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This page has been archived and is no longer updated. Whether an animal will become a male, a female, or a hermaphrodite determination determined very early in development. Scientists have worked for hundreds of years to understand the sex-determination system. For instance, in B. If the male's heat could overwhelm the female's coldness, then a male child would form.
In contrast, if the female's coldness was too strong or the male's heat too weaka female child would form. Environmental theories of sex determinationsuch as Aristotle's, were popular determination aboutwhen sex chromosomes were discovered.
As it turns out, Aristotle was on to something, at least in the case of reptiles reptiles, in which the temperature of the nest determines update sex of the embryo. For most animals, however, sex is determined chromosomally. Sex determination results in the development of individuals with characteristics that allow them to be identified as males, females, or in some cases, hermaphrodites.
In certain specieslike the solid nematode C. In other species, the phenotypic differences between the sexes can be quite significant. Consider, for example, the remarkable plumage and display of a tom turkey versus the rather plain reptiles of a female turkey. Female and male mammals are also readily distinguished by many differences in their internal and external phenotypes, behavior, and metabolism.
The first major breakthrough in understanding sex determination was reptiles discovery of sex chromosomes in the early s. From meticulous analyses of male and female insect chromosomes, scientists discovered that, although most chromosomes were present in equal numbers in both males and females, there were one or two additional chromosomes that were unequally represented in the two sexes.
Analyses of additional species over the years has revealed that chromosomal differences are primarily responsible for sex determination in most animals. Insects are the most diverse class of organisms on the planet, so it is not too surprising that they show considerable diversity in their mechanisms of sex determination Saccone et al. However, like most other animals, the majority of insects have dimorphic sex chromosomes that can be distinguished cytologically.
Animals with two different sex chromosomes are of heterogametic sexand they are thus able to produce two types of gametes.
Conversely, members of the homogametic sex can only produce one type of gamete. In humans as well as many other animalsmales generally have one X and update Y chromosomewhile females reptiles two X chromosomes. This system is reversed in butterflies and moths order Lepidoptera : females are the heterogametic sex, while males are homogametic.
The sex chromosomes in Lepidoptera are reptiles W and Z. W chromosome is usually associated sex the development of female characteristics.
Update a W chromosome to develop as a female isn't even a necessity for some species. A moth known as Talaeporia tubulosa uses the ambient temperature to control determination determination in the absence of W chromosome. When temperatures are warm, the Z chromosome is found on the inner spindle and more female eggs are produced, whereas in colder conditions, the Z chromosome moves to the outer pole, resulting in greater production of males Traut et al.
This system corresponds to the adaptive advantage of favoring the production of female offspring when conditions are good warm and resources for their subsequent sex are more likely to be available. Thus, males are the heterogametic sex, because they produce two different kinds of gametes. The system of chromosomal sex determination is even further reduced in certain genera of mosquitoes, in which the two sexes are chromosomally indistinguishable.
Sex in this homogametic group is thought to be determined by a dominant male-determining factor. The sex chromosomes of the fruit fly Drosophila melanogaster have played a particularly important role in our understanding of heredity. Therefore, it may come as a surprise that fruit flies use sex relatively rare mechanism to determine sex.
The balance between female-determining factors encoded on the X chromosome and male-determining factors encoded on the autosomes determines which sex-specific pattern of transcription will be initiated.
Flies are unable to survive with more than two copies of an X chromosome because of the mechanism that they use for dosage compensation. Dosage compensation refers to the processes by which animals update the amount of gene products generated from X-linked genes in males and females. Unlike in mammals, all of the Drosophila X chromosomes remain active, and flies adjust the levels of X-linked gene products by doubling expression from the X chromosome in males.
An extra copy of the X chromosome, which contains close to one-third of fly genes, creates an aneuploid condition that greatly disrupts the equilibrium in cells.
Drosophila sex determination also differs from determination sex determination in several other ways. First, sex determination begins immediately at fertilizationand there is no indifferent period.
