We provided fruit flies (Fig. 2B): one vial of each type of flies for each pair of students.. It takes about one week for the flies to hatch upon fecundation, and two weeks for each subculture to increase the fly population to reach the size necessary for classroom use. We proposed to students some activities: Observation of D.melanogaster morphology -Drosophilae are moved from the test-tube where they live to "anesthetics" tube“ where they are left few minutes with CO2 (alka seltzer tablets or dry ice). -Flies are left on a thin cardboard or a Petri plate and observed under a stereomicroscope. Students observed Drosophila sexual dimorphism: the female is bigger than male and abdomen is light . The male has got a dark spot on posterior extremity of the body and a structure like a comb on the anterior leg to block the female during the copulation (Fig. 2B).

Other activities: -Observation of the Drosophila life cycle. At school students performed crosses. Crossed wild type (wt) male and female Drosophilae in a test-tube, made a series of passages in other test-tubes on the 2th, 4th, 6th, 8th and 10th day, and, after 10 days, they observed embrions, larvas, pupas and adults in different test-tubes. . The Drosophila egg is about half a millimiter long. It takes about one day after fertilisation for the embryo to develop and hatch into a worm-like larva. (Fig. 3A): The larva eats and grows continuously, moulting one day, two days, and four days after hatching (first, second and third instars). (Fig. 3B). -Observation of D.melanogaster mutants: regarding the colour of the eyes, regarding the colour of the body, regarding the structure of the wings. We provided fruit flies mutants and they try and describe the mutations.

The fruit fly, Drosophila melanogaster, has been the most popular eukaryotic organism used in classrooms. It is a small fruit fly about 3mm long, similar to the ones you see attracted to your bananas and other fruit. It has short life cycle of two weeks, making it possible to study numerous generations in a short period. It is easy to culture and inexpensive to house large numbers. Its size is amenable for cultivation in school laboratories. Also, it is large enough that many attributes can be seen with the naked eye or under low-power magnification. Moreover, it has a very long history in biological research and there are many useful tools to facilitate genetic study. It was recognised by the award of the Nobel prize in Physiology or Medicine to Thomas Hunt Morgan in 1933, to Hermann Muller in 1946 and to Ed Lewis, Christiane Nusslein-Volhard and Eric Wieschaus in 1995. The use of Drosophila is a powerful tool also in teaching the Life Sciences. In fact it allows the observation of sexual dimorphism, of mutants and of the life cycle. Moreover it permits the realization of crosses aimed to demostration of sexual linked characters and crosses aimed to estabilish if a mutation is conferrel by a dominant or a recessive gene. There are also similarities with the human species. From a genetic point of view humans and fruit flies are quite similar. About 60% of known genetic diseases may occur in the gene of the fruit fly. And approximately 50% of the proteins of Drosophila have a similar in mammals. The Drosophila is used as a genetic model for several human diseases, including neurodegenerative disorders such as Parkinson's disease, Huntington's disease and Alzheimer's disease. It is used to study the biological mechanism of the immune system, diabetes, cancer, and even substance abuse.

THE ROLE OF DROSOPHILA IN TEACHING LIFE SCIENCES M. E. Pasini*,Y. Intra, P. Fasano

Department of Biosciences, University di Milano, Via Celoria 26, 20133 Milano, Italy. E-mail: maria.pasini@unimi.it

embryo 2° and 3° instar larva prepupa

Fig. 3A,3B -The Life cycle of Drosophila melanogaster and the various larval instars

Extraction of salivary glands and staining giant chromosomes: Polytene chromosomes are over-sized chromosomes which have developed from standard chromosomes and are commonly found in the salivary glands of Drosophila melanogaster. Specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), to increase cell volume, forming a giant polytene chromosome. Polytene chromosomes form when multiple rounds of replication produce many sister chromatides that remain synapsed together. In addition to increasing the volume of the cells' nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreduplication, to produce large amounts of glue before pupation. Polytene chromosomes have characteristic light and dark banding patterns that can be used to identify chromosomal rearrangements and deletions. Dark banding frequently corresponds to inactive chromatin, whereas light banding is usually found at areas with higher transcriptional activity. We provided the Petri plates with larvae at 3rd instar in acetic acid solution and the students have extracted and colored cells of salivary glands .

Polytene chromosomes stained with acetic orcein and visualized by light microscopy (100X).

Extraction of salivary glands and staining giant chromosomes:

The Core Lab

Fig.1A Students in the LAB Fig.1B Vials of flies

Fig.2A Stereomicroscope Fig.2B Drosophila sexual dimorphism

Fig. 4A,4B Observation of D.melanogaster mutants

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Results and conclusions To understand a complex problem, it is sometimes useful to exploit an easier "model". Model organisms play an important role in conveying biological concepts. The proposed activities in the context of various events of scientific communication organized by the University of Milan, result effective for learning about topics in biology, zoology and genetics.