Sex Determination

Abstract

A wide array of sex determination mechanisms, encompassing genetic and non-genetic pathways (i.e., hormonal, environmental, and epigenetic factors), have been found among different organisms. The presence of two complementary sexes, male and female, is an ancient feature in biology. Triggering the differentiation of male and female reproductive organs is a conserved ontogenic process, and sex determination is an inherently fascinating process. Sex determination is dependent on molecular signaling whether the male and the female differentiating pathway is activated, and different triggering elements such as genetic, non-genetic, and epigenetic factors control the whole process. This chapter describes various aspects of sex determination, such as historical development, the evolution of sex chromosomes, and different sex determination systems in other organisms.

1. Introduction

Sexual reproduction is a historical process of life on earth, and the most popular heterogametic system (X and Y sex chromosomes) in humans and many other organisms leaves an imprint that sex determination mechanism is ancient and conserved [1]. Sex determination is inherently an integral part of reproduction that separates reproductive organs responsible for male and female gamete production [2, 3]. It is an intricate developmental process that describes whether the individual will be developed as male or female. At the same time, sexual differentiation is the subsequent development of phenotypic differences (primary and secondary sexual characters) between male and female individuals from an undifferentiated zygote [2]. Sex differentiation stages are decided by the sex determination, that is, the gender-specific response of different tissues to hormones produced by the gonads (male or female reproductive organs) distinctly in both genders [4, 5].

Various pathways decide males and females, and these pathways have been evolved rapidly in many species or genera/taxa. Sex determination is regulated by several different genetic (i.e., sex chromosomes) and non-genetic pathways (i.e., hormonal, environmental, and epigenetic factors). In different animal and plant species, genetic systems have been classified as homogametic or heterogametic sex types [2]. In most species, heteromorphic sex chromosomes are present, which are the results of evolutionary changes in size or shape of the sex chromosomes. Similarly, non-genetic pathways also play a key role in determining the sex in fern species (hormonal regulation) and crocodiles, alligators, and turtles (thermo-regulation).

Sex determination is an important evolutionary process as it encourages the genetic fitness of an individual. The ultimate aim of sex determination is to promote the heterozygosity or accumulation of diverse alleles in a species, which is vital for creating genetic variation in living organisms. It is pivotal in plant breeding to design a specific breeding program as per the need and demands of diverse stakeholders to improve plant productivity and nutritional quality. The exact mechanism or genes determining the sex or reproductive organ is unclear. Therefore, it is an important area of study in developmental and evolutionary biology, as well as in ecology. Sex determination in various plant and animal species is not under the control of the universal model. Thus, this chapter provides a brief overview of the different mechanisms of sex determination in plant and animal species.