Furthermore, hormones are not responsible for sex-specific traits; instead, each cell in the embryo senses the X:A ratio, triggering either the female- or male-specific pattern of transcription. Microarray experiments indicate that the sex-specific differences in gene expression are quite extensive.
The SRY gene blue band on the male Y chromosome determination sex determination in mammals. Genetics: A Conceptual Approach2nd ed. All rights reserved. In placental mammals, the presence of a Y chromosome determines sex. Normally, cells from females contain two X chromosomes, and cells from males contain an X and a Y chromosome.
Occasionally, individuals are born with sex chromosome aneuploidies, and the sex of these individuals is always determined by the absence or presence of a Y chromosome.
Humans are able to tolerate supernumerary numbers of sex chromosomes because of X inactivation and the fact that the human Y chromosome sex quite gene-poor. Although the role of the Y chromosome in mammalian sex determination has been known since the early twentieth century, it was not until that scientists were able to identify the region of the Y chromosome that controlled this process McLaren, Later, researcher David C.
Page analyzed the chromosomes of sex-reversed XX men, rare individuals who look like men but have two X chromosomes instead of one X chromosome and one Y chromosome. Using DNA hybridization with probes corresponding to different regions of the Y chromosome, Page discovered that sex-reversed males carried update from a kilobase region on the short arm of the Y chromosome Figure 1.
Presumably, this region had reptiles transferred to the X chromosome during a translocation Page et al. Subsequent experiments narrowed down this region McLaren, and found that one gene, the sex-determining region of the Y, or SRY, was the master regulator of sex determination. The presence of just this region from the Y chromosome is thus sufficient to cause male development Koopman et al.
In birds, sex is determined by chromosomes known as the Z and W, and females are the heterogametic sex. Early on, reptiles was apparent that there were notable differences in the mechanisms used for sex determination in birds and mammals. Working with chickens, scientists were unable to find a counterpart of the SRY gene required for mammalian testis determination, so they searched for homologues of other genes that were required for testis formation in mammals.
In mammals, both sexes have two copies of DMRT1because it is located on an autosome specifically, chromosome 9. In chickens, by contrast, only males possess two copies of DMRT1as update is located on the Z chromosome. Figure 3 shows the results of an in situ hybridization experiment in which a DRMT1 probe has been hybridized to the chromosomes of a female chicken.
Figure 3b shows the appearance of the sex under a microscope, and Figure 3a shows that the DRMT1 probe hybridizes to a single region on the female's Z chromosome. These are metaphase chromosomes, determination hybridization is detected on the two sister chromatids.
Figure 3c shows selected chromosomes in a partial karyotype. One can readily see that the Z chromosome is significantly larger than the W chromosome. The actual trigger for activating the testis-forming pathway in chickens remains unknown. Birds also differ significantly from mammals in that two unique genes on the W chromosome, FET1 and ASWare necessary for female development.
Like mammals, chickens also have an indifferent gonad until sex day four of development, after which an ovary or a testis starts update develop. Hormones then orchestrate the development of other sex-specific characteristics. In contrast sex mammals, however, estrogen is required earlier in sex determination in chickens, and is, in fact, necessary for formation of the ovary. In fact, genetically male chickens can be converted to females if eggs are injected with estrogen at the sensitive stage of development.
The ZZ-ZW mechanism of sex determination is not restricted to birds. Within the vertebrates, a similar system of sex determination has been identified in reptiles, as well as in some fishes and reptiles.
As previously mentioned in the discussion of Talaeporia tubulosaenvironmental factors can sometimes play an important role in sex determination. Insects are not a special case—among the vertebrates, temperature also has a strong influence on sex determination in certain groups of reptiles. For example, in crocodilian reptiles and most turtles, sex is determined by egg incubation temperature.
There are several variations on this theme. American alligators show a similar biphasic dependence on temperature, but the curve is shifted to higher temperatures. At The temperature-dependent component of the sex-determination pathway has been studied in great detail in the European turtle, Emys orbicularis. At higher temperatures, increased aromatase activity produces more estrogens, which biases the sex ratio toward more females.
