Organization of polygenic systems: Cell death modifiers from natural populations of Drosophila melanogaster

MicroRNA miR-252-5p regulates the Notch signaling pathway by targeting Rab6 in Drosophila wing development

The Notch signaling pathway plays a central role in the development of various organisms. However, dysregulation of microRNAs (miRNAs), which are crucial regulators of gene expression, can disrupt signaling pathways at all stages of development. Although Notch signaling is involved in wing development in Drosophila, the mechanism underlying miRNA-based regulation of the Notch signaling pathway is unclear. Here, we report that loss of Drosophila miR-252 increases the size of adult wings, whereas the overexpression of miR-252 in specific compartments of larval wing discs leads to patterning defects in the adult wings. The miR-252 overexpression-induced wing phenotypes were caused by aberrant Notch signaling with intracellular accumulation of the full-length Notch receptor during development, which could be due to defects in intracellular Notch trafficking associated with its recycling to the plasma membrane and autophagy-mediated degradation. Moreover, we identified Rab6 as a direct target of miR-252-5p; Rab6 encodes a small Ras-like GTPase that regulates endosomal trafficking pathways. Consistent with this finding, RNAi-mediated downregulation of Rab6 led to similar defects in both wing patterning and Notch signaling. Notably, co-overexpression of Rab6 completely rescued the wing phenotype associated with miR-252 overexpression, further supporting that Rab6 is a biologically relevant target of miR-252-5p in the context of wing development. Thus, our data indicate that the miR-252-5p-Rab6 regulatory axis is involved in Drosophila wing development by controlling the Notch signaling pathway.

Measuring selective effects of modifier gene polymorphisms on the Bare locus of Drosophila subobscura

An attempt to quantify the effects of modifier gene polymorphisms on the operation of natural selection on a major locus has been carried out. The modifier system we have investigated is constituted by a set of polygenic modifier loci affecting the morphological expression of the Bare (Ba) bristle mutant of Drosophila subobscura. Ba is a dominant mutant that is lethal in homozygous condition and both the polygenic modifiers and Ba are located on the O chromosome of this species. Experimental populations were found with Ba/+ individuals and two different types of populations were started according to their modifier genetic background: populations with wild O chromosomes of either high or low modifier effect (cages H and L, respectively). Fitness estimates (total fitness, viability and fertility) for genotypes of the Ba locus were obtained under the two different modifier backgrounds. In the populations with high modifier background the total fitness of the Ba/+ heterozygote was very similar to that of the +/+ homozygote (fitness equal to 1). However, in cages with low modifier background a strong selection against the Ba/+ heterozygote was detected (average of total fitnesses over generations was 0.66 +/- 0.10), and fertility appears to be the fitness component responsible for this effect (mean fertility was 0.55 +/- 0.08). These findings demonstrate that modifier gene polymorphisms affecting the expression of the Ba mutant may be associated with large selective effects on the major locus.

Genetic variation in a modifier system affecting the expression of bare mutant of Drosophila subobscura

Genetic variation affecting the expression of Bare (Ba), a morphological mutant of Drosophila subobscura that reduces the number of bristles and is located on the O chromosome of this species, is reported. Our results show that O chromosomes extracted from a natural population (El Pedroso, Santiago de Compostela, Spain) show considerable genetic variation in modifier effect upon Ba expression. The amount of modifier variability is dependent on the chromosomal arrangement (OST and O3 + 4 + 7), since modifier variation is higher in OST than in O3 + 4 + 7 chromosomes. Investigations of the genetic architecture of this modifier system on the O chromosome carried out using biometrical methods indicate that a relatively small number of genetic "factors" can explain the differences in modifier effect between a wild O chromosome of high modifier effect and a marker chromosome of low score. In addition, the modifier effects show a non-uniform distribution along the O chromosome and some indication of clustering of modifier "factors" around the major locus (Ba) is also observed.

Developmental effects of modifiers of the vg mutant in Drosophila melanogaster

An analysis of the modifiers affecting the expression of the vg gene was performed. We selected for weak and strong expression of the vg mutant in F2 segregating populations obtained by crossing a vestigial stock with an Oregon laboratory stock (O) and with a wild strain (B) captured near Bologna, Italy. The selection for enlarged wings was more effective in the vg B population where wild wings appeared from the 10th generation. The assay of the three major chromosomes showed that the modifiers are located on chromosomes 2 and 3. The mutant imaginal disc cell death phenotype is evident in vg/vg strains that have a wild-type wing phenotype. It is suggested that the selected modifiers do not prevent cell death but induce regenerative growth.