2. Historical development

Whether a plant or animal will become a male, a female, or bisexual is determined during the initial development of an organism. Hundreds of years ago, researchers have started studying the mechanism of sex determination. For example, in 335 B.C.E., Aristotle anticipated that sex is controlled or ruled by the heat of the male partner during intercourse. A male child will be born when the male parent’s heat overwhelms the female parent’s coldness and vice versa. Environmental theories of sex determination (i.e., Aristotle’s theory, in reptiles, temperature during embryo development regulates the gender) were popular until about 1900 and meticulous scientific research began after the discovery of sex chromosomes during 1900, and gradual scientific improvements followed during the next century. In 1891, Hermann Henking [6], a German Biologist studying spermatogenesis in the insect firebug (Pyrrhocoris apterus; 2n = 24), detected that, as a result of meiosis, half of the spermatozoa have not received all 12 chromosomes and ended with 11 chromosomes only. It means one of the chromosomes was not involved in meiosis. This chromosome seemed and behaved differently from others; he was not able to speculate the significance of this element and named it “X element or X body”. In 1902, Clarence Erwin McClung [7] cleared the Henking assumption through cytological observations on several grasshopper species and demonstrated that the somatic cells in female grasshoppers are different in chromosome number than do corresponding cells in the male, which he referred to as “accessory or supernumerary chromosome” and demonstrated their association with sex determination [8]. Later on, American geneticist Edmund Beecher Wilson (1905) observed differences, either in the presence or in the absence of one chromosome or in the size of one chromosome pair in germ cells of both the sexes of protenor species [9]. Another American geneticist Nettie Maria Stevens (1905) studied germ cells of both sexes of mealworm beetle (Tenebrio molitor) and found that in males, one chromosome was smaller than the other chromosome, and she confirmed that this chromosome must be regulating the sex in males [8]. Later on, this smaller chromosome was named as “Y chromosome” and the larger one as “X chromosome” by Stevens (1905). After Stevens died in 1912, Wilson was the first to designate the name “sex chromosome” for the pair of XX and XY chromosomes. Hermann Joseph Muller (1914), an American geneticist, speculated that differentiation of sex chromosomes would arise from lack of recombination due to the appearance of sex-determining genes on the Y or W chromosomes [10]. By the end of the 1950s, the male-determining function was established on the small arm of the Y chromosome and was named “testis determining factor” (TDF in humans and Tdf in mice). Further, Ohno (1967) proposed the concept of ancestral sex chromosomes and their progress to evolve modern-day sex chromosomes by degeneration of the Y or W chromosomes [11]. As science progressed, the major breakthrough in sex determination was achieved through sequencing and transgenic approaches. Gene-sequencing approaches revealed an open-reading frame (ORF) coding a single exon gene in a male mouse (XY) and named it as a sex-determining region of the Y chromosome (SRY in humans and Sry in mice). Conclusive evidence of functionality of SRY gene was developed through transgenic approach by generating sex-reversed mouse (transgenic XX mouse having Sry gene).

3. Evolutionary differentiation of sex chromosomes

Sex chromosome evolution is linked with dosage compensation of sex-linked genes [11]. For example, human sex chromosomes evolved around 300 million years ago. The Y chromosome underwent inversions that inhibited large regions from recombination between homologous regions of X and Y chromosomes. This leads to the gradual spread of regions with reduced recombination. Sequence-based analysis shows the six evolutionary strata on the X chromosome, and each gene on it diverged from their Y paralogs for the same length of time. Same evolutionary strata were also found in other mammals and even in birds. It is evident that Y chromosome in mammals and W chromosome in birds are poor in gene richness, and also have lost several functional genes. The human X chromosome maintains 98% of genes, while Y chromosome retains only 3% of the genes located on the proto-sex chromosomes. The human Y chromosome is rich in palindromic duplicated sequences that help in the retention of specific Y-linked genes, which are essential for male fertility. These sequences also endorse deletions in chromosomes and tend to male sterility due to functional gene loss. Thus, these sequences maintain the integrity of Y chromosomes. H.J. Muller [10] suggested the origin of sex chromosomes from a pair of autosomes (Figure 1).

Figure 1.

Evolutionary differentiation of X and Y sex chromosomes from ancient autosomes [12]. There was end-to-end pairing between ancient X and Y chromosomes. During evolution sex-determining locus such as TDF (testis-determining factor) accumulated in one chromosome of pair (step-1). Further, there was accumulation of male-specific gene/s (step-2), which was responsible for chromosomal recombination repression (step-3) and led to development of male-specific region (MSY) on Y chromosome. Mutations and deletions (step-4) in the non-recombining region rapidly degraded the sex-specific chromosomes. The pseudoautosomal region (PAR) is present on both X and Y chromosomes in small portion, which helps in partial chromosomal pairing between X and Y chromosomes at anaphase I (modified from Graves, 2006 [5]).

4. Sexual differentiation in animals and plants

In animals, primary sex characters are associated with male and female gametes producing organs such as gonads, and their development depends on the genes of their zygotes. Secondary sex characters are associated with different attributes, which differentiate males and females, such as the development of mammary gland, genital duct, pitch of voice. The development of these traits is mainly due to hormones produced by diploid gonads.