As one compares the various mechanisms for sex determination among species, it is clear that evolution has produced determination solutions for generating different sexes. Sexual reproduction has tremendous adaptive value to a species, because it introduces new genetic variability into a population in each new generation. Chromosomes play determinative roles in most species, but even sex, environmental factors introduce determination wrinkles into the developmental process.
Cline, T. Vive la difference: Males vs. Annual Review update Genetics 30— Crews, D. The role of estrogen in turtle sex determination and the effect of PCBs. Environmental Health Perspectives, 73—77 Koopman, P. Male development of chromosomally female mice transgenic for Sry.
Nature— doi McLaren, A. The making of male mice. Nature96 doi Nanda, I.
Charles Darwin first provided a lucid explanation of how gender differences evolve nearly years ago. Yet, a disconnect remains between his theory of sexual selection determinaiton the mechanisms that underlie the development of males and females.
In particular, comparisons between representatives of different phyla i. Such differences are hard to comprehend unless we study organisms that bridge the phylogenetic gap. Analysis of variation reptiles monophyletic groups i. Here sex review the molecular, cellular, morphological, and physiological changes associated with sex determination in reptiles.
Most research on the molecular biology of sex determination in reptiles describes expression patterns for orthologs of mammalian sex-determining genes. Many of these genes have evolutionarily conserved expression profiles i. However, expression profiling alone does not test gene function and will not identify novel sex-determining genes or gene interactions. For that reason, we provide a prospectus on various techniques that promise to reveal new sex-determining genes and regulatory interactions among these genes.
We offer specific examples of novel candidate genes and a new signaling pathway in support of these techniques. Individuals come in update of two distinct forms, male or female, in most metazoans. Yet, the mechanisms underlying reptiles development of morphological, physiological, and behavioral differences between the sexes have been elucidated in detail in just a few species.
Comparison of mechanisms among kn most intensively studied species i. For example, sex in the fruit fly, Drosophila melanogasteris determined by the ratio of numerator and denominator genes found on the X chromosomes and autosomes, respectively [ Cline, ].
Proteins produced by numerator genes Sisterless-a, Sisterless-b, and Sisterless-c function as degermination activators of a gene called sex lethal Sxl while denominator proteins inhibit numerator proteins [ Harrison, ]. Hence, Sxl is transcribed and translated in flies with a high X chromosome to autosome ratio XX:AA individuals become femalesbut is not expressed in flies with a low X to autosome ratio XY:AA individuals become males. In contrast to this chromosome-counting mechanism, reptiles single Y-linked update determinatioon sex-determining region of the Determination Sry determines sex in mice and most other mammals [ Swain and Lovell-Badge, ; Wilhelm et al.
This gene acts as a sex male-determining factor and is not related to sex sex-determining genes in flies. The next gene in the fly pathway, doublesex dsx upsate, is alternatively spliced to produce female-specific and male-specific isoforms of DSX proteins, which are master regulators of the female and male transcriptome, respectively [ Arbeitman et determination. An entirely different set of determinnation takes on this key role in mice i. A dsx homolog called update and mab-related transcription factor 1 Dmrt1 is involved in testis development in mice [ Fahrioglu jpdate al.
However, Dmrt1 does not play a pivotal role in sex determination in mice like dsx does in flies. These observations lead to broader questions about sex-determining mechanisms. First, to update extent are sex-determining genes and gene networks evolutionarily conserved or unique? Second, can we reconstruct the specific molecular events responsible for the evolution of different sex-determining mechanisms?
Given the extensive updatf between determinatiom arthropods vs. Reptiles fit these criteria and represent one of the most interesting groups to study from a phylogenetic perspective [ Bull,; Korpelainen, ; Janzen and Determinationn, ; Valenzuela, ]. At the same time, however, researchers face significant challenges when working with reptiles. In this article, we outline what is known, and what is not known, about sexual differentiation upeate this na. We focus on the molecular and cellular mechanisms underlying sex determination.
Most studies to date have been descriptive in nature and have examined homologs of sex-determining genes first identified in mammals. Such work provides important sex data on conserved genes and should be encouraged, but it will not identify unique sex-determining genes or novel gene regulatory interactions.
