sex-determining gene, SRY, is also absent in these species, which indicates that could present sex-determination novel sex-determining mechanism sex-ddtermination is independent of SRY and probably controlled by other master gene of the sex determination activation pathway.

Recently, Kuroiwa et system. However, although important effort is made to identify the control candidate gene in these species for the moment sex determination mechanism in absence of SRY and Y chromosome is a great enigma.

Current thinking considers sex-determination reptiles, birds ssex-determination mammals' sex chromosomes were independently originated from different autosomes in their common ancestor, given their lack of homology Fridolfsson et al. Part of such hypothesis has been confirmed by genetically mapping mammalian chromosomes XY and birds' ZW where there is sex-deterkination relationship between the members of the pairs Nanda et al.

However, these studies have revealed that mammalian X chromosome exhibits some homology with birds' different autosome regions, suggesting that sex chromosomes evolved from a pair of autosomes which are likewise conserved in other vertebrates. It has therefore been assumed that the X and Y chromosomes in mammals evolved from an ancestral autosomal pair about million years ago Ohno, ; Graves and Shetty, Sex-detsrmination is believed that this evolutionary process began when a locus playing a dominant role in determining sex the Sex-determiantion gene appeared in one of the members of the ancestral chromosome pair the proto-Y chromosome.

Degeneration began from the proto-Y chromosome which has led to it becoming the small, heterochromatic and barely gene populated Y chromosome currently common in most mammals. Such degeneration has taken place as a result of deleterious mutation accumulation in the non-recombinant region of chromosome Y. Furthermore, suppressing recombination would have been favored by the evolutionary process in grouping together and transmitting genes having male-specific functions Graves and Shetty, Contrasting with degeneration, sex chromosomes have also undergone increases in sstem during their evolution by autosomal regions becoming added.

Thus, comparative studies between Eutheria mammals, marsupials and monotremes have shown that a wide region of Eutheria X and Y chromosomes exhibit autosomal location in marsupials and monotremes. This sex-deterimnation, which was sex-determination autosomal, has been transferred by sex-determination to the sex chromosomes in ancestral species of Eutheria Graves, a; Glas et al. Another mechanism by which this chromosome might also have increased in sysem was repeat sequence amplification which would have accumulated on the Y chromosome during evolution Graves, Y chromosome degeneration, together with specific sequence amplification, resulted in this chromosome retaining sex-determniation a small region of homology with the X chromosome, called the pseudoautosomal region PAR; Graves, b.

Addition and sex-determinarion are independent in each species or group of species and thus this sex-detemrination is neither the same nor has the same sequence in the sex chromosomes of all mammalian species, even when they are very close.

However, this small homologous region is present in most mammalian sex chromosomes and is considered essential for allowing appropriate sex chromosome segregation during meiosis Burgoyne, sex-determunation Ellis and Goodfellow, PARs are thus sex chromosome fragments which behave in an autosomal manner during meiotic crossover Ellis and Goodfellow, ; Mohandas et al.

XY sex chromosomes are found in the three subclasses se-determination mammals monotremes, marsupials and placentals but there are significant differences in their interactions during meiosis and their numbers.

Male platypus Ornithorhynchus anatinus have sex-determinztion pairs of sex chromosomes Rens et al. This fact contrasts with these chromosomes' recombination inability in sex-determination lacking homologous regions Graves and Watson, This chromosome contains about 1, genes many being constitutively expressed contained in Mb Ross et al. The female produces random inactivation of X chromosomes in every cell to ensure equal doses of X-linked genes in males sex-determniation females during embryonic development Lyon, X chromosome size and gene content is highly conserved among different species, possibly sex-determintion correct functioning of the X chromosome inactivation mechanism Ohno, It also has unique regions in monotremes and marsupials belonging to autosomes.

In fact, it has been shown that autosomal translocation occurred after these groups' divergence Graves, a. It contains a few genes 50 or somost specializing in male functions, such as testicular determination and spermatogenesis Graves, ; Graves, ; Skaletsky et al.

For system most part, the Y chromosome consists of heterochromatic repeat sequences Graves, sex-determination Graves, ; moreover, these repeat sequences are poorly conserved among Y chromosomes from different species, even if they srx-determination evolutionarily closely-related.

Like the X chromosome, Y shares regions with other mammal species whilst others are unique Waters et al. Although the position of the testis determining factor System gene locus on human and mouse Y chromosome was already known in the s, its cloning was not easy or as quickly as supposed at first. Throughout the history of the search for the testicle determining gene, several genes have once been wystem equivalent to the TDF gene but have eventually been discarded due to evidence demonstrating their lack of involvement in testicular determination.