In most plant species, both male and female reproductive organs are present in same flower (i.e., hermaphrodite or bisexual plants) or in different flowers of same plant (monoecious plants), and in some cases such as papaya, date palm, spinach, asparagus, male and female sex organs are present on flower of different plants (i.e., dioecious or unisexual plants). The monoecious plants produce either staminate (male)- or pistillate (female)-type flower in the same plant such as maize [13]. However, there are different kinds of flower combinations in monoecious plants such as andromonoecious (many Umbelliferae)—has staminate and hermaphrodite flowers, gynomonoecious (Atriplex and many Compositae)—has pistilate and hermaphrodite flowers, and androgynomonoecious or trimonoecious (Acer campestre)—has staminate, pistillate, and hermaphrodite flowers [13, 14].

5. Sex determination systems

Conventionally sex determination systems are classified based on the mechanism or causative factors involved in the specification of individual sex. Broadly, it has been classified into four categories:

• Genetic sex determination (GSD)—when sex is determined early in the development by genetic factors (sex chromosomes, genes or alleles).

• Environmental sex determination (ESD)—when the sex of an individual is influenced by environmental parameters such as temperature, photoperiod, nutrition.

• Maternal sex determination (MSD)—when the sex of an offspring is determined by genotype or physiological condition of the mother.

• Mixed sex determination—when both genetic and environmental factors determined the sex of an individual.

• Further, each sex determination system has been classified into different categories as depicted in Figure 2 and detailed elaboration is given below.

Figure 2.

General classification of sex determination system.

5.1 Genetic sex determination (GSD)

Genetic sex determination system is also recognized as genotypic sex determination, and the development of an individual as male or female is triggered by the presence or absence of one or more genes or chromosomal segment or the entire chromosomal complement. These gene/s or chromosome/s is responsible for the primary and secondary sexual characters associated with each sex. Indeed, genes responsible for the development of male or female sex are located on a single pair of homologous chromosomes (sex chromosomes). In both sexes, they occur distinctly and are characterized by specific genes or by a different allelic constitution at homologous loci. The evolution of separate sexes (male and female) is the result of the evolution of anisogamy that is, sexual reproduction by the fusion of dissimilar gametes. The hermaphroditism/gynandromorphism (male or female sex organs within an individual) is common phenomenon in most of flowering plants (more than 90%), whereas it is a very rare phenomenon in animals (in some individuals of Drosophila). Separate sexes have evolved individually in both plants and animals, which suggests that there must be an evolutionary penalty for hermaphroditism. In animals, genetic sex determination is very well established in most of the species and this system is very well studied in Drosophila melanogaster flies, Caenorhabditis elegans nematodes, and humans. Commonalities among these bring us to a general impression of genetic regulations of sex determination and conservation of sex determination mechanism.

Sex determination mechanisms are very complex and evolved with a remarkably diverse array among plant species than among animals. In plants, a number of forms are present in functional hermaphroditism in flowering plants, varying from “perfect flower- male and female reproductive organ in each flower” to “monoecy-separate sex flowers on same individuals.” Many other forms are like gynomonoecious (both female and hermaphrodite flowers), andromonoecious (both male and hermaphrodite flowers), dioecious (separate sex individuals), gynodioecious (either female or hermaphrodite), and androdioecious (either male or hermaphrodite). In plants, rapid progress is achieved in learning genetics and molecular mechanism of sex determination by comparing the monoecy and dioecy.

5.1.1 Chromosomal sex determination system

In chromosomal sex determination systems, male and female individuals differ from each other by either in morphology or in a number of one pair of chromosomes these are known as sex chromosomes or allosomes or heterochromosomes, which are dissimilar to the normal chromosomes (autosomes). On the basis of structure, there are two types of sex chromosomes such as: i) homomorphic—both X and Y chromosomes are structurally similar and ii) heteromorphic—both X and Y chromosomes are distinct morphologically. In diploid species, where male or female individual produces different types of gametes is known as heterogametic sex (Table 1), whereas individual producing similar kind of gametes is known as homogametic sex.

