Sex Determination in Animals - Essay Example

Sex Determination in Animals Name Institutional Affiliation Date Sex Determination in Animals Introduction One of the ways to observe differences in species in the Animal Kingdom is by looking at the sexes. Determination of the sex of an animal is usually localized on a specific chromosome or conditions that affect the developmental pattern of the organism in regards to development of sexual organs or glands (Haag & Doty, 2005). Sex is determined by various mechanisms that can be genotypic-based, environmental-based, or other rare methods of sex-determination. Sexual determination is a vital mechanism in reproductive biology as it helps to tell whether an animal is male, female or hermaphrodite in early stages of development (Hake & O’Connor, 2008). This essay is about various sex determination systems in animals including the XY, ZW and other types of chromosomal sex determination systems as well as temperature-dependent sex determination and other rare sex determination systems such as the sex change, or sex reversal. XY chromosomal sex determination Hake & O’Connor (2008) confirm that XY chromosomal sex determination system is found in humans and is occurs with females having two similar sex chromosomes (XX) while the males having two distinct sex chromosomes (XY). Whereas somatic chromosomes are found in equal numbers in both females and males, the sex chromosomes are unequally represented depending whether the animal is male or female (Hake & O’Connor, 2008). The XY chromosomal sex determination system is genotypic-based and sex is established by the presence and number of X and Y chromosomes. The presence of Y chromosome in placental mammals determines sex. Cells from the male parent contains X and Y chromosomes while cells from the female parents contain XX. Thus, the offspring that inherits X genes from the mother and Y gene from the father develops into a male while the one that inherits the X genes from both the mother and father takes the development pathway of a female (Hake & O’Connor, 2008). The basic sex determination mechanism in mammals arises from the gene activator function SRY located on the arm of the Y-chromosome (Manolakou, Lavranos, & Angelopoulou, 2006). In some abnormal cases, the SRY may be present in cells with two X chromosomes causing the person inherent of this to appear male. In other cases, the SRY could be mutated or non-functional in a person with XY genes, causing the individual to appear female (Manolakou, Lavranos, & Angelopoulou, 2006). Normally, the mere presence of a Y chromosome makes a person male, while its absences make a person female. According to Forsyth (1993) the biological law that states that XY results in a male while XX results in a female is true in humans and most other mammals. ZW chromosomal sex determination The ZW sex-determination mechanism is present mostly in birds and also some insects, some reptiles, fishes, and amphibians (Hake & O'Connor, 2008). Whereas in humans the male is heterogametic, that is having two distinct sex chromosomes and the female is homogametic, the opposite is true for birds. The male has two similar sex chromosomes (ZZ) while the female has two distinct ones (ZW). The sex of the offspring is therefore likely to be determined by the female. According to Manolakou, Lavranos, & Angelopoulou (2006) it is still under investigation as to whether sex depends on the Z chromosome dosage. For example, when the gene DMRT1 on the Z chromosome skips dosage compensation and is emphasized specifically in the gonads, thus capable of joining the number of Z gametes with gonad differentiation (Manolakou, Lavranos, & Angelopoulou, 2006). On the other hand, sex determination can be done by the presence of W chromosome which is feminizing, following the example of Y chromosome in the placental mammals. Just like the SRY on the human Y chromosome, the W chromosome in birds contains the genes FT1 and ASW necessary for female development (Hake & O'Connor, 2008). Although the function is unknown, FET1 is expressed in the gonads resulting up to the moment of sexual differentiation, and it is also smaller in size to the Z chromosome as the Y chromosome to the X in humans (Hake & O'Connor, 2008). On the other hand, the Z chromosome bears a lot of similarities with the X chromosome in terms of being larger in size, and containing more genes that determine functionality (Manolakou, Lavranos, & Angelopoulou, 2006). Other types of chromosomal sex determination Other types of chromosomal sex determination include XO and Haplodiploidy sex determination systems. In the XO sex determination system, the female has two copies of the sex chromosome (XX) but the male has only one copy of X, denoted as XO to show absence of a second sex chromosome (Saccone, Pane, & Polito, 2002). This system of sex determination applies to a number of insects including cockroaches, grasshoppers, and crickets. There are also some few mammals that lack the Y chromosome and these include the Tokunoshima spiny rat, Amami spiny rat, and some species of the shrew. C. elegans, a nematode species is male when with one chromosome (XO) and a hermaphrodite when with the pair of chromosomes (XX) (Saccone, Pane, & Polito, 2002). In this method, sex is determined by the amount of genes expressed across the two gametes, as determined by the ratio between chromosomes and autosomal sets number (Saccone, Pane, & Polito, 2002). In the order Lepidoptera (butterflies and moths) Z and W are used to connote sex chromosomes where the female develops from ZO while the male develops from ZZ (Hake & O’Connor, 2008). The haploiddiploid system is found in the insect order Hymenoptera such as bees, wasps, and ants and is where sex is determined on whether or not the eggs are fertilized (Saccone, Pane, & Polito, 2002). In honey bees for example, the drone (male bee) is established from unfertilized haploid eggs. The male honey bee has a single set of 