[BojanaL]

Few more details on Drosophila development and sex determination

Drosophila melanogaster is well studied and often used model organism in various experiments. Drosophila life cycle and development is temperature dependent - it’s ectothermic species. At temperature of ~29° C, drosophila will live for 30 days. Transition from egg to adult can last from 7 to 50 days, depending on the outer temperature. At 25° C (ideal temperature), adult stage will be reached in 8.5 days. At 30° C, heat stress will prolong development to 11 days. At 19° C development will last 18, or even 50 days if temperature drop to 12° C.
Drosophila is holometabolous insect (undergoes complete metamorphosis). Female is lying 400 eggs, that will hatch after 12-15 hours. During 4 days of larval stage, two molting will happen. In the following stage, encapsulated in the puparium, drosophila will finish metamorphosis and emerge as fully developed adult after 4 days.

8-12 h after emerging, females are ready for courtship and sexual intercourse. Mating ritual is divided in couple of stages. First, male will position himself in front of the female and serenade (sound is produced by wing vibration) to attract female attention. In the next phase, he will turn to the abdominal part of the female and excite her sexually by taping and licking her genitalia. At the end, he will attempt copulation by curling his abdomen. Copulation will last 15-20 minutes and seminal fluid (containing very long sperm cells) will be transported to the female’s body. Females are mating with more males, and sperm cells are competing for fertilization. Tubular receptacles and 2 spermathecae are used for sperm storing. Incapacitation (one sperm is incapacitated by other) and displacement of the sperm (done by female) will finally “decide” which sample will be the most successful in fertilization. It’s believed that ~80% of offspring is result of the last mating.

Drosophila gender determination is interesting as well. Genome consists of four chromosomes: three autosomal and X/Y pair. Y chromosome is carrying genes that will encode sperm production but it doesn’t determine the male sex like in humans. Ratio of X to autosomal chromosomes will determine the gender. More importantly, each cell in the body will “decide” whether to be male or female depending on the mentioned ratio. Normal female will develop each time X to autosomal chromosome ratio is 1, despite the number of chromosome pairs present (XXXX:AAAA, XXX:AAA or XX:AA). Normal male will develop when X to autosomal ratio is 0.5 (X:AA). When 3 X chromosome pairs are present (XXX:AA), 1.5 ratio will result in metafemale will development. This genetic combination is associated with impaired development and inability of fly to emerge from the pupae. If autosomal chromosomes are dominating (X:AAA), 0.33 ratio will lead to metamale development. Drosophila with this genetic combination is weak and sterile. Finally, when ratio of X to autosomes is 0.66 (XX:AAA), intersex will be formed. Organism with both male and female characteristics is known as gynandromorph.

All this combinations are associated with three genes present on autosomal and X chromosome that will determine the gender. Deadpan is autosomal gene and it will inhibit expression of the Sex-lethal gene. Sisterless is located on the X and it will inhibit activity of the Deadpan. When autosomal chromosomes are present in higher ratio (over X chromosome), Deadpan will inhibit Sex-lethal, and male will develop. When same amount of X and autosomal chromosomes are present, Sisterless will inhibit the action of Deadpan and female will develop. Different forms of Sex-lethal gene are present in males and females. This gene is controlling further stages of the gender development by splicing its own mRNA and producing truncated (males) or fully developed (females) protein that are affecting expression of Doublesex gene, which is responsible for yolk production in females.

Drosophila genome is sequenced back in 2000. Out of 15,016 genes, 60% are associated with gene expression control. Mating is not the only thing where Drosophila is showing high similarity with humans. Genes for 75% of known human diseases are matching Drosophila genes. Besides genetic experiments, Drosophila is excellent model organism for the study of cancer, diabetes, oxidative stress or even drug abuse. Very interesting and more than useful creature, for sure.