S.N.	Chromosomal mechanism	Male	Female	Example (Animals)	Example (Plants)
1.	XX (female) and XY (male)	Heterogametic (XY)	Homogametic (XX)	Humans, mice, Diptera, Hemiptera, Coleoptera, most common in animals	Asparagus, Spinach, Hemp, White Campion, Sorrel, Humulus
2.	XX (male) and XY (female) or ZZ and ZY system	Homogametic (XX)	Heterogametic (XY)	Birds, silkworm	Maidenhair tree, California poplar, Wild strawberry
3.	XX (female) and X0 (male)	Heterogametic (X0)	Homogametic (XX)	Grasshopper, protenor, Orthopteran insects	—
4.	X0 (female) and XX (male)	Homogametic (XX)	Heterogametic (X0)	Insects such as Fumea	—

Table 1.

Different mechanisms of the chromosomal sex determination in animals and plants.

There are different chromosomal mechanisms for sex determination and illustrated below:

Chromosomal sex determination is widespread, but not ubiquitous, in the animal kingdom. Autosomes are present in two copies in diploid organisms, three copies in triploids, and so on. Generally, males are XY and females are XX in most mammalian species. In XY system, X chromosome is large and gene-rich, while Y chromosome is small and heterochromatic, that is, almost devoid of genes. Generally, animal cells comprise two types of sex chromosomes, that is, X chromosomes present in both male and female, while Y chromosomes present in male only. Homogametic parent produces one type of gametes, while heterogametic two different types of gametes (Figure 3a and b).

Figure 3.

a and b. Homogametic and heterogametic nature of males in birds and humans, respectively. Homogametic male (XX) produces one kind of haploid gametes only, that is, X type, while heterogametic male produces two different types of gametes, that is, X and Y type during gametogenesis. In both case, male and female progenies are produced in equal proportion.

In land plant species, heteromorphic sex chromosomes are found in most of the species and homomorphic sex chromosomes are restricted only to the gymnosperm and angiosperm. In asparagus, papaya, and spinach, the X and Y chromosomes are homomorphic but functionally distinct. Asparagus also shows distinct YY male, which is unique in its type. In the case of spinach, females are homogametic (XX) and males are heterogametic (XY) as mammals. The Y chromosome consists of genes that are responsible for the suppression of carpel development and for activation of stamen development. In Ginkgo biloba (Maidenhair tree) and Populus trichocarpa (California poplar), Fragraria elateria (wild strawberry) female is heterogametic, while male is homogametic.

The heteromorphic sex chromosomes system is present in Cannabis sativa (Hemp), Silene latifolia L. (white Campion or liverwort), Rumex acetosa (Sorrel), Humulus spp., etc. The X and Y chromosomes are morphologically and functionally dissimilar and show a lack of complete pairing with each other during meiosis. In the above species, females are homogametic (XX) and males are heterogametic (XY). Some of the organisms have multiple heterochromosomes either in one or in both sex. By analogy (partially or non-homologous) in homogametic and heterogametic (XX and XY or ZZ and ZW) conditions, the gender is defined. Multiple heterochromosomes are the result of mutation (translocations) and these chromosomes are derived from the existing heterochromosomes systems (XX or ZW). Female (XX) and male (XY₁Y₂) are found in Humulus japonicas and R. acetosa etc., whereas female produces one type of gamete (X) and male produces two types of gametes (X and Y₁Y₂). Mating between egg cell (X) and male gamete (X) tend to produce diploid female (XX), while a fusion of egg cell (X) and male gamete (Y₁Y₂) tends to produce male (XY₁Y₂). Some strains of Humulus lupulus show homogametic female (X₁X₁X₂X₂)—produces one type of gamete (X₁X₂), and heterogametic male (X₁X₂Y₁Y₂)—producing two different type of gametes (X₁X₂ and Y₁Y₂). These chromosomes are generally present in orthopteran, crustacean, coleopterans, and mammals.

