The chapter is devoted to the consideration of sex determination in vertebrate groups of nonmammalians: fish, amphibians, reptiles, and birds. Attention is drawn to the fact that all these groups of animals, unlike mammals, are implemented hormonal control options for primary sex determination, and there is a possibility of sex reversion. Determination of gonadal development in vertebrates like testis or ovary was initially controlled mainly by sex hormones (fish and amphibians). Later, various sex determining genes were involved in this process. The system was quite plastic and was able to respond to changes in external conditions (reptiles). The appearance of heteromorphic sex chromosomes (birds) has led to the emergence of some specific W chromosomal signal, which provides estrogen control of the development of a heterogametic sex. In mammals, the control of the primary determination of sex (the appearance of the gonad) becomes purely genetic, and the role of sex hormones is reduced to the differentiation of testis or ovaries.
- sex determination
- sex hormones
- sex chromosomes
- sex determining genes
Gender is a set of morphological and physiological characteristics of the organism, providing reproduction, the essence of which is to fertilization, i.e. the fusion of male and female germ cells (gametes) in zygote, which develops into a new organism. Differentiation of sex (its phenotypic manifestation) includes two successive stages: the primary determination of sex and the appearance of secondary (external) sexual characteristics (actual differentiation). It is believed that the concept of this process is conservative. Sex determination is both a genetic and ecological process, with the sex of the individual being determined by an alternative physiological solution. It is assumed that there are two main mechanisms for determining sex: genetic (GSD—genetic sex determination) and environmental (ESD—environmental sex determination). Genetic sex is determined at the time of conception and depends on genetic differences between males and females, and ecological sex depends on external conditions in the absence of significant genetic differences and is determined after fertilization in response to environmental conditions. For birds and mammals, only the GSD is characteristic, and for crocodiles—TSD (one of the forms of ESD). In addition, there are two varieties of the genetic sex determination system: with heterogametic males (XY, mammals) and heterogametic females (ZW, birds). It should be noted that amphibians have both genetic systems, and for lizards, snakes, turtles, and bony fish, all possible variants of sex determination are described [1, 2, 3].
Sex steroid hormones including androgens, estrogens, and progesterone are present in all vertebrates which play essential roles in modulating a variety of behavior and processes, such as embryonic development, sexual differentiation, growth, aggression, reproduction, learning, memory, social communication, and so on. Many signaling actions of these sex steroid hormones are mediated by their receptors that belong to the superfamily of steroid nuclear receptors. Once a sex steroid hormone ligand binds to its receptor, the receptor becomes phosphorylated and is translocated into the nucleus, where it binds to specific DNA sequences and activates gene transcription. Androgens have a critical physiological role in reproductive biology and sexual differentiation, particularly in the development of male secondary sex characteristics [4, 5].
It is assumed that sex determination is a combination of hormonal and genetic factors and is divided conditionally into appropriate stages. This phenomenon is reflected in the possibility of sex inversion—the possibility of its complete or partial hormonal alteration. For fishes and amphibians, there is the sensitivity of normal development of the gonads to androgens and estrogens. In reptiles, birds and marsupials, only estrogens are effective. The appearance of the gonads of placental mammals does not depend on sex hormones. This trend is associated with the stability of growing offspring or incubation of eggs .
The proposed chapter will consider the system of sex determination in fish, amphibians, reptiles, and birds in comparing the role of hormonal and genetic mechanisms, possibilities, and mechanisms of sex inversion.
