12.2: Sex determination and sex ratios

Sex determination

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes.

In some species there are hermaphrodites (Rosenfield, 2018). There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization.

In many species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is generally triggered by a main gene (a "sex locus"), with a multitude of other genes following in a domino effect.

In other cases, sex of a fetus is determined by environmental variables (such as temperature). The details of some sex-determination systems are not yet fully understood. 

Some species such as various plants and fish do not have a fixed sex, and instead go through life cycles and change sex based on genetic cues during corresponding life stages of their type. This could be due to environmental factors such as seasons, temperature, or even social context. In some gonochoric species, a few individuals may have sex characteristics of both sexes, a condition called intersex (Minelli & Fusco, 2019).

Figure \(\PageIndex{1}\): Some chromosomal sex determination systems in animals.

Discovery

Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in 1903 (Nature Education, n.d.; Ogilvie & Choquette, 1981; Smith, 2010).

Figure \(\PageIndex{2}\): Nettie Stevens.

 

Chromosomal systems

XX/XY sex chromosomes

Figure \(\PageIndex{3}\): Drosophila sex-chromosomes.

The XX/XY sex-determination system is the most familiar, as it is found in humans. The XX/XY system is found in most other mammals, as well as some insects. In this system, most females have two of the same kind of sex chromosome (XX), while most males have two distinct sex chromosomes (XY). The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes (autosomes), and are sometimes called allosomes. In some species, such as humans, organisms remain sex indifferent for a period of time after fertilization; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized (Hake, 2008).

X-centered sex determination

Some species, such as fruit flies, use the presence of two X chromosomes to determine femaleness (Penalva & Sánchez, 2003). Species that use the number of Xs to determine sex are nonviable with an extra X chromosome.

XX/X0 sex chromosomes

In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0 denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis) and Sorex araneus, a shrew species. Transcaucasian mole voles (Ellobius lutescens) also have a form of XO determination, in which both sexes lack a second sex chromosome (Chandra, 1999). The mechanism of sex determination is not yet understood (Kuroiwa et al., 2011).

The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite (Majerus, 2003, p. 60). Its main sex gene is XOL, which encodes XOL-1 and also controls the expression of the genes TRA-2 and HER-1. These genes reduce male gene activation and increase it, respectively (Kuwabara et al., 1992).

Figure \(\PageIndex{4}\):  Heredity of sex chromosomes in XO sex determination.

ZW/ZZ sex chromosomes

The ZW sex-determination system is found in birds, some reptiles, and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on the expression of DMRT1 (Smith et al., 2009). In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY (Hake, 2008). However, not all species depend upon the W for their sex. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW (Majerus, 2003, p. 60). Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of Lepidoptera, the males produce double the normal amount of enzymes, due to having two Z's (Majerus, 2003, p. 60). Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex (Majerus, 2003, p. 60). However, reportedly, the silkworm Bombyx mori uses a single female-specific piRNA as the primary determiner of sex (Kiuchi et al., 2014). Despite the similarities between the ZW and XY systems, these sex chromosomes evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9 (Stiglec et al., 2007). The chicken's Z chromosome also seems to be related to the X chromosome of the platypus (Grützner, 2004). When a ZW species, such as the Komodo dragon, reproduces parthenogenetically, usually only males are produced. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term (BBC News, 2006).

In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex (Annenberg Media, 2004).

Haplodiploidy

Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Sex determination is controlled by the zygosity of a complementary sex determiner (csd) locus. Unfertilized eggs develop into haploid individuals which have a single, hemizygous copy of the csd locus and are therefore males. Fertilized eggs develop into diploid individuals which, due to high variability in the csd locus, are generally heterozygous females. In rare instances diploid individuals may be homozygous, these develop into sterile males. The gene acting as a csd locus has been identified in the honeybee and several candidate genes have been proposed as a csd locus for other Hymenopterans (Beye et al., 2003; Privman, 2013; Miyakawa, 2018). Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their spermatheca and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony (Van Wilgenburg, 2006).

Figure \(\PageIndex{5}\): Haplodiploid sex chromosomes.

Environmental systems

Temperature-dependent

Many other sex-determination systems exist. In some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated during a temperature-sensitive period. There are no examples of temperature-dependent sex determination (TSD) in birds. Megapodes had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex (Göth & Booth, 2005). For some species with TSD, sex determination is achieved by exposure to hotter temperatures resulting in the offspring being one sex and cooler temperatures resulting in the other. This type of TSD is called Pattern I. For others species using TSD, it is exposure to temperatures on both extremes that results in offspring of one sex, and exposure to moderate temperatures that results in offspring of the opposite sex, called Pattern II TSD. The specific temperatures required to produce each sex are known as the female-promoting temperature and the male-promoting temperature (Torres Maldonado et al, 2002). When the temperature stays near the threshold during the temperature sensitive period, the sex ratio is varied between the two sexes (Bull, 1980). Some species' temperature standards are based on when a particular enzyme is created. These species that rely upon temperature for their sex determination do not have the SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature (Torres Maldonado et al., 2002). The sex of some species, such as the Nile tilapia, Australian skink lizard, and Australian dragon lizard, is initially determined by chromosomes, but can later be changed by the temperature of incubation (Schartl, 2004).

It is unknown how exactly temperature-dependent sex determination evolved (Valenzuela & Janzen, 2001). It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season (Valenzuela & Janzen, 2001). Environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent amniote was the common ancestor of amniotes with sex chromosomes (Janzen & Phillips, 2006).

Figure \(\PageIndex{6}\): All alligators determine the sex of their offspring by the temperature of the nest.

Other systems

There are other environmental sex determination systems including location-dependent determination systems as seen in the marine worm Bonellia viridis – larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin (Gilbert, 2006). Some species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses (Thalassoma bifasciatum) are the reverse. Some species, however, have no sex-determination system. Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and Komodo dragon that can reproduce both sexually and asexually, depending on whether a mate is available (Watts et al., 2006).

Other unusual systems include those of the green swordtail (a polyfactorial system with the sex-determining genes on several chromosomes); the juvenile hermaphroditism of zebrafish, with an unknown trigger; and the platyfish, which has W, X, and Y chromosomes. This allows WY, WX, or XX females and YY or XY males (Schartl, 2004).

 

Sex ratios

The sex ratio is the ratio of males to females in a population. In most sexually reproducing species, the ratio tends to be 1:1. This tendency is explained by Fisher's principle (Hamilton, 1967). For various reasons, however, many species deviate from anything like an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps such as Polistes fuscatus and Polistes exclamans, bees, ants, and termites (Kobayashi et al., 2013). In most species, the sex ratio varies according to the age profile of the population (Coney & Mackey, 1998).

Fisher's principle

Fisher’s principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher’s argument in his 1967 paper on “Extraordinary sex ratios” as follows, given the assumption of equal parental expenditure on offspring of both sexes (Hamilton, 1967).

1. Suppose male births are less common than female.
2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
4. Therefore the genes for male-producing tendencies spread, and male births become more common.
5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS) (Smith & Price, 1973). This ratio has been observed in many species, including the bee Macrotera portalis. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio (Danforth, 1991).

 

References

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Contributors and Attributions

Modified by Dan Wetzel from Wikipedia: https://en.wikipedia.org/wiki/Sex-determination_system, https://en.wikipedia.org/wiki/Sex_ratio