5.1.2 Sex determination due to active Y chromosome in plants

In contrast to the animal Y chromosome, plant Y chromosome is large and contains a high proportion of genomic DNA. In Rumex acctosa, the Y chromosome is rich in condensed heterochromatin, while it is rich in euchromatin in S. latifolia. In case of S. latifolia, X and Y chromosomes contain sex-determining genes; however, autosomal genes also play a significant role in sex determination. The Y chromosome can be divided into the four major functional fragments and they rule the sex differentiation (Figure 4) such as i) female suppressor region—contains genes of female suppression and positioned at one end of chromosome; ii) male promoter region—contains genes for promotion of maleness (i.e., development of stamen); iii) male fertility region—contains genes for initiation of male fertility and anther maturation; iv) pairing region—helps in chromosomal pairing with one end of X chromosomes. Thus, normal disjunction of X chromosomes and Y chromosomes occurs during anaphase I. Mutation in first region leads to production of both male and female flowers on same plant. Mutation in second region only leads to the development of asexual flower. Mutation in third region only tends to develop male sterile XY plant. Mutation in fourth region only leads to chromosomal anomalies during meiotic cell division. The X chromosome consists of two functional regions. First region covers major portion of X chromosome and is nonhomologous to the Y chromosome. It consists of genes that are responsible for the development of pistillate flower. However, function of this segment is suppressed by first fragment of Y chromosome. Sometime, one Y chromosome can inhibit the effect of four X chromosomes and produce male flower in XY plants. In second region, small end portion of X chromosome helps to pair with Y chromosome due to their homology. Thus, two genes are essential for sex determination in plants—one gene for suppression of carpel development and other gene for the development of stamen. However, mammalian cell carries single gene (SRY), which controls sex determination.

Figure 4.

Structure of X and Y chromosomes in plants. The length and genome content is higher in X chromosomes than Y chromosomes. The X and Y chromosomes partially pair with IV fragment of X chromosome and II fragment of Y chromosome, and helps to regular normal segregation of sex chromosomes at anaphase I during gametogenesis.

There is an abundant diversity in chromosomal sex determination systems; however, there are some different chromosomal sex determination systems which fall under the category of “Miscellaneous or Other category.” The UV system is also a part of this category. UV system chromosomal sex determination is determined at haploid phase of the life cycle. Females and males are haploid and characterized by the possession of a sex chromosomes U and V, respectively. This system is generally present in organisms with haplontic and haplodiplontic system (some algae and bryophytes) having anisogamous and heterosporous condition. In UV system, sex is determined during meiosis not at the time of fertilization. In case of fungus gnat Sciara, all zygotes have similar genotypes (XXXAA) and the loss in one or more paternal chromosomes will determine that the zygote will develop into a female (XXAA) or male (XAA).

The Y chromosome in XY system and W chromosome in ZW system may have gone through the degeneration process and lost some of the original genes that are present in the another sexual chromosome (X or Z). Therefore, in homogametic sex (XX or ZZ), some genes are present in double copy (as like the autosomes), whereas in heterogametic sex (XY or ZW) they are in single copy. Genetic imbalance affects all the genes on sex chromosome in XO and ZO system. The genes not involved in sexual differentiation require identical level of expression in the two sexes. Dosage compensation is the phenomena, which balances the level of expression of genes in both the genders [15]. Dosage compensation phenomenon is very well understood in the Drosophila, Caenorhabditis, birds, and lepidopterans. In multiple heterochromosomes, the dosage compensation system becomes more complex, for example, platypus and birds.

5.1.3 Genic sex determination system

Sex determination is governed by separate genes or alleles present on specific locus of the chromosomes of both males and females [16]. In this system, sex determination is in control of distinct alleles rather than sex chromosomes; therefore, such a system may also refer to as a multiple allele sex determination system. In case of polygenic sex determination system, a set of the factors (genes) distributed on several chromosomes were involved and have masculinizing or feminism effects and collectively, they govern one sex or other.

