Animal Sex Determination and Manipulation
In this article, you will be studying about sex determination and manipulation in farm livestock, birds and some reptiles. In fact you will learn how to predetermine the sex of the offspring.
Sex: Sex is defined as the sum total of those differences in structure and function on the basis of which an organism is classified as male or female.
Sex Determination: Two main theories have been advanced in explaining the mechanism of sex determination.
Chromosomal Theory of Sex Determination
Under this theory, sex is said to be determined at fertilization by the sex chromosomes. Each gamete contains the haploid number of chromosomes. In mammals and in most insects, all gametes produced by the female are similar having X chromosomes.
Males on the other hand produce two types of gametes in approximately equal numbers, one type bearing the X chromosome and the other type bearing the Y chromosome.
In mammals and most insects, the female is known as the homogametic (xx) sex while the male is heterogametic (xy). At fertilization, a zygote with XX results is a female, while that with XY results is a male. In birds, the female is the heterogametic sex while the male is homogametic. These are illustrated in table 1 below;
Table1: Kinds of Chromosomal Sex Determination
| SEX | SPERM | EGGS | FEMALE | MALE | |
| Mammals and most insects | Male | X and Y | All X | XX | XY |
| Grass hoppers | Male | X and O | All X | XX | X |
| Birds, some reptiles and amphibians | Female | AIIZ | Z and W | ZW | ZZ |
Adopted and Modified from Osinowo, 2006, Introduction to Animal Reproduction.
Aberrations of genetic sex do occur either as a result of non-disjunction of sex chromosomes translocation, deletion or mutation. In maternal non-disjunction, fertilization of the ovum will lead to formation of an XXX or an XXY zygote.
In paternal non-disjunction, zygotes of XXY or XO genotypes result. Such cases of aneuplody often result in gonadal or endocrine defects.
In man, the Klinefelter‟s syndrome (XXY) is characterized by gonadal hypofunction. In women, Turner‟s syndrome (XO) is characterized by gonadal agenesis or aplasia.
Genic Balances Theory of Sex Determination
This theory purports that sex is a phenotypic trait, determined by interaction between genotype and environment. Individuals vary in their degree of maleness or femaleness.
C.B. Bridges in 1922 proposed the genic balance theory to explain apparent quantitative variability in sexual character. According to this theory, sex is determined by the autosomes as well as by the X chromosomes, the ratio of autosomes to X‟s being the significant relation.
In Drosophila, the X chromosome carries more genes for maleness (Table 2). Which sex actually develops is decided by the balance between the two sets of genes.
Table 2: Sexual Types in the Fruit Fly, Drosophila melanogaster
| SEX | X CHROMOSOME | SETS OF AUTOSOMES (A) | SEX INDEX (X/A) |
| Super female | 3 | 2 | 1.5 |
| Normal Female; tetraploid triploid diploid haploid | 4 3 2 1 | 4 3 2 1 | 1.0 1.0 1.0 1.0 |
| Intersex | 2 | 3 | 0.67 |
| Normal male | 1 | 2 | 0.50 |
| Super male | 1 | 3 | 0.33 |
Adopted from O.A. Osinowo, 2006, Introduction to Animal Reproduction
Other Theories of Sex Determination Include:
Genetic Dosage Compensation X–Inactivation Model of Sex Determination
The ZFY gene, thought to constitute the primary sex-determining signal, was identified within a very small segment of the Y chromosome by Page and co-workers in 1987.
However, the presence of a similar gene, ZFX, on the X chromosome prompted the propounding of the dosage compensation X-inactivation theory of sex determination.
According to this theory, both ZFX and ZFY produced functionally interchangeable proteins. It therefore means that XY cells would have two active copies of the gene (ZFX and ZFY) while XX cells would only have one active (ZFX) copy, due to X- inactivation. Embryos with two copies would thus develop into males while those with a single copy of the gene would develop into females.
In this way, the theory held that gene dosage determined sex. Though elegant, the theory became untenable when it later became evident that the ZFX gene escapes X- inactivation, thus contradicting the dosage compensation model.
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Sex Determination in Birds
In order to distinguish animals with female heterogamety from those with male heterogamety, the distinguishing chromosome is known as W in the former (in mammals, Y), while the common chromosome is called Z (X in mammals).
Temperature Dependent Sex Determination in Alligators
Many reptiles, including alligators and crocodiles, exhibit no sex chromosome dimorphism. Rather, the sex of the offspring is determined by the temperature at which the eggs are incubated.
In alligators (Alligator mississippiensi), incubation of eggs from: 29-31.5°C – results in 100% female offspring;
32.5-33°C – results in 100% male offspring
33.5-34.5°C – different sex ratios
35°c – 100% females
Below 29°C and above 35°C, high mortality results.
Sex Determination in Fishes
Most fishes function either as male or female throughout life, i.e. most are bisexual or gonochoristic, as opposed to hermaphroditic, a condition in which an individual produces both eggs and sperm at some stage of its development.