2. Features of sex determination in fishes
Fishes are perhaps the most complex group of animals in the mechanism of sex determination. Only bony fish include over 30,000 species. It is the largest group of vertebrates. They are divided into three groups in accordance with the laws of sex determination: (1) gonochoristic species whose sex is determined genetically or through environmental factors; (2) sequential hermaphrodites (about 2% of all existing species), changing the sex of males to females (protandrous), the sex of females to males (protogynous), or in both directions (serial) in the process of ontogenesis; (3) unisexual type of sex determination (characteristic only for Amazon mollies (
The Japanese medaka (Oryzias latipes) and Maebashi medaka (
Only two sex determining genes in vertebrates were described:
In this species, the sex determined region of the Y chromosome is only 260 kb (1% of the total length of the Y chromosome (59 Mbps)). In this area, there is suppression of recombination. In medaka, all XY individuals carry mutations in the gene
The zebrafish testes derived from
In other fishes, e.g., salmonids, there appear to be an early stage of differentiation of sex chromosomes. In rainbow trout (
In vertebrates, until recently, only four sex determining genes were discovered:
Sex determining genes in fish are not conservative. It is believed that the reason for this is the more frequent variation of sex chromosomes in fish than other cold-blooded animals and mammals ( Figure 1 ).
These objects sex determination has a high plasticity and is, therefore, possible sex reversal, even in species with established regulatory genes. Striped Danio (
In fish, there are two systems of sex determination: XX/XY and ZW/ZZ. The most common one is the last. Exploring the flatfish
Fish is characterized by plasticity of germ and somatic cells. This plasticity is maintained throughout the life cycle. Furthermore, they have described the influence of factors on this process such as temperature, pH, density of population, etc. It should be noted that the temperature sensitivity of fish is different from that of reptiles, especially because these types of monosexual populations are rare, even under extreme conditions. TSD in fish is less common than previously thought. The effect of estrogens, acting via estrogen receptors (ER) and directly or indirectly regulating P450arom and AMH, is particularly noticeable. It is noted that the analysis of the differences between gonochoristic and hermaphroditic fish species will help to understand the mechanism of plasticity of sex determination in vertebrates. In addition, there is the idea that gender in fish depending on species is a complex trait under the control of one or many genetic factors in addition to environmental effects [9, 14]. In the Chinese tongue sole (
However, hermaphroditic fishes have a plastic sex, and a stable sex is difficult to maintain with sex steroids. The black porgy regulated the dynamic development of both sexes; only one sex can grow while the other sex exists in a rudimentary stage (
). The sexual fate of the digonic gonad is determined by the male fate maintenance and through the Gnrh—Gth—Dmrt1 signaling. Altogether, testicular
3. Sex in amphibians
Amphibians have two sex determined systems: XX/XY and ZZ/ZW. Most tailed amphibians (order
Models of sex differentiation in amphibians can be divided into three types: (1) a direct development of the undifferentiated gonads into testes or ovaries, (2) the development of the undifferentiated gonad into the ovary and subsequent development of the testis through the ovary, and (3) the development of the testes through the intersex phase (prodifferentiating type) . For a long time, genes that determine sex could not be found in amphibians. Recently, for smooth clawed frog (
In the northern crested newt (
The dominant hypothesis of sex determination for amphibians is proposed in relation to the
4. Sex in reptiles: determination of sex under the influence of temperature
Sex determination by environmental factors is mainly known in reptiles. The most well studied temperature sex determination (TSD) is occurring in three of the five main taxonomic groups of reptiles: turtles, crocodiles, and lizards, but it is not found in snakes. The adaptive significance of such sex determination mechanism is shown. During early embryonic development of gonad, epithelial cells are divided and unite in the epidermal strip of mesonephros mesenchyme. Further, during the so-called temperature-dependent period under the level of endogenous estrogen, such strip forms seminiferous tubules with Sertoli cell epithelium or gaps with squamous epithelium. The mechanism of this sex determination is poorly understood. Obviously, it is found in species with undifferentiated Y chromosome. The transition from the female promoting temperature (FPT) to male promoting temperature (MPT) is carried out in a temperature-period (TSP), during the so-called “window” of vulnerability .
In some species of reptiles, GSD is not fixed for life, and the original gender may change during development without changing the genotype. This phenomenon is known as environmental sex reversal (ESR) and observed also in insects, fish, and amphibians  ( Figure 6 ).