Genic balance theory (GBT) was given by Calvin Blackman Bridges (1921) [17] for sex determination in Drosophila melanogaster (2n = 2x = 8). In Drosophila, instead of XY sex chromosome, sex is determined by the genic balance or sex index ratio between X-chromosomes and autosome genomes (sets).

$$\mathit{Sex}\mathrm{index}\mathrm{ratio}=\frac{\mathit{No}.\mathrm{of}\mathrm{X}\mathrm{chromosmes}}{\mathit{No}.\mathrm{of}\mathrm{autosomal}\mathrm{sets}}=\frac{\mathrm{X}}{\mathrm{A}}$$E1

In Drosophila, Y chromosome is heterochromatic. Thus, it is not active in sex determination (Table 2). However, gene for male fertility is located on Y chromosome and Y chromosome also plays a major role in spermatogenesis and development of male reproductive organ. Hence, Y chromosome is essential for restoring male fertility. The gene of femaleness is located on X-chromosome and gene associated with maleness is located on autosomes. It is also applicable to some other animal species such as nematodes (Caenorhabditis elegans).

Sex index ratio	Sex type	Fertility status	Examples
X/A = <0.5	Super male or meta-male	Sterile male	3A + X0; 3A + XY
X/A = 0.5	Male	Fertile (Y chromosome present) or sterile male (Y chromosome absent)	2A + XY (fertile); 2A + X0 (sterile); 4A + XX (sterile); 4A + XXY (fertile)
X/A= >0.5 and < 1.0	Intersex	Sterile	3A + XX; 3A + XXY; 4A + XXX
X/A = 1.0	Female	Fertile female	2A + XX; 3A + XXX; 2A + XXY; 3A + XXXY
X/A= >1.0	Super female or meta-female	Sterile female	3A + XXXX; 2A + XXX

Table 2.

Sex index ratio of genic balance mechanism in Drosophila [Bridges, 1921].

The sterile meta-females and meta-males have been entitled as glamour girls and boys of fly world by Dodson.

5.1.4 Male haploidy or haplodiploidy sex determination system

Haplodiploidy is most commonly used in insects of Order-Hymenoptera (honey bees, ants, and wasps) and Thysanoptera (thrips) for sex determination. Sex determination takes place by sets of chromosomes of an individual receives [18]. Two sets of chromosomes (diploid) tend to female and one set (haploid) tends to male sex formation [18, 19]. For example, in honey bee male individual (i.e., drone) formed from unfertilized egg cells (i.e., haploid). Thus, male develops from the process of parthenogenesis and called as arrhenotoky (where haploid egg cell develops males rather than females through parthenogenesis). However, female (queen and worker bees) develops from diploid egg cells (i.e., fertilized egg cell). Thus, male has half number of chromosomes than female and is haploid. The male (drone) is solely derived from queen and in some cases from worker honey bees. The chromosomes number in diploid queen is 32, while 16 chromosomes in haploid drones. Drone produces sperm cells that consist of whole genome and sperm cells are genetically identical. Thus, the genetic makeup of female workers is derived half from mother and other half from father, while genetic makeup of drone is solely derived from mother. Byes and coworkers [18] cloned complementary sex-determining (cds) locus in the Apis meliifera and proved that this gene is responsible for sex determination cascade of honeybees. Interestingly, firstly, in haplodiploidy system male has no father and cannot have son but it has grandfather and can have grandson. If there is only one queen in a hive, then the relatedness between workers will be ¾ rather than ½, which is common between siblings in other sex determination systems. Thus, it shows more eusocial behavior of honey bees. Secondly, there will be rapid elimination of recessive lethal and deleterious alleles from the population due to haploid genomic nature of males, while dominant lethal and deleterious alleles will be removed every time of their occurrence because of their phenotypic expression in each stage (Figure 5).

Figure 5.

Haplodiploidy system of sex determination in honey bees. Drones are haploid and produce genetically similar sperm cells by mitosis, while queen is diploid and produces egg cells that are genetically dissimilar and generated through meiotic cell division. Formation of queen or worker will be controlled by the feed stuff to the developing zygote after fertilization between male sperm cell and female egg cell. Drones are formed through parthenogenesis.