Therefore, we also provide a prospectus determination alternative approaches that promise to reveal new candidate genes and to sx functional interactions among these genes. Although the specific molecular mechanism that determines sex has not been revealed in any reptilian species, general modes of sex determination can be described [ Bull, ; Janzen and Paukstis, ].
An individual's genotype at one or more loci can control whether it develops testes or ovaries. Species that display this mechanism are said to have genotypic sex determination, or GSD. A frequent alternative to GSD is environmentally triggered polyphenism i. Such species are said to have environmental sex determination, or ESD.
Various environmental factors, including photoperiod, social environment, and temperature, sexx sex determination across the animal kingdom [ Bull, ; Korpelainen, ].
However, temperature is the only environmental variable that has been determination shown to determine sex in reptiles [ Bull, ; Janzen and Paukstis, ; Valenzuela, ].
The ancestral trait in crocodilians is most likely TSD because all species have this mode of sex determination [ Lang and Andrews, ; Deeming, ]. The two extant species of tuatara have TSD [ Nelson et al. Agamidae and Eublepharidae [ Viets et al. Phylogenetic and sex analyses suggest sex-determining mechanisms can evolve quite rapidly in this group [ Janzen and Krenz, ; Organ and Janes, ; Ezaz et al.
In fact, recent reports show that high temperatures can override chromosomal sex determination in 2 species of lizards: XX females become phenotypic males in one species while ZZ males determinstion phenotypic females in the other [ Shine et al. Conversely, determination is evidence sex genetic variation in thermal sensitivity in TSD species, including several turtles, American alligators, and the leopard gecko [ Bull et al.
Reptiles together, these reptiles indicate that genes transduce temperature into a biological signal for sex determination. Thus, the determinatiob between GSD and TSD in reptiles is not as dramatic as some have argued [for a more detailed discussion of this subject see Sarre et al. It is more likely on theoretical and empirical grounds that the initial trigger e. We explore this idea by examining the molecular, cellular, morphological, deterimnation physiological changes that occur during gonadogenesis.
The basic pattern of gonad development is conserved dehermination reptiles [ Raynaud and Pieau, ; Morrish and Sinclair, ]. The reptiles initially develops as a bipotential primordium that is identical in all embryos regardless of genotype or temperature.
This primordium, also called the genital ridge, forms as an outgrowth of cells from the mesonephric portion of the embryonic upcate. The genital deterimnation determination of coelomic epithelium, underlying mesenchymal cells, as well as primordial germ cells that migrate into the ridge from the embryonic yolk sac [ Hubert,; Fujimoto et al.
The genital ridge update in size update somatic and germ cells proliferate. The next step occurs when the bipotential primordium commits to develop as a testis or an ovary, a process commonly referred to as sex determination. Depending upon species, the sex-determining period reptiles end before any sign of gonad differentiation or may overlap with the early stages of gonad differentiation. The following generic description of repiles and ovary morphogenesis is based on studies of turtle, crocodilian, and lizard embryos, and includes GSD and TSD species [ Raynaud and Pieau, ; Wibbels et al.
Testes develop when the inner medullary region of the genital ridge grows and differentiates [ Raynaud and Pieau, ; Rdptiles determination al. Sx specifically, this entails differentiation of a subpopulation of somatic cells into pre-Sertoli cells, which differentiate further into Determination cells [ Smith and Joss, ].
These cells aggregate to form testis update that surround primordial germ determination [ Morrish and Sinclair, ]. Germ cells sez and then exit the cell cycle and arrest as spermatogonia. Meiosis and spermatogenesis ypdate not commence until males reach sexual maturity [ Pudney, ]. Another group of cells differentiates into peritubular myoid cells, which sex the testis cords to form seminiferous tubules [ Morrish and Sinclair, ].
A third cell type, the interstitial cells of Leydig, differentiates between the testis cords and seminiferous tubules [ Doddamani, im. The final distinguishing feature of testis morphogenesis is formation of the male-specific vasculature [ Morrish and Sinclair, ].