16 chromosomes. In contrast, the female honey bees are generated from fertilized eggs and thus diploid. The females contain two sets of chromosomes hence totalling to 32. Thus, in the case of the honey bees, the female sex is produced by sexual reproduction and hence there are biparental diploid females and uniparental haploid males (Manolakou, Lavranos, & Angelopoulou, 2006). Table 1: Chromosomal sex determination Species Sex chromosomes in: Females Males Placental mammals XX XY Marsupial mammals XX XY Birds ZW ZZ D. melanogaster XX XY C. elegans XX XO (From: Klug & Cummings, 2004, p.75). Figure: 1. Haplodiploid reproduction (From: Manolakou, et al. 2006, p. 59) Temperature-dependent sex determination Environmental factors such as temperature can sometimes play a vital role in sex determination (Janzen & Phillips, 2006).The temperature-dependent sex determination mechanism takes place when the environmental temperature condition determines whether the egg will develop as a female or male during the critical period of embryonic development (Quinn, 2007). It is a common method of sex determination in reptiles such as turtles, alligators, and reptiles. The egg-laying to embryonic development period is thermo-sensitive, and this leaves reptilian sex determination at the mercy of the ambient conditions that impact egg clutches in their nests (Quinn, 2007). For instance, among various turtle species, males hatch from eggs in cooler nests (below 22 degree Celsius) while females hatch from eggs in warmer nests (above 28 degree Celsius) (Janzen & Phillips, 2006). In crocodile species, such as the some types of alligators, both high and low temperatures result in the female sex while intermediate temperatures result in the male sex (Quinn, 2007). It is a widely held perspective that temperature dependent and genotypic sex determination are incompatible mechanisms, mutually exclusive, and that a reptile’s sex can never be under the influence of both environment and genotypic mechanisms at the same time (Quinn, 2007). Thus, there is no genetic predisposition for the temperature-sensitive reptilian embryo to establish as either female or male, until it enters the thermo-sensitive period of the development journey. Nevertheless, some recent studies suggest that in some cases, incubation temperature and sex chromosomes may have a degree of interaction in determining the sex of the reptilian embryo (Quinn, 2007). Animals that change sex In some animals, the sex is not always permanently fixed. There are animals that experience sex change, for example coral reef fish such as clownfish (Holbrook & Schmitt, 2005). Clownfish are all born male. A clownfish harem is led by a large female, a medium-sized reproductive male, and a number of small non-reproductive juvenile males. When the female dies, the reproductive male changes to become the dominant female, and the largest of the juvenile males matures to takes position of a dominant reproductive male. The dominant male usually has functioning testes and some latent cells that can change to ovaries under the appropriate conditions. In the demise of the female, the dominant male’s testes degenerate and ovaries develop from the latent ovarian cells. Thus, clownfish are protandrous hermaphrodites, in that they can switch from male to female. Unlike in placental animals, chromosomal differences do not determine the opposite sexes of clownfish. Both female and male clownfish have the same composition of chromosomes (Holbrook & Schmitt, 2005). It is possible for a juvenile clownfish to switch sexes because it has both male and female immature sexual organs. Hormonal levels of testosterone and estradiol-a derivative of estrogens-, control the expression of particular genes (Holbrook & Schmitt, 2005). Usually, the first sexual change that happens in a juvenile clownfish is that of a male as the testosterone levels are too high causing the male organs to mature. Later, when the female of the harem dies and the position has to be filled, the estradiol level in the reproductive male surpasses the testosterone leading to altered gene expression where female organs develop (Holbrook & Schmitt, 2005). Conclusion The paper has discussed the various systems for sex determination among species including the XY, ZW, and temperature-dependent sex determination. Clearly, evolution has generated several solutions for determining different sexes. Reproduction has remarkable adaptive value to the animal species because it introduces genetic variability to the population in each generation, and this is by having male, female, and even hermaphrodite sexes. Not only chromosomes, but environmental and other rare factors as well play determinative roles in most species’ sex identification. References: Forsyth, A. (1993). A natural history of sex. Victoria: Chapters Publishing, Ltd. Haag, E., & Doty, A. (2005). ‘Sex determination across evolution: Connecting the dots,’ PLoS Biology, 3(1), e21. Hake, L. & O'Connor, C. (2008). ‘Genetic mechanisms of sex determination’, Nature Education 1(1). Holbrook, S. J. & Schmitt, R. J. (2005). ‘Growth, reproduction and survival of a tropical sea anemone (Actiniaria): Benefits of hosting anemonefish,’ Coral Reefs, 24(1), 67-73. Janzen, F. J., & Phillips, C., P. (2006). ‘Exploring the evolution of environmental sex determination, especially in reptiles" Journal of Evolutionary Biology 19 (6): 1775– 1784. Klug, W., & Cummings, K. (2004). Essentials of genetics, 5th edition. UK: Pearson Education Manolakou, P., Lavranos, G., & Angelopoulou, R. (2006). ‘Molecular patterns of sex determination in the animal kingdom: A comparative study of the biology of reproduction’, Reproductive Biology and Endocrinology, 4 (1): 59. Quinn, A. (2007). ‘How is the gender of some reptiles determined by temperature?’ Scientific American, June. Web. Saccone, G., Pane, A., & Polito, L. C. (2002). ‘Sex determination in flies, fruit flies and butterflies’, Genetica, 116: 15–23. Read More