5.1.5 Single gene sex determination

There are evidences where single autosomal genes affect the sex type in animals. For example in Drosophila, one autosomal recessive gene-transformer (tra) affects the pattern of sex. If this is present in homozygous recessive state in XX zygotes, then it convert females into males but sterile. However, tra gene does not affect in male (XY) or when it is present in heterozygous state (Tra/tra) in female, when a female Drosophila having heterozygous tra gene (XX Tra tra) was mated with male having homozygous tra genes (XY tra tra). In F₁ generation, 1/4 progeny will be normal female (XX Tra tra), while 3/4 progeny will be male. Among male progenies, 1/3 progenies comprises XX chromosomes but found to be sterile male due to recessive homozygous tra genes. Another example is human, where recessive autosomal gene—testicular feminization—induces breast and vagina in males (XY). These male individuals also have rudimentary testis and are sterile. Single gene sex determination also occurs in dioecious plant species. For example, in papaya (Figure 6), sex determination occurs due to single gene with three alleles (m, M1 and M2).

Figure 6.

Determination of sex in dioecious papaya through single gene. Genetic constitution of female, male, and hermaphrodite plants is mm, M₁m, and M₂m, respectively. Crossing between female and male plants tends to produce 50% female (mm) and 50% male (M₁m) progenies. Crossing between female and hermaphrodite plants tends to produce 50% female (mm) and 50% hermaphrodite (M₂m) progenies. Selfing in hermaphrodite tends to produce 2/3 hermaphrodite and 1/3 female progenies, while 1/4 progenies will be nonviable due to expression of lethal genes (M₂M₂).

5.2 Environmental sex determination (ESD)

In many species, sex of an individual is governed by the environmental circumstances on a zygote of unstated sex and sex is determined by the effect of environmental factors on embryonic and post-embryonic developmental stages. The ESD generally occurs in unicellular eukaryotes and among multicellular organisms, it is found mainly in non-avian reptiles, amphibians, and some fishes. Among the different environmental factors, temperature plays a key role on sex determination. However, other environmental factors such as social environment, nutrition, and pH also play decisive role in sex determination. These ESD systems are more often labile than the genetic sex determination system; evolutionary drivers can force to shift ESD to the GSD system and changes are due to variation in the threshold temperature and nutrition, etc. Mainly, sexual liability is encountered in lizards (Bassiana duperreyi by temperature) and ferns (gametophtyic age). Based on different environmental factors, ESD mechanisms are classified, as mentioned below:

5.2.1 Temperature-dependent sex determination

Sex is irreversibly determined by the incubation temperature during embryogenesis. Temperature affects the sex in most of the species of the turtle, crocodiles, lizards, and snakes. Based on incubation temperature for eggs, there are following three different reactions that may occur (Table 3).

High temperature	Low temperature	Intermediate	Examples	Temperature range
Male	Female	Both male and female in variable proportion	Crocodiles, alligator, and lizards	High—30–35°C; Low—23–28°C
Female	Male	Both male and female in variable proportion	Most species of turtles	High—30–35°C; Low—23–28°C
Female	Female	Males	Chleydra serpentine (turtle spp.) and few crocodile spp.	High—30–35°C; Low—23–28°C
Female	Female	Both male and female in variable proportion	Australian crocodile	High—>30°C; Low—< 25°C

Table 3.

Sex determination reactions based on incubation temperature for eggs.

5.2.2 Size of egg or body size

Egg size decides sex differentiation in many species like sea worm (Dinophilus). Big egg size tends to develop female, while small egg size tends to produce male progeny. In many plants of the genus Arisaema (Araceae), the sex depends on the body size of plants (small plants only bear male flowers, and large plants only female flowers, while intermediate ones will have both male and female flowers).

5.2.3 Interaction with conspecifics or social sex determination

The fate of an individual as a male or female will be decided based on spatial proximity of an individual relative to other members of its own species or the interaction with other conspecifics. Chemical (pheromones) or other communication channels (tactile or visual) stimulates the developmental response for one sex or the other. Bonellia viridis (marine annelid) will develop as a female if its larvae settle on a sea floor area in isolation from other individuals. In contrast, if larva attached to proboscis of an adult female, it starts to progress into a male through the effect of male pheromones released by female. In case of many sequential hermaphrodite fishes, they start their life as one sex and later on converted to another sex based on social interactions such as anemone fish (the largest male in the group become the dominant female after the death of dominant female) and homosporous fern (Ceratopteris richardii).

