Sex Ratio
The sex ratio is the ratio of males to females in a population. It is expressed as a percentage of males to females or males per 100 females. There are four types of sex ratios applied in animals during different stages of life: Primary sex ratio at fertilization; secondary sex ratio at birth, tertiary at puberty or, adult sex ratio (ASR) and quaternary sex ratio in post- reproductive stages.
Theoretically, the primary sex ratio is 1: 1 or 50%. But, it depends upon many factors. Supplementation of omega-6 polyunsaturated fatty acids (PUFAs) to dam can increase the probability of male calves in cattle and sheep by influencing oocyte cellular functions and developmental potential. It is also evidenced in sheep and pigs that females with good physical conditions could produce more male offspring. Due to altered feeding patterns, the primary sex ratio can also be altered under the seasonal influence. Artificial insemination at the initiation of the oestrus period increased the probability of the female calves in cattle.18.5.1 Manipulation of Sex Ratio
Assisted reproduction techniques can be applied to modulate primary sex ratio considering animal economic traits and ecological importance. The use of sex-sorted semen is now emerged as a promising technique to generate animals of the desired sex. This method is based on the sorting of X and Y chromosome bearing spermatozoa fluorescence-activated cell sorting (FACS) techniques. The molecular markers to identify X chromosome bearing spermatozoa are X-linked enzymes, like glucose-6-phosphate dehydrogenase (G6PD), hypoxanthine-guanine phosphoribosyltransferase (HPRT), phosphoglycerate kinase and alpha-galactosidase. The Y-chromosome bearing spermatozoa can be separated by the presence of sex-specific proteins (SSPs), which have distinct antigenic properties due to its H-Y antigen. The X- and Y-chromosome bearing spermatozoa are identified by detecting the chromosome-specific sperm protein constituents, like kinases, transmembrane proteins and chaperones.
PCR, fluorescence in situ hybridization (FISH) and Raman spectroscopy techniques are also used to identify sexed spermatozoa through proteomics.Learning Outcomes
• Development of sex organ: The embryonic development of the gonads occurs in two distinct phases. In the first indifferent phase, the bipotent genital ridge having characteristics of both male and female gonads is developed. The second phase is called the sex determination phase, in which the bipotent genital ridge undergoes differentiation to develop either testis or ovaries. The testis is developed under the genetic control of ‘testes-determining factor’ (TDF) encoded on the short arm of the Y chromosome. The ‘Z’ factor in the XX karyotype triggers ovarian development. The testis is developed from the medullary part of the genital ridge and ovaries from the cortex. In avian species, the males are homozygous (ZZ), females are heterozygous (ZW), and the testisdetermining factor is DMRT1. The male accessory sex organs are developed under the influence of testosterone and its metabolites secreted from developing testes from the Wolffian duct, and the Mullerian duct differentiates into female accessory sex organs.
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• Disorders of sex development (DSD): The disorders of sex development (DSD) and impaired fertility (IF) are serious concerns in animal breeding, particularly where artificial insemination is generally performed. The sex chromosome DSD is seen in freemartins and is characterized by sex chromosome aneuploidies, sex chromosomal structural rearrangements and lymphocyte chimerism (XX/XY). The XX DSDs are analogous to Turner’s syndrome and characterized by the presence of both male and female reproductive organs. The XY DSDs (analogous to Klinefelter’s syndrome) include androgen insensitivity syndrome (AIS) and persistent Mullerian duct syndrome (PMDS) and characterized by cryptorchidism and hypospadias.
• Sex differentiation in animals and birds: A number of biotechnological tools and imaging techniques can be applied to determine foetal sex in animals during the various period of embryonic life by identifying sex-specific genes, amplicons, enzymes and proteins, whereas ultrasonography is the best imaging technique to determine foetal sex through phenotypic characteristics.
It is effectively used in multiple ovulation embryo transfer (MOET) nucleus breeding programmes.• Sex ratio: The ratio of males to females in a population is called the sex ratio. It can be expressed in four types according to different stages of life of the animals. The ratio can be altered naturally and by several manipulation techniques. These techniques are applied to conserve the wildlife and target the economic population for increasing production.
