Life cycle of Drosophila melanogaster
Drosophila melanogaster is a common fruit fly used as a test system and has
contributed to the establishment of the basic principles of heredity. It is also
called the “Cinderella of Genetics”.
Drosophila melanogaster is a dipterous, holometabolous insect. It has a
characteristic larval stage preceded by the egg and succeeded by the pupalstage.
Egg is about 0.5 mm in length, ovoid in shape, and white. Extending from the
anterior dorsal surface, there is a pair of egg filaments. The terminal portion of
these filaments are flattened into spoonlike floats. This floats keep the egg from
sinking into the semi-liquid medium.
The larva hatches out from the egg. It is white, segmented, and wormlike. The
head is narrow and has black mouth parts (jaw hooks). The larva undergoes
2 moults, so that the larval phase consists of 3 instars. After this stage, the larva
crawls out of the medium and finally attaches to the inner drier surface of the
bottle. This culminates in pupation.
Soon after the formation of the “pupal horn” from the anterior spiracle, the
larval body is shortened and the skin becomes hardened and pigmented. The
pupa is considered a reorganization stage. During this process, most of the
adult structures are developed from the imaginal disc. A fully transformed adult
fly emerges out through the anterior end of the pupal case.
At the times of eclosion, the fly is greatly elongated and light in color, with
wings yet to be unfolded. Immediately after this, the wings unfold and the body
gradually turns dark and brown. After 6 hours of emergence, the adult fly
attains the ability to participate in reproduction.
The body is divided into head, thorax, and abdomen. The head has a pair of
compound eyes and a pair of antennae. The thorax is divided into 3 segments—
prothorax, mesothorax, and metathorax, each with a pair of legs. The mesothorax
has a pair of wings and the metathorax has a pair of halters. The abdomen is
segmented in 4 or 5 sections in males and 6 or 7 in females. The abdominal
tip in males is darkly pigmented.
Morphology of Drosophila Melanogaster
The body of an adult Drosophila melanogaster is divided into 3 parts, namely the
head, thorax, and abdomen.
- Head. The head is composed of 6 fused segments, a pair of antennae with
plumose aristae, and a licking proboscis without mandibles. On the dorsal
side of the head between the compound eyes are 3 simple eyes called
ocelli. Bristles are found on the head.
- Thorax. It is composed of 3 fused segments, namely the prothorax,
mesothorax, and metathorax. All 3 segments have a pair of wings. The
metathorax has halters (reduced wings).
- Abdomen. The abdomen consists of 7 or 8 visible segments in the female
and 5 or 6 segments in male.
Differentiation between Male and Female Drosophila
Drosophila melanogaster, like other animals, requires an optimum temperature for
its survival, growth, and breeding. The optimum temperature for the maintenance
of Drosophila melanogaster is between 20°C to 25°C. The temperature around and
above 31°C makes the flies sterile and reduces the oviposition, and may result
in death. At any lower temperature, the life cycle is prolonged and the viability
may be impaired.
The routinely used food media for the maintenance of Drosophila melanogaster
is cream of white agar medium. The ingredients of this media are:
1000 mL of distilled water
100 gm of wheat flour (sooji)
100 gm of jaggery
10 gm of agar agar
7.5 mL of propionic acid, and
Heat-sterilized bottles should be used for preparing culture. Similarly,
sterilized cotton has to be used to plug the bottles. As the condition of the
medium deteriorates with time, the flies have to be transferred from old to new
bottles, with fresh culture medium at least once every 3 weeks.
Take a clean vessel and boil 1000 mL of water. Then add 10 gm of jaggery and
stir well. To this add 10 gm of agar agar, which acts as a solidifying agent.
Once it boils, add 100 gm of sooji. Then add 7.5 mL of propionic acid, which
acts as an antimicrobial agent. By constant stirring, the medium becomes a
viscous fluid. The hot mixture is transferred into the culture bottle. The bottles
are left for cooling, yeast is added, and they are plugged with cotton. Bottles
are ready to use only after adding the yeast solution.
When flies have to be analyzed, either for routine observation or for experiments,
they are anaesthetized to make them inactive. The procedure is to transfer flies
from the media bottle to another empty wide-mouthed bottle, referred to as an
etherizer. The mouth of this bottle is to be covered with a stopper sprayed with
ether. It takes a minute or so to anaesthetize flies. After this, if etherized flies
revive before completion of observation, they have to be re-etherized by using
re-etherizer. The re-etherizer is an ether-soaked filter paper fitted in a petri plate,
which has to be placed over the flies on the glass plate.
Adult flies are 2–3 mm long, while females are slightly larger than males. The
males carry a sex comb on the first tarsal segment of the first leg. Males can
also be identified by the presence of black pigmentation at the tip of the rounded
abdomen. The tip of the female’s abdomen is pointed and nonpigmented. After
the separation of the 2 sexes, the unwanted flies should be discarded
immediately into the morgue (a bowl of mineral oil or detergent in water).
Isolation of Virgins
In many experiments with Drosophila, it is essential that the sperm of a particular
genotype (male) is used for fertilization of a particular female. To ensure this,
it is often essential to isolate virgin females. The females can store and utilize
the sperms from 1 insemination for a large part of the reproductive life. Females
that have any chance of being nonvirgins should not be used for crosses. For
ensuring virginity, females are removed before 6 hrs of their emergence and
males are removed from the culture bottle before 12 hrs of emergence.
Making and Conducting Crosses
To make crosses between different strains 1–10 virgins from the first strain are
mated in a culture bottle with the corresponding number of males from other
strains. The reciprocal mating of the 2 strains is also done. As soon as the cross
is made, the bottle is marked with the nature of the cross and the date of
crossing. If the larva does not appear after 5–7 days, then the culture is discarded.
If the culture is successful, then the parents are discarded and the already-laid
eggs are allowed to develop into adults.
Analysis of the Progeny
The aim is to understand the pattern of inheritance of a character from parents
to offspring and to subsequent generations. Therefore, each progeny (F1, F2, and
test cross) has to be carefully analyzed and classified according to the phenotype
and sex of each individual. Utmost care must be taken to record the number of
flies in each category. From each experiment bottle, the counting must be restricted
to the first 7 days from the third day of eclosion. A minimum of 200 flies must
be analyzed from each of the F1, F2, and test cross progenies.
Statistical Test and Confirmation of Results
The data obtained from the analysis of the progeny have to be tested with an
established hypothesis. This has to be done to ascertain whether the observed
data work with the hypothesis or not. The routinely used statistical test for such
an experiment is the Chi square test:
|x2 = ε (0 – E)2
The chi square test obtained from this test and the degree of freedom
(df ) df = n – 1 are checked for the level of significance in the chi square
If the calculated value of the chi square is less than the table value at a
particular level of significance, the difference between the observed and expected
frequency is not significant. Then we have to accept the hypothesis.
On the other hand, when the calculated value is more than the table value,
the difference between the observed and expected value is significant, then we
have to reject the hypothesis.