5.2.4 Photoperiod, nutrition, parasitism, water pH, and social interaction

Under the ESD system, photoperiod is also a sex-decisive factors and in case of brackish water amphipod (Gammarus duebeni), the sex ratio varies according to photoperiod exposure during the post-hatching. During elongated dark period, the proportions of male individuals will be higher in comparison with the female. Nutrition, parasitism and water pH also determine the sex in few species. Nutritional control of sex determination occurs in calanoid copepods and mermithid nematodes, whereas parasitism plays as decisive role in sex determination of some of the isopods, coenopods, and copepods. In the South American cichlid fishes and some poecilids, water acidity has an effect on sex determination and individuals growing in acid waters will be predominantly males and those develop in neutral or slightly basic water will be females. Many fishes are sequential hermaphrodites, where they start their life as one sex, but change sex later in development. In the anemone fish (Amphiprion akallopisos), which lives in social groups with one dominant breeding pair as well as several subordinate males, sex change occurs when the dominant female dies and the largest male in the group becomes the dominant female.

Environmental factors (such as light, temperature, humidity, day length, GA₃, and ethylene) have effect on limited plant species in their sex determination. For example, plant-equisetum develops as female under normal environmental condition, while as male under stress condition. In cucurbits such as melons and cucumbers, sex is also affected by the application of growth hormones such as GA₃ and ethylene induce femaleness. Thus, environmental factor has more impact on males rather than female.

5.3 Maternal and cytoplasmic sex determination system

In this category, the sex of the progeny depends on the mother of the individual and their interaction effect with genetic and environmental factors. Maternal sex determination occurs in two different forms—one where sex is established by its mother rather than the individual’s genotype, whereas in second form, physiological conditions of the mother and their specific signals determine that an offspring will be male or female. In the dipterans insects such as Chrysomya albiceps and Calliphora rufifacies, two forms of female are present, one is producing only male offspring known as androgenic females (ff) and the other is exclusively producing female offspring known as gynogenic females (Ff). In cecidomyid midge, the sex is governed by the nutritional condition of the mother and in response to the nutritional conditions, female brain secretes a factor and it reaches to ovaries to determine the gender of an individual.

In genetic sex determination (GSD), sex-determining factors are typical nuclear genes and show the Mandelian inheritance. On the contrary, some of the sex-determining causes are inherited by the cytoplasm and transmitted only from mother to daughter and not by the males. Sex ratio distortion toward the female sex has been observed in members of crustaceans group and is unique in this group [20].

5.4 Mixed sex determination system

When the sex of an individual is ruled by the combined effect of genetic and environmental factors at various degrees, in case of American salamander (Pleurodeles) and fish (Menidia menidia), sex is determined by the joint effect of both the sex karyotype (ZW and ZZ) and incubation temperature of eggs.

6. Conclusion

The several mechanisms of sex determination reveal the diverse pathways governing sex determination in both plants and animals and these pathways are also very well understood in various model organisms. The highly evolved system of sex determination is heterogametic sex determination in animals, that is, XX/XY. Even though there are several unsolved mysteries related to the sex determination system such as why heterogamety is more common in male then female? Why degeneration of sex chromosomes occurs only in few organisms not in all? With the progress in molecular techniques over the past decades, several puzzles were solved like discovery of Sry and a ray of hope arises to learn more about molecular basis of sex determination, evolution of sex chromosome, mapping of gene, sequencing, gender-dependent expression of sex-regulating gene, and relationship between the evolution of genetic degeneration and dosage compensation. Multiple “-omics” data and integrative approaches will allow scientists to address the unresolved questions and finding the new sex-determining genes as well as genetic networks involved in sex determination.