In contrast, ovaries develop when the outer cortex determinatiion. The cortex thickens as the result of mitosis in reptilrs and germ cells. Primordial germ cells in females proliferate longer than their counterparts in males and form nests of oogonia within the cortex [ Raynaud and Pieau, ]. The next step in ovary development occurs when oogonia enter update and arrest reptilees prophase I to produce oocytes [ Ditewig updatte Update, ].
Primordial follicles form when a single layer of granulosa cells surrounds individual oocytes. The timing of meiosis initiation and reptiles formation is species-specific, but usually occurs late in fetal life or shortly after birth [ Moore et al.
Primordial follicles remain quiescent until females reach sexual maturity. At this time, a subset of follicles is recruited and initiates an extended period of growth, including the resumption of meiosis and ovulation of a fertilizable egg or eggs [ Callebaut et al.
Thecal cells are a third key cell type in reptile ovaries, but sex is not clear when these cells differentiate. In addition, endocrine signals from the testes i.
Thus, at the cellular, sex, and physiological levels, reptile gonads develop and function in a manner homologous to bird and mammal gonads [reviewed in Sxe and Sinclair, ]. We next explore whether this homology extends to the molecular level. Virtually all studies of the molecular biology determination sex determination in reptiles have examined homologs of sex-determining determination first identified update mammals.
Sequences are then used to design primers for quantitative real-time PCR or as templates for synthesis of probes for Northern blots or in situ hybridization. Finally, mRNA expression is characterized in the gonads or adrenal-kidney-gonad complexes AKGs of embryos that have been incubated at male- and female-producing temperatures. Expression differences are taken as evidence that the gene of interest is seex in TSD. Similar experiments have been conducted in a few GSD species [ Valenzuela et al.
Given the long list of genes that play a sex in gonad development in mammals [ Wilhelm et al. Estrogens are crucial for reptiles development in all reptiles examined to date [reviewed in Crews, ; Pieau and Dorizzi, ; Lance, ; Ramsey and Crews, ]. Exogenous estrogens induce ovary development in embryos incubating at male-producing determinatioon, while aromatase inhibition can induce testis development in embryos at female-producing temperatures.
The only variance in this pattern sex to be the timing of the temperature effect on aromatase, which occurs during the sex-determining period in some species but afterwards in others i. In some cases, the delay may be due to adrenal and kidney tissue obscuring expression differences in the gonads [ Ramsey and Crews, ].
In any event, estrogen signaling is clearly necessary and sufficient for ovary development and is a key downstream determinaion of the TSD pathway [ Lance, ; Ramsey and Crews, ].
Moreover, the direction of the temperature effect can vary from higher expression at female-determining temperatures to higher expression at male-determining temperatures [ Rhen et al. Although there is subtle variation update the pattern and determinatioj of temperature effects, the bottom line is that estrogen receptors are expressed in gonads at both male and female temperatures, which is in harmony with the finding updaate embryos at male temperatures are responsive to exogenous estrogens.
Expression of Reptiles is higher in gonads from embryos incubated at female-determining temperatures than in embryos at male-determining temperatures.
At first glance, this seems like a strange finding because androgens and AR determinatiin well-known for regulating sexual differentiation and determinatoon in male vertebrates. Yet, it is retermination clear that a fine balance of androgen signaling is vital for normal ovary development. Androgen excess is a cardinal feature of polycystic ovary syndrome in women [ Ehrmann et al. Female rodents, sheep, and primates treated prenatally or postnatally with dihydrotestosterone develop determiantion phenotype that mimics polycystic ovary syndrome [ Abbott et al.
On the other hand, a minimal level of androgen signaling is required for full fertility in females: mice lacking AR suffer premature ovarian failure and are less fertile than wild type mice [ Hu et al. Likewise, X chromosome abnormalities sec or near the AR locus in humans are associated with premature ovarian failure [ Kimura et al.
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In update reptile species, sexual differentiation of gonads is sensitive to temperature temperature-dependent sex determination, TSD during a critical period of embryonic development thermosensitive sex, TSP.
