Sex determination in animals

Sex determination systems in animals

1. DARSHANA A S 2018-11-039 SEX DETERMINATION IN ANIMALS

2. INTRODUCTION Sex determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create offspring using sexual reproduction have two sexes . There are hermaphrodites in place of one or both sexes. There are also some species that have only one sex due to parthenogenesis (female reproducing without fertilization.)

3. HISTORY 1891 – H.Henking While studying spermatogenesis of wasp noted a particular nuclear structure. Half of the sperm receives this structure & half didnot . He didnot speculate on the significance of this body but called it “ X-body ”. First experimental evidence that led to discovery of sex chromosomes.

4. 1902 – C. E. McClung Made extensive observations of spermatogenesis in Grasshopper & suggested that “ X-body was involved in determination of sex ” . He reported that somatic cells of female grasshopper contains 24 chromosomes & male have 23 chromosomes.

5. 1905 – E. B. Wilson & N. Stevens Noted that females of Protenor have 7 pairs of chromosome & male have 6 & an unpaired chromosome. They called it X – chromosome. 1908 – Discovered Y chromosome and so named by Stevens.

6. SEX DETERMINATION Establishment of male and female individuals or male and female organs of an individual is called sex determination. Mechanism of sex determination-3 types Environmental Chromosomal Genic

7. Zygotes do not differ genetically. Sex differentiation determined by environmental factors. 2 types Temperature dependent Location dependent 1. ENVIRONMENTAL / NON-GENETIC SEX DETERMINATION

8. 1. Temperature-dependent sex determination In some animals (turtles, alligators, crocodiles, some lizards etc.) the temperature at which eggs are incubated has a decisive effect on sex of animals that hatch from them. These effects are of 3 types: Only females at high (30-35°C) and only males at low (23-28°C). Incubation of eggs at intermediate temperatures(28-30°C) produce both males and females. Eg : most species of turtles

9. Only males at high(>33°C) and only females at low(<30°C) temperatures . Intermediate temperatures(30-33°C) produces both males and females E g : many species of crocodiles and alligators and in some species of lizards Only females at high(30-35°C) and low(<25°C) temperatures . At intermediate temperatures, both males and females are produced, but at some intermediate temperatures, only males are produced. Eg : Australian crocodile, snapping turtles

10. 2. Location-dependent sex determination Eg : Bonellia viridis – marine (green spoon worm) ♂  and ♀ have same chromosomes. If a young worm is raised from an isolated larva it becomes a female. If larva comes in contact with female then larva turns into male and eventually migrates to female reproductive tract. Possibly a hormone is secreted from the proboscis of female to initiate male sexual differentiation .

11. 2. CHROMOSOMAL SEX DETERMINATION Male and female individuals differ from each other in respect to either the number or morphology of the homologues of one chromosome pair, referred to as sex chromosome or allosome – X & Y X chromosome found in both males(only one) and females(has two X chr .) Y chromosome occurs only in one of two sexes of a species. Chromosomes whose number and morphology do not differ between males and females of a species are called autosomes .

12. Mechanisms of chromosomal sex determination: XX-XO system [XX female, XO male XO female, XX male] XX-XY system [XX female, XY male XY female, XX male] ZZ-ZW system Haplodiploidy system [Diploid (2n) female, Haploid (n) male]

13. XX FEMALE, XO MALE Females possess two X- chromosomes (XX) - homogametic females. Males possess only one X-chromosome - heterogametic males. O or zero in XO chromosome →absence of another X- chromosome. Found in grasshoppers, Protenor and many other insects, esp. those belonging to Orthoptera .

14. If X carrying ovum fertilized by X carrying sperm -zygote develop into a female. If sperm containing no chromosome unites with ovum -zygote formed is XO- develop into male. Maternal gametes always contain an X chromosome - sex of offspring depends on whether a sex chromosome is present in male gamete.

15. XO FEMALE, XX MALE Known in a few insect species, e.g., Fumea Females are heterogametic (producing two kinds of eggs, half with a X chromosome and half without any X chromosome) and males are homogametic (producing single type of sperms, each carrying a single X chromosome). Union of a sperm with X chromosome containing egg -XX zygote -males. Fertilization of an egg devoid of X chromosome with a sperm -XO zygote -females.

16. XX FEMALE, XY MALE Most common among animals Found in humans, mice, Diptera ( Drosophila , house fly, etc.), some fishes, some amphibia , etc. Females are homogametic (XX) produce one kind of eggs, each with one X chromosome. Males are heterogametic (XY) produce two kinds of sperms : half with X chromosome and half with Y chromosome.

17. Fertilization of egg by a sperm having X chromosome -XX zygote-female. Union of egg with sperm having Y chromosome -XY zygote -males.

18. XY FEMALE, XX MALE Found in birds, reptiles, some insects, e.g., silk worm, etc. Females have XY chromosome constitution - heterogametic sex - half the eggs have X, rest have Y chromosome. Males have two X chromosomes (XX) homogametic sex -all sperms have one X chromosome.

19. Fertilization of X containing egg with sperm -XX zygote-male. XY zygote -when Y containing egg is fertilized by a sperm-female.

20. ZW FEMALE, ZZ MALE Occur in certain insects & vertebrates like amphibians, reptiles, birds & plants Female have one Z and one W chromosome- produce two types eggs. Male - two homomorphic Z chromosomes.

21. Sex of offsprings depends upon the kind of egg Z bearing egg produce male & W bearing egg produce female.

22. Found in Hymenoptera(honeybees, ants, termites, etc) An individual’s sex determined by number of sets of chromosomes. First demonstrated by Dzierzon in 1850 Somatic chromosome no. of females is diploid, males is only haploid When an egg is fertilized by a sperm, the developed organism will be diploid (2n)- female If egg is not fertilized, organism is haploid(n) -male. DIPLOID FEMALE, HAPLOID MALE

23. During spermatogenesis, males produce haploid sperms Normal meiosis during oogenesis produces all haploid eggs Fertilization of eggs produces diploid zygotes - develop into diploid larvae - give rise to workers which are sterile females Diploid larvae fed on royal jelly develop into fertile females called queen Unfertilized eggs develop parthenogenically - produce haploid larvae - fully fertile haploid males called drones

24. Some specific genes located in X and Y chromosomes are involved in sex determination. Explained by Genic balance theory of sex determination in Drosophila proposed by Bridges in 1921. Theory states that sex of an individual is determined by a balance between the genes for maleness and those for femaleness present in the individual In Drosophila, genes for maleness are present in autosomes and those of femaleness are located in X chromosome 3. GENIC SEX DETERMINATION

25. ie , sex of an individual is determined by the ratio of no. of its X chromosome and that of its autosomal sets, termed as sex index Sex index = Number of X chromosomes(=X) = X/A Number of autosomal sets (=A) Sex index = 1 normal females = 0.5 normal males = 1.0 – 0.5 intersex = > 1.0 super females/meta females = < 0.5 super males

26. GYNANDROMORPHS Individuals showing male characteristics in a part of their body and remaining parts show female phenotype In Drosophila gynandromorphs are always mosaics for X chromosome; parts with male phenotype are always XO and female phenotype are XX They arise from XX zygotes.