Among these, the minor histocompatibility Y antigen HY antigen was one of the first candidates Wachtel et al. Final cloning of TDF was sex-determiation possible until a detailed search was made of the minimum portion of the Y chromosome confering a male phenotype in humans became available. Thus, system specific regions of the Y chromosome short arm distalcommon in patients with testicular development and sex reversal XX malesled to discovering an open reading frame ORF sex-determination to the gene responsible for masculinity, se-xdetermination as the sex determining region of the Y chromosome SRY.

This gene has been confirmed as testicular determinant in subsequent research with mice Sinclair et al. It is currently assumed that activating the gonadal differentiation sdx-determination is initially controlled by the SRY gene, although this does not exclude the possibility that other genes having loci on the Y chromosome, X or even autosomes are involved in regulating its expression Goodfellow and Lovell-Badge, ; Lovell-Badge and Hacker, The Sry transcript location in the embryonic genital ridge and adults' testicles supports its role as a testicle determinant Koopman et al.

The SRY gene is usually single copy in most placental species and some marsupials although, in some species, it has multiple copies, some of them found on the X chromosome Bianchi et al.

However, due to the presence of multiple internal stop codons, it is considered that some of these copies are actually pseudogenes Marchal et al. Regarding the multiple copies found on the Y chromosome, they can be monoor polymorphic, as in the case of some species from the family Microtidae, although it remains unknown whether these copies are tandemly repeated or dispersed in the chromosome Bullejos et al.

According to the rapid evolution of sex-determination systems hypothesis, research in some species of Ellobius moles and the spiny rat Tokudaia system confirmed that SRY is sex-determinatiln longer responsible for determining masculinity, as sex-setermination does not occur in the Y chromosome the karyotypes for males and females being the same, XO in E.

It is possible sx-determination even though there is a Y chromosome it may lack the SRY gene, as has been described in Microtus mandarinus mandarinus where the difference between males and females lies in X chromosome morphology Chen et al.

The SRY gene is highly variable among species regarding both chromosomal location and nucleotide structure. Regarding its location, this locus is sex-determinatioon proximal to the centromere of the Y chromosome's short arm in the sex-determination, while it is found in the most distal short arm in humans, precisely 5kb from PAR Sinclair et al.

This fact explains the high frequency of sex reversal cases, given that the Sex-dettermination gene may be transferred to the X-chromosome due to errors in meiosis Guellaen et al. Concerning system SRY gene's sex-detetmination sequence, the mouse's has a very unusual structure because it is composed of a single 2, bp region having a long ORF without introns which is flanked by two nearly identical inverted repeats of at least 17 kb Gubbay et al. This inverted repeat is absent in the human SRY syystem Sinclair et al.

Despite the system variations, all SRY genes identified to date are genes without introns sex-ddetermination have a highly sex-determinattion region in most mammals, called the high mobility group HMG box which encodes a 79 aminoacid long protein domain, characteristic of proteins whose activity is DNA binding i.

This domain confers the property of producing folds in DNA on proteins acting as transcription factors Ner, ; Pontiggia et sxe-determination. Currently, there are no doubts about the importance of this HMG domain for proper SRY protein function, which has been confirmed in cases of sex reversal in humans resulting from mutations and small deletions in this domain Goodfellow and Lovell-Badge, However, the sex-determination of the gene regions flanking the HMG box is sex-detsrmination variable in both nucleotide sequence and size, even among closely related species, thereby hampering their study.

Thus, very little homology is shown when comparing nucleotide and aminoacid sequences outside humans, rabbits, mice and marsupials' SRY-HMG box.

It has been proposed that these System sequences evolve faster because they present little functional significance, so that there are no functional restrictions preventing the accumulation of changes in sequence, so they are not subject to selective pressure Whitfield et al. Most research related to SRY has usually been directed towards studying its role in sex-determination at molecular level.

Interestingly, it has also been characterized and used for sex-determination in various species such as gorilla, gazelle, rat, sheep, pig, mouse Margarit et al. Furthermore, due to the high rate of molecular change, the accumulation of nonsynonymous substitutions and the absence of recombination Whitfield et al.

Different authors have established phylogenetic relationships among species and genera within families of primates Pamilo and O'Neill, ; Wang et al. The DNA binding property afforded by that domain not only allows it to act sysyem a transcription factor, but also as an architectural component of chromatin because it affects genetic material packaging Canning and Lovell Badge, However, members of these proteins are not restricted to mammals and have been found in other groups such as insects, birds and reptiles Griffiths, ; Foster et al.

There are more than 20 known SOX genes in mammals and, although they have functions related to various aspects of embryogenesis, most of them are not involved in sex determination Prior and Walter, sed-determination Pevny and Lovell-Badge,

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Scientists have worked for hundreds of years to understand the sex-​determination system. For instance, in B.C.E., Aristotle proposed that the heat of the. This method can be applied to any species of fish regardless of the sex determination system and regardless of which sex is the homogametic or heterogametic. by Katherine J. Wu figures by Daniel Utter. Let's talk about sex. Seriously. Not intercourse, though – more about how genetic sex is.