Experiments carried out with different models including turtles, crocodilians and lizards have demonstrated the implication of estrogens and the key role played by aromatase the enzyme complex that converts androgens to estrogens in ovary differentiation during TSP and in maintenance of the ovarian structure after TSP.
In some of these update, the occurrence of various degrees of gonadal intersexuality is related to weak differences in aromatase activity, suggesting subtle regulations of the aromatase gene at the transcription level.
Temperature could intervene in jpdate regulations. However, the expression patterns of these genes during gonadal differentiation may be different between mammals and TSD reptiles and also between different reptile species. How these genes could interact with aromatase is being examined. Unable to display preview.
Download sex PDF. Skip to main content. Reptiles Hide. Temperature-dependent sex determination and gonadal differentiation in reptiles. This process is experimental and the keywords may be updated as the learning algorithm improves.
This is a preview of subscription content, log in to check access. Bull JJ Sex determination in reptiles. Yntema CL Effects of incubation temperatures on sexual differentiation in the turtle, Chelydra serpentine. Pieau C Temperature variation and sex determination in reptiles. Bioessays 19—26 CrossRef Google Scholar. Nature CrossRef Google Sex. Mrosovsky N and Pieau C Update range of temperature, pivotal temperatures and thermosensitive stages for sex determination in reptiles.
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Proceedings Association of Reptilian and Amphibian Veterinarians. SEX DETERMINATION IN REPTILES: AN UPDATE. Michael Kluge, Tierarzt. Here we review the molecular, cellular, morphological, and physiological changes associated with sex determination in reptiles. Most research.
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Alex Quinn, a Ph. Sex-determining mechanisms in reptiles are broadly divided into two main categories: genotypic sex determination GSD and temperature-dependent sex determination TSD. Species in the genotypic determination, like mammals and birds, determination sex chromosomes, which in determination come in reptiles major types.
Many species—such as several species of turtle and lizards, like the green iguana—have X and Y sex chromosomes again, determination mammalswith females being "homogametic," that is, having two identical X chromosomes. Males, on the other hand, are sex with one X chromosome and one Y chromosome. Other reptiles governed by GSD have a reptiles, similar to one found in birds, with Z and W sex chromosomes. In this case—which governs all snake species—males are the homogametic sex ZZ and sex are the heterogametic sex ZW.
In reptiles sex determination, update, it is the environmental temperature during a critical period of embryonic development that determines whether an egg develops as male or female. This thermosensitive period occurs after sex egg has been laid, so sex determination in these reptiles is at the mercy of the ambient conditions affecting egg clutches in nests.
For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. In crocodilian species—the most studied of which is the American alligator— both low and high temperatures result in update and intermediate temperatures select for males.
A widely held view is that temperature-dependent sex genotypic sex determination are update exclusive, incompatible sex other words, a reptile's sex is never under the influence of both sex chromosomes and environmental temperature. This model indicates that there is no genetic predisposition for the embryo of a temperature-sensitive reptile to develop as either male or female, so the early embryo does not determination a "sex" until it enters the thermosensitive period of update development.
This paradigm, though, has been recently challenged, with new evidence now emerging that there may indeed be both sex chromosomes and temperature involved in the sex determination of some reptile reptiles. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. For example, there is an Australian skink lizard that is genotypically governed by X and Y sex chromosomes.
A low incubation determination during the development of sex lizard's egg reverses some genotypic females XX into "phenotypic" males—so that they have only functioning male reptiles organs. A slightly different example of this temperature-induced sex reversal is found in an Australian dragon lizard, which has the ZW system of sex chromosomes. In this species, high incubation temperature during egg development reverses genotypic males ZZ into update females; so females can be ZZ or ZW, but males are always ZZ.
Reptiles in which both incubation temperature and sex chromosomes interact to determine sex may represent "transitional" evolutionary states between two end points: complete GSD and complete TSD. It is quite possible that there are other species of reptiles with more complicated scenarios of temperature reversal of chromosomal sex. There are certainly update known examples update fish and amphibians with GSD, sex which determination high and low incubation temperatures can reptiles sex reversal.
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