Chromosomes in Drosophila :
The fruit fly Drosophila melanogaster has 3 pairs of autosomal chromosomes and an X and Y chromosome. Each autosome has two arms that are simply referred to as left (L) and right (R). Each chromosome arm is numbered as follows:X (1-20), 2L (21-40), 2R (41-60), 3L (61-80), 3R (81-100), and chromosome 4 (101-102) as per giant ploytene chromosomes. Polytene chromosomes (X, 2L, 2R, 3L, 3R, 4) of Drosophila melanogaster consist of 102 divisions according to the standard drawn reference maps of Bridges. Chromosome maps are needed and used when, for example, the position of a gene, the breakpoint in a chromosome rearrangement, or the location of DNA by in situ hybridization, is determined.
Sex determination in Drosophila is based on the ratio of X chromosome to autosomal sets. Therefore, females, which have two X chromosomes, have a ratio of 1 whereas; males, which have only 1 X chromosome, have a ratio of 0.5.
What is the need for collecting Virgin females for Crosses?
Females in fruit flies mate with several males and has the ability to store sperms in spermathecae ( organ of the female reproductive tract in insects, some molluscs, and certain other invertebrates. Its purpose is to receive and store sperm from the male) This forces researchers to use virgins when setting up a genetic cross. Using non-virgins results in progeny that do not have the expected genotypes or phenotypes predicted by simple Mendellian genetics.
The identification of virgins can be done by their body colour (they lack dark colouration of matured females- its easy to identify when one work with flies often) virgin females are much larger than older females and in the early hours after eclosure, there will be visible a dark greenish spot (the meconium, the remains of their last meal before pupating) on the underside of the abdomen.
Once females are recognised as virgins its better to store them for at least one day before adding males. Generally, males will mate more efficiently if they have matured 3 days or longer. Be ensure to select robust, healthy males; the older the flies, the lower the mating efficiency. Mating occurs quickly and the behaviour is interesting to watch.
Temperature dependent life cycle:
The life cycle of Drosophila is quite simple: eggs are laid which develop into larvae, which develop into pupae, which develop into adult flies. Flies do not “hatch” from pupal cases they “eclose”. The above life cycle is dependent on the temperature at which the flies are grown. The generation time for flies grown at 25°c is 10 days, at room temperature (21-22°c) it is 12-13 days, and at 18 o C the generation time gets more which is around 19 days.Generally stocks of Drosophila should be maintained at this temperature so that you can wait for three weeks before transferring the stocks to fresh vials or else need to be turned once in two weeks at 25°c-Turning stocks might be most boring but important thing in Drosophila genetics.
There are some rules to be followed while writing the genotype of a fly stock and this very important aspect of drosophila genetics and that is the reason why it should be learnt thoroughly.
- Chromosomes are always written in order, with a semi-colon separating each chromosome. like
X/Y; 2; 3; 4
- Genotypes are listed only when a mutation is present and are italicized.Anything not listed is assumed to be wild type.
- Dominant mutations are capitalized (e.g. cyo for curly wings).
- Recessive mutations are written in lower case (e.g. w for white gene)
- If a particular allele is present that allele is super-scripted .
- If more than one mutation is present on a chromosome, the mutations are listed from left to right
corresponding to the left and right arms of each chromosome. cn bw ( cinna bar and brown)
- If a mutation is homozygous then the mutation is written only once as cn bw
- If heterozygous then written as cn/+ bw /+
For more detailed view of Nomenclature like details about writing genotypes of deletions, inversions, translocations and transpositions check FLYBASE
Image credit : Life cycle –
The Natural History of Model Organisms: The secret lives of Drosophila flies by T. Ann Markow
Drosophila Chromosomes : John Roote and Andreas Prokop – A basic training package for Drosophila Genetics.