In reptiles, there is an “open” sex determination program that is different from a “closed” program, characteristic of birds and mammals. It is believed that in this case, the gender depends on the ratio of estrogens and androgens during sexual differentiation of the gonads. The temperature of incubation may change the activity of genes encoding aromatase, estrogen receptor, and reductase. It is not excluded that different taxonomic groups of animals with TSD have different mechanisms of regulation of sex. There may be temperature-sensitive genes
For Mississippi alligator (
Sex reversal has not yet been demonstrated in nature for any amniote, although it occurs in fish and rarely in amphibians. There is only one report about sex change in reptiles in the wild (Australian bearded dragon (
5. Sex determination in bird
In birds, estrogens play an important role in sex determination. They regulate expression of key sex determining genes during the first 3 days of embryonic development and further. At the same time, the set of sex chromosomes is equally important. Embryos with two Z chromosomes in birds develop as males, and those with ZW chromosomes develop as females. At present, two hypotheses on sex determination in birds compete. One of these hypotheses considers the number of Z chromosomes as a key sex determining factor, while the other hypothesis supposes the presence in W chromosome of the key gene controlling ovarian development or suppressing the appearance of testes. The presence in Z chromosome of a strong candidate gene for sex determination (DMRT1 gene) supports the dose scheme.
presents a hypothetical scheme of genetic control of primary sex differentiation in
In birds, sex determination depends on sex hormones and sex-hormone-specific receptors. Estrogen receptors are also important in this process. In a recent study, the gonads and endocrine profile of a gynadromorphic chicken were described. It had male features on the right and female features on the left. At sexual maturity, the gonads of this bird were largely testicular. The right gonad was a testis, with SOX9+Sertoli cells, DMRT1+germ cells, and active spermatogenesis. According to histology, the left gonad was primarily testicular, but with a few number of peripheral aromatase follicles. The gynandromorph had low levels of serum 17β-estradiol (39 pmol/L). In contrast, the gynandromorph had very elevated levels of serum testosterone (41.3 nmol/L). Despite the elevated testosterone, the bird was female on one side of the body. The right male side was almost entirely ZZ (96%), whereas those from the left female side were a mixture of male (77% ZZ) and female (23% ZW) cells. It had a low percentage of ZW cells on the female side, but still had female sex-linked feathering, smaller muscle mass, smaller leg and spur, and smaller wattle. This indicates that sexually dimorphic structures such as the wattle, spur, and feathering must be at least partly independent of sex steroid effects. Even a small percentage of ZW cells appear sufficient to support female-type sexual differentiation [39, 40, 41]. Studies of chimeric embryos also support the hypothesis that avian sexual differentiation is largely, or partly, cell autonomous, involving direct genetic factors and steroid hormones.
So, estrogens and androgens play important roles in sexual differentiation and reproduction, particularly in the development and expression of male and female sexual characteristics. These effects are principally mediated by the estrogen and androgen receptors (ESRs and ARs), which belong to superfamily of the nuclear receptors . The nature of the relationship between sex hormones and gender determining genes and the patterns of their interaction remains unclear. For some amphibians, the absence of appropriate genes and the replacement by control factors of steroid hormones and receptors are postulated. For birds, we can assume a special role of heteromorphism of sex chromosome and the presence of a specific interaction of the W and Z chromosomes. In this regard, we should mention the phenomenon of detection of specific chromosomes (germ line restricted chromosomes, GRS) found only in the germ cells of songbirds.
In mammals, aromatase is expressed later in embryonic development and the gonadal sex is formed independently of sex hormones and differentiation can occur in the absence of steroidogenesis. For mammals, two-step primary sex determination is typical. At the first stage, its determination is carried out by the
So, determination of gonadal development in vertebrates like testis or ovary was initially controlled mainly by sex hormones (fish and amphibians). Later, various sex determining genes were involved in this process. The system was quite plastic and was able to respond to changes in external conditions (reptiles). The appearance of heteromorphic sex chromosomes (birds) has led to the emergence of some specific W chromosomal signal, which provides estrogen control of the development of a heterogametic sex. In mammals, the control of the primary determination of sex (the appearance of the gonad) becomes purely genetic, and the role of sex hormones is reduced to the differentiation of testis or ovaries.
This research was supported by a grant 17-04-01321A Russian Foundation for Basic Research (RFBR). The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be considered as a potential conflict of interest.
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