Sex Determination in Drosophila

Arun M. ChilkeDepartment of Zoology,

Division of Toxicology and Biomonitoring,Shree Shivaji Arts, Commerce and Science College, Rajura-442905

(India)Email: achilke.2011@rediffmail.com

Web: www.sscollegerajura.org

Introduction

Drosophila melanogaster is a fruit fly or vinegar fly belongs to order-Diptera. This species is a model organism for the study of developmental biology, physiology and genetics. However immense study has been carried on D. melanogaster in the field of genetics and hence it is named as “Cinderella of Genetics”.

Importantly this organism exhibit sexual dimorphism and its life span is of short duration. Moreover it has short developmental period. Therefore, this animal is quite suitable for studying the several couple of generation. It has been good study material in the hands of biologist.

Sexual Dimorphism

Male Drosophila

Female Drosophila

Male and female in the population of Drosophila melanogaster can be easily recognized on the basis of external features. And the character on The basis of which the sexes can be differentiated is called sexual dimorphic characters. In general both the male and female are having Brick red color eyes and consists of black rings across the abdomen. Male is smaller than the female. Male has characteristically sex-comb and the reproducing parts are surrounded by cluster of spiky hairs (claspers).

Chromosomes

Sum four chromosomes together constitute genome of D. melanogaster. It is a diploid organism and hence total number of chromosome in normal individual are eight. Thus they consists of four pair of chromosomes. Out of these three pair is of autosome and one pair belongs to heterosome or sex chromosome.

Male in heterozygous and contain X and Y- chromosomes. Female is homozygous and contain both the X chromosome.

Male Karyotype Female Karyotype Haploid Set (Genome)

Role of Chromosome in Sex determination

Autosome and express somatic characters whereas the heterosome express sexual characters.

In the female X-chromosome is very important as it expresses the feminine characters. But the Y-chromosome does not express the masculine character in the male. It is known that in the D. melanogaster masculine character is almost and always determined by autosome. It means there is no role in the determination of sex by Y-chromosome. But for the survival of male the presence of Y-chromosome is must. In the male and female the sex is determine by the ration between autosome to the X-chromosome which can be well explained by Genic Balance Mechanism (C.B.Bridges).

When a normal female with genotype (2A+XX) is allowed to cross with normal male with genotype (2A+XY) the normal male and female is produce.

Female X Male

(2A+XX) (2A+XY)

Gametes A+X A+XA+Y

(2A+XX) Female (2A+XY) Male

Female X Male

Gametes

Parents

Offspring's

Formation of Triploid Female

Sometime the diploid female lay the abnormal eggs due to non-disjunction. Non-disjunction is the process during which chromosomes fail to separate precisely. Unfortunately the whole set of chromosome enter into a gamete (diploid gamete) and other gamete do not receive any chromosomes.

If such diploid egg get fertilized by normal haploid male the triploid female is produce.

Female X Male

(2A+XY) (2A+XX)

Gametes 2A+XX

A+XA+Y

(3A+XXX) Triploid Female

Non-disjunction

NondisjunctionNormal Gametogenesis

Triploid Female

Gametes

Parents

Female Male

Genic Balance Mechanism

In Drosophila melanogaster sex is determined by autosome and X-chromosome. Autosomes are responsible to determine the male sex and X-chromosome female. Y-chromosome is inert in the determination of male sex but its presence is must for the survival of male itself.

The ration (X/A) of X-chromosome to the autosomes (A) determine the sexes both the male and female. But it the ration is equals to 1 then it produces the female and 0.5 produce the male. However, above 1 produces the super-female and below 0.5 produce the super-male. When the ratio is between 1 and 0.5 it produces the intersexes.

X/A = 1 is Female

X/A= 0.5 is Male

X/A> 0.5 is super-male

X/A< 1 is super-female

And when X/A is in between 1 and 0.5 then it is Intersexes.

Triple X-Female X Diploid Male

(3A+XXX) (2A+XY)

(A+X)(2A+XX) (A+X) (A+XX) (2A+X) (A+Y)

Female Gamete's (Four typed) Female

Male

2A+XX A+X A+XX 2A+X Gametes

Male Gamete's

(Two type)

3A+XXX

Triploid

female

2A+XX

Diploid female

2A+XXX

Super-female

3A+XX

Intersexes

A+X

3A+XXY

Intersexes

2A+XY

Normal male

2A+XXY

Female

3A+XY

Super-male

A+Y

Triploid Female Super-femaleDiploid femaleIntersexes

Normal male Super-maleIntersexes Female

Eight Genotypically different Individuals are obtained in the Cross between Triploid Female and Diploid male

2A+XX2A+XY

Balance inclinedTowards femaleness

Balance notinclinedTowards maleness

Schematic Presentation of Genic Balance Mechanism

Triploid Female or Meta-female

Triploid female is genotypically abnormal and phenotypically appear normal

like diploid female. However she is fertile and there are maximum chances to produce four different types of

gametes. It produce normal and abnormal offsprings with inter sexes, super male, super female, triploid female, normal male and normal female.

Intersexes

Drosophila with an intersexes genotype ratio express the phenotype of male and female sexes. May possesses sex comb, genitalia which show a mixture of male and female parts (have 1 ovary and 1 testis).

Supersexes

Super male: It is similar to male in their phenotype but having lowered viability, notched wings, disturbed wing veins and bristle patterns. They are sterile.

Super female: Similar to normal female but having low viability, notched wings, disturbed wing vein and bristle patterns. They are also sterile.