Development

Drosophila is a holometabolous insect; in other words, during the life cycle the animal undergoes a complete metamorphosis. The life cycle of Drosophila consists of four phases: embryo, larva, pupa and adult (Figure 2.6). The first phase comprises 17 defined stages of embryogenesis which take place inside the eggshell (chorion), after which the 1st instar larva hatches. The second, larval phase consists of 3 instars during which nutritional resources are accumulated and stored for the pupal phase, during which there is no feeding (Yamaguchi & Yoshida, 2018). This is why the third instar larvae are of particular interest in behavioral studies on foraging and feeding (Belay et al., 2007; Dombrovski et al., 2017). The metabolic rate of Drosophila in metamorphosis follows a U-shaped curve, according to which energy consumption is highest in the beginning of the transformation, declines in the middle and increases again during the last phases before eclosion (Merkey et al., 2011). The energy needed for the process is stored in the form of triglycerides in the fat body. In addition to fueling the metamorphosis, the fat body, unlike most larval tissues, does not undergo remodeling but is dispersed as free-floating fat cells. These cells have been postulated to enable a stress response in the newly eclosed adult to either avoid starvation until the first feeding or protect from dehydration until full development of the cuticle (Aguila et al., 2007; Storelli et al., 2019). Once having gained its flight ability shortly after eclosion, the adult will find a mate to restart the cycle. The duration of the cycle is highly dependent on temperature; the higher the temperature, the faster the cycle.

44 Figure 2.6. Life cycle of Drosophila.

2.5.1.1 Spermatogenesis

The male reproductive organs in Drosophila consists of two coiled, tube-shaped testes. The process of spermatogenesis starts in the germinal proliferation center at the apical tip of the testis. A stem cell undergoes mitosis producing a new stem cell and a primary spermatogonial cell. While the stem cells remain attached to the apical hub, the primary spermatogonial cell is enclosed by two somatic cyst cells and through gonial mitotic divisions produces 16 spermatocytes forming the spermatogenic unit known as the spermatogonial cyst, within which meiosis and spermatid differentiation will occur (Fuller, 1993). Spermatogenesis is regulated by non-apoptotic programmed cell death, whereby almost one third of the formed cysts are eliminated, never completing the spermatogenesis. In the remaining cysts, spermatocytes undergo meiosis and finally differentiate into 64 elongated spermatids

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(Yalonetskaya et al., 2018). The spermatids then go through the invidualization process where a cystic bulge moves along the spermatids from head to tail, a process during which, in an apoptosis-like process, the spermatids lose the major part of their cytoplasm, which is collected in a ‘waste bag’ structure, after which the spermatids become detached from the cyst (Arama et al., 2003). Mature sperm is then released into the seminal vesicle located in the proximal end of the testis. After copulation, sperm is stored in the spermathecal and seminal receptacle of the female reproductive tract for later use (Fuller, 1993).

2.5.1.2 Nutrition

In nature, the main diet for many Drosophila species including Drosophila melanogaster is decaying fruit and vegetables. Volatiles such as ethanol attract the flies to arrive at the source on a specific time point of the decay trajectory. The flies, as both adults and larvae, take their essential nutrition from the plant and from microbes involved in the decomposition process (Markow, 2015; McKenzie & Parsons, 1972; Nunney, 1996). A recent study by Brankatschk et al. (2018) suggested that the dietary behaviour of flies is temperature dependent i.e. in a colder climate Drosophila prefer feeding on plants over microbes or yeast. Polyunsaturated fatty acids (PUFAs) of plant origin increase the fluidity of lipid membranes and are proposed to help acclimatize the fly’s metabolism to temperature changes in the environment (Brankatschk et al., 2018). The latter study was conducted under laboratory conditions on laboratory fly strains and, therefore, comparisons with feeding behaviour in the natural habitat should be made with caution.

Under laboratory conditions, the environment is highly controlled in regard to temperature, humidity and diet. Standard laboratory diet tends to be rich in calories and treated with antimicrobial and antifungal agents, leading to a lack of diversity in microbes and micronutrients (Piper, 2017). One issue in developing an optimal Drosophila diet for laboratory use is the variation in nutritional needs in fly strains of

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different genetic backgrounds (Aw et al., 2018; Melvin et al., 2018). For example, the optimal protein:carbohydrate ratio for development and egg laying seems to be affected by the mitochondrial haplotype of the fly strain. This, in turn, is linked to the level of mitochondrial metabolism that enables efficient intake and use of nutrients, suggesting that the genotypic background defines, at least to some extent, the metabolic response and stress caused by different macronutrients. Flies also seem to differ in their preference for different types of sugars (Aw et al., 2018).

Comparison of the recently diverged Drosophila species, Drosophila simulans and sechellia, demonstrated significant differences in sugar tolerance with changes in genes encoding components of the mitochondrial ribosome (Melvin et al., 2018).

Nutritional needs of Drosophila, although modest, are difficult to define but linked, at least partially, to mitochondrial metabolism.

Despite the variation in nutritional requirements between different strains, Drosophila presents a valuable model for studies connecting diet with phenotypic traits and clinical symptoms of diseases such as diabetes and Parkinson’s disease (Bajracharya et al., 2017; Birse et al., 2010; Havula et al., 2013). In addition to conserved metabolic pathways compared to those of mammals and the availability of well-established tools for metabolic studies described above, manipulation of environmental and dietary conditions of flies in order to study genotype-phenotype relations is relatively uncomplicated.

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