Establishment of sex depends on sex chromosomes, designated X and Y in most fishes.
Usually, XX individuals are female and XY individuals are male
There are exceptions, e.g. among the poeciliidea in which the homogametic individual is male. Chromosome designated W and Z are recognized in some of these fishes.
Although the sexes have similar appearances in many species, sexual dimorphism or dichromatism is common in fishes and may be especially well marked in those species with internal fertilization or elaborate reproductive behavior.
Occasional hermaphrodites are found in many gonochoristic species as an abnormality, but numerous species are normally hermaphroditic, with some even capable of self- fertilization.
Synchronous (or simultaneous) hermaphrodites have ripe ovaries and testes at the same time but usually spawn with one or more other individuals, alternately taking the role of male and female. Synchronous hermaphrodites are known from the following families:
Chlorophthamidae, Bathypteroidae, Alepisauridae, etc. Protandrous condition is known in members of Gonostonatidae, Serranidae, Sparidae, etc. Protogynous hermaphrodites have been noted in the Synbranchidae, Serranidae, Meanidae and Labridae.
Sex reversal and hermaphroditism are controlled by the endocrine system which is genetically programmed in normally hermaphroditic species to act on the gonads in response to the proper stimuli, which can be internal or external.
Pre-Determination of Sex
X and Y chromosome bearing sperm have differences in quality of DNA. The X having more DNA than Y chromosome-bearing sperm.
Bull, boar and ram X and Y chromosome-bearing sperm population can be separated by flow-cytometric procedures.
Measurements of DNA in separated X and Y-bearing sperm populations can be used to predetermine the sex of offspring.
Centrifuging of semen results in the Y-chromosome bearing sperms being in the upper half of the supernatant, while the X-chromosome bearing sperm are in the remaining half supernatant below.
In humans semen deposition into the female reproductive tract at peak body temperature or the sudden fall in temperature in normal female cycles results in a male zygote.
Ovulation is said to take place at either peak body temperature or the sudden fall just after the peak. Y-chromosome bearing sperm are said to be lighter than X and therefore swim and reach the egg before the X, but the Y are less viable than the X.
Sex Reversal, Gamete and Embryo Sexing
Sex reversal is common in some species of fishes as earlier discussed. See 2.4.4
With the development of the embryo transfer technology, interest in the sexing of gametes before AI or embryos before transplantation has become substantial. Gamete sexing differentiating the Y-chromosome bearing sperm has been earlier discussed. Possible cytological approaches in sexing of embryos before transplantation are:
Biopsy of bovine embryos on day 6 or 7
Observation of the Barr body
Trophoblast biopsy of bovine embryos between days 1 and 15, and
Halved embryos on days 6 and 7
Of these four methods, only trophoblast biopsy on days 12-15 and embryo bisection on days 6 or 7 can be considered practicable. The first excludes the possibility of long term storage while the second requires additional expense and expertise.
Embryo sexing using immunological techniques has also been reported. Experimental evidence suggests that H-Y antigen is present in early embryos of the bovine, and that serological reagents that identify H-Y antigen can be used to identify males and females in embryo transfer systems.
The Sex Chromatin
The sex chromatin is a well-developed DNA-positive chromocentre, observed in the nucleus of most cells from certain tissues in most mammals. It represents a heterochromatic X chromosome which is genetically inactive. It usually lies against the inner surface of the nuclear membrane or adjacent to the nucleolus.
The maximum number of sex chromatin bodies in a given interphase nucleus is one less than the number of X chromosome. Thus, XY or XO individuals are sex-chromatin negative, while XX or XXY have one sex chromatin body per nucleus.
In farm animals, sexual dimorphism as indicated by presence of the sex chromatin is detected mainly in neurons of the brain and spinal cord. The sex chromatin is also detectable in foetal cells from the amniotic fluid of cattle, sheep and pigs.
In summary, sex determination and manipulation in farm animals is a complex technique, some require specialized equipment. While in some species the males are heterozygous, in others the females are heterozygous.
How is sex determined in fishes? Most fishes function either as male or female throughout life. i.e. most are bisexual or gonochonistic, as opposed to hermaphroditic, a condition In which an individual produces both eggs and sperm at some stage of its development. Establishment of sex depends on sex chromosomes, designated X and Y in most fishes.
Usually, XX individuals are female and XY individuals are male. There are exceptions, e.g. among the poeciliidea in which the homogametic individual is male. Chromosome designated W and Z are recognized in some of these fishes.
Although the sexes have similar appearances in many species, sexual dimorphism or dichromatism is common in fishes and may be especially well marked in those species with internal fertilization or elaborate reproductive behaviour.
Occasional hermaphrodites are found in many gonochoristic species as an abnormality, but numerous species are normally hermaphrodic, with some even capable of self – fertilization.
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