A Genetic Screen for Dominant Modifiers of a Small-Wing Phenotype in Drosophila Melanogaster Identifies Proteins Involved in Splicing and Translation

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Studies in the fly, Drosophila melanogaster, have revealed that several signaling pathways are important for the regulation of growth. Among these, the insulin receptor/phosphoinositide 3-kinase (PI3K) pathway is remarkable in that it affects growth and final size without disturbing pattern formation. We have used a small-wing phenotype, generated by misexpression of kinase-dead PI3K, to screen for novel mutations that specifically disrupt organ growth in vivo. We identified several complementation groups that dominantly enhance this small-wing phenotype. Meiotic recombination in conjunction with visible markers and single-nucleotide polymorphisms (SNPs) was used to map five enhancers to single genes. Two of these, nucampholin and prp8, encode pre-mRNA splicing factors. The three other enhancers encode factors required for mRNA translation: pixie encodes the Drosophila ortholog of yeast RLI1, and RpL5 and RpL38 encode proteins of the large ribosomal subunit. Interestingly, mutations in several other ribosomal protein-encoding genes also enhance the small-wing phenotype used in the original screen. Our work has therefore identified mutations in five previously uncharacterized Drosophila genes and provides in vivo evidence that normal organ growth requires optimal regulation of both pre-mRNA splicing and mRNA translation.

NORMAL biological development and homeostasis require tight control of growth at the level of individual cells, organs, and the whole organism. For example, unregulated cellular proliferation may result in too few or too many cells, leading to inappropriately sized, nonfunctional organs, which in turn can result in a variety of pathological conditions. Significantly, individuals of the same species raised in a given environmental niche grow to similar final sizes. This means that growth is controlled, at least to some extent, genetically. Such "growth genes" might encode key components or effectors of discrete growth-regulatory signaling pathways.

In recent years, the fruit fly, Drosophila melanogaster, has been used as a model organism to investigate the genetic basis of growth control (reviewed in EDGAR and NIJHOUT 2004). Most of these studies have focused on the growth of the larval imaginai discs. Imaginai discs are epithelial structures that undergo massive growth during the ^4 days of larval life. The size of the disc at the end of the larval period largely determines the size of the adult appendage (eye, wing, etc.) into which it ultimately develops (reviewed in JOHNSTON and GALLANT 2002).

Key regulators of Drosophila imaginai disc growth have been discovered through three main approaches. First, classical genetic research has produced many mutant strains that exhibit growth phenotypes. For example, Minute mutants, which correspond to at least 50 different genetic loci, have a slower growth rate and sometimes an altered adult size (LAMBERTSSON 1998). These phenotypes are thought to be the result of a reduced capacity for protein synthesis. Indeed, several Minute mutations have been demonstrated to disrupt genes that encode ribosomal proteins (RPs) (LAMBERTSSON 1998).

A second way in which Drosophila growth regulators have been discovered is through the study of proteins or signaling pathways whose mammalian orthologs have been implicated in cell proliferation and/or growth. An example of this approach is the characterization of the insulin receptor/phosphoinositide 3-kinase (InR/ PI3K) pathway (reviewed in LEEVERS and HAFEN 2004). For example, clonal loss of DpIlO, which encodes the catalytic subunit of the Drosophila class IA PI3K, reduces cell size and clonal growth in imaginai discs (WEINKOVE et al. 1999). Conversely, overexpression of DpIlO increases cell size, promotes cell cycle progression, and causes tissue overgrowth (LEEVERS et al. 1996). Similar results have been obtained through modulating the activity of other components of the pathway (LEEVERS and HAFEN 2004). …