Exercises
Objective Questions
Q1. Which phase of the sex development is continued up to puberty?
Q2. What is the major role of the supporting cells of the bipotent gonad in the ovary?
Q3. Which gonad is developed from the medullary part of the genital ridge?
Q4. Which gene is primarily responsible for male gonad development in mammals?
Q5. What is the full name of DMRT1?
Q6. Why do mammals having Klinefelter’s syndrome can be developed male gonad and in Turner’s syndrome developed female gonad?
Q7. What is the fate of the embryonic genital tubercle in male and female mammals?
Q8. Why some interspecies mammalian hybrids are fertile, but some are sterile?
Q9. Which type of gonad is developed in the animals having a Y chromosome, but, sry gene is not fully expressed?
Q10. Which kind of sex differentiation can easily be recognized?
Q11. What is the sex ratio?
Q12. Which sex chromosome is larger and why?
Q13. Which chromosome carries the key candidate gene in the bird?
Q14. Which mammals lack the sry gene in the Y chromosome?
Subjective Questions
Q1. Write the developmental process of male gonad during embryonic life in mammals.
Q2. Write the developmental process of female gonad during embryonic life in mammals.
Q3. How are gonads developed in the bird?
Q4. Write the role of various endocrines and growth hormones in sex differentiation.
Q5. Explain: ‘A tigon is fertile, but the mule is sterile’.
Q6. Why freemartins heifer can be sterile?
Q7.
How does the sex differentiation can be recognized in mammals and birds and its importance?Q8. What are the different sex ratios, and how can they be altered or manipulate?
Answer to Objective Questions
A1. The second phase or phase of differentiation of the gonad
A2. Steroidogenesis
A3. Testis
A4. sry gene
A5. Double-sex and Mab-3 related transcription factor #1
A6. Mammals having Klinefelter’s syndrome belong Y chromosome, and Turner’s syndrome do not belong Y chromosome
A7. In males, it forms the penis and in the female clitoris
A8. Fertile animals will occur when the involving species have compatible chromosome numbers, and sterility develops when they belong to the incompatible number of chromosomes.
A9. Ovary
A10. Male
A11. It is the ratio of males to females in a population and expressed as a percentage of males to females.
A12. The X chromosome, as it contains more DNA mass
A13. Zchromosome
A14. Mammals under order monotremes
Keywords for the Answer to Subjective Questions
A1. Y chromosome and sry gene, endocrines, growth factors A2. XX karyotype, absence of sry gene, growth factors A3. Karyotype ZZ or ZW, SOX9 gene, DMRT1
A4. Role of androgens, Mullerian inhibitors, TGF-β, FGF9, INSL3
A5. Hybrid, chromosome number, functional characters of the gonad
A6. Heterogeneous twin foetuses, masculinizing factors, suppression of ovarian development
A7. PCR, USG, MOET
A8. Four types, dietary manipulation, using ART
Acknowledgement We are grateful to Prof. Sagar Sanyal, Ex. Professor and Head, Department of Veterinary Physiology, West Bengal University of Animal and Fishery Sciences, Kolkata, West Bengal, India for his valuable guidance during the preparation of the chapter. We are also greatly indebted to the helps from Dr. Parthib Roy, Master Scholar, Department of Veterinary Gynaecology and Obstetrics for preparation of the figures.
Further Reading
Textbooks
Budik S (2017) Genetics in domestic animal reproduction.
In: Abubakar M (ed) Trends and advances in veterinary genetics. https://doi.org/ 10.5772/67132Pineda MH (2003) The biology of sex. In: Pineda MH, Dooley MP (eds) McDonald’s veterinary endocrinology and reproduction, 5th edn. Iowa State Press, pp 201-238
Research Articles
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Chue J, Smith CA (2011) Sex determination and sexual differentiation in the avian model. FEBS J 278:1027-1034
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