Karen G. Hales

Developmentally Regulated Mitochondrial Fusion Mediated by a Conserved, Novel, Predicted GTPase

The Drosophila melanogaster fuzzy onions (fzo) gene encodes the first known protein mediator of mitochondrial fusion. During Drosophila spermatogenesis, mitochondria in early postmeiotic spermatids aggregate, fuse, and elongate beside the growing flagellar axoneme. fzo mutant males are defective in this developmentally regulated mitochondrial fusion and are sterile. fzo encodes a large, novel, predicted transmembrane GTPase that becomes detectable on spermatid mitochondria late in meiosis II, just prior to fusion, and disappears soon after fusion is complete. Missense mutations that alter conserved residues required for GTP binding in other GTPases inhibit the fusogenic activity of Fzo in vivo but do not affect its localization. Fzo has homologs of unknown function in mammals, nematodes, and yeast.

Asunder is a critical regulator of dynein-dynactin localization during Drosophila spermatogenesis.

Spermatogenesis uses mitotic and meiotic cell cycles coordinated with growth and differentiation programs to generate functional sperm. Our analysis of a Drosophila mutant has revealed that asunder (asun), which encodes a conserved protein, is an essential regulator of spermatogenesis. asun spermatocytes arrest during prophase of meiosis I. Strikingly, arrested spermatocytes contain free centrosomes that fail to stably associate with the nucleus. Spermatocytes that overcome arrest exhibit severe defects in meiotic spindle assembly, chromosome segregation, and cytokinesis. Furthermore, the centriole-derived basal body is detached from the nucleus in asun postmeiotic spermatids, resulting in abnormalities later in spermatogenesis. We find that asun spermatocytes and spermatids exhibit drastic reduction of perinuclear dynein-dynactin, a microtubule motor complex. We propose a model in which asun coordinates spermatogenesis by promoting dynein-dynactin recruitment to the nuclear surface, a poorly understood process required for nucleus-centrosome coupling at M phase entry and fidelity of meiotic divisions.

Genetics on the Fly: A Primer on the Drosophila Model System

Fruit flies of the genus Drosophila have been an attractive and effective genetic model organism since Thomas Hunt Morgan and colleagues made seminal discoveries with them a century ago. Work with Drosophila has enabled dramatic advances in cell and developmental biology, neurobiology and behavior, molecular biology, evolutionary and population genetics, and other fields. With more tissue types and observable behaviors than in other short-generation model organisms, and with vast genome data available for many species within the genus, the fly’s tractable complexity will continue to enable exciting opportunities to explore mechanisms of complex developmental programs, behaviors, and broader evolutionary questions. This primer describes the organism’s natural history, the features of sequenced genomes within the genus, the wide range of available genetic tools and online resources, the types of biological questions Drosophila can help address, and historical milestones.

Iron testes: sperm mitochondria as a context for dissecting iron metabolism.

A recent paper in BMC Developmental Biology reports that a mitochondrial iron importer is required for Drosophila male fertility and normal mitochondrial shaping in spermatids. This suggests that mitochondrial morphogenesis during insect spermatogenesis may be a useful new context in which to study iron metabolism.See research article http://www.biomedcentral.com/1471-213X/10/68

Testis-specific ATP synthase peripheral stalk subunits required for tissue-specific mitochondrial morphogenesis in Drosophila

BackgroundIn Drosophila early post-meiotic spermatids, mitochondria undergo dramatic shaping into the Nebenkern, a spherical body with complex internal structure that contains two interwrapped giant mitochondrial derivatives. The purpose of this study was to elucidate genetic and molecular mechanisms underlying the shaping of this structure.ResultsThe knotted onions (knon) gene encodes an unconventionally large testis-specific paralog of ATP synthase subunit d and is required for internal structure of the Nebenkern as well as its subsequent disassembly and elongation. Knon localizes to spermatid mitochondria and, when exogenously expressed in flight muscle, alters the ratio of ATP synthase complex dimers to monomers. By RNAi knockdown we uncovered mitochondrial shaping roles for other testis-expressed ATP synthase subunits.ConclusionsWe demonstrate the first known instance of a tissue-specific ATP synthase subunit affecting tissue-specific mitochondrial morphogenesis. Since ATP synthase dimerization is known to affect the degree of inner mitochondrial membrane curvature in other systems, the effect of Knon and other testis-specific paralogs of ATP synthase subunits may be to mediate differential membrane curvature within the Nebenkern.

The Molecular and Genetic Characterization of Second Chromosome Balancers in Drosophila melanogaster

Balancer chromosomes are multiply inverted and rearranged chromosomes used in Drosophila melanogaster for many tasks, such as maintaining mutant alleles in stock and complex stock construction. Balancers were created before molecular characterization of their breakpoints was possible, so the precise locations of many of these breakpoints are unknown. Here, we report or confirm the positions of the 14 euchromatic breakpoints on the 2nd chromosome balancers SM1, SM5, CyO, and SM6a. This total includes three breakpoints involved in a complex rearrangement on SM5 that is associated with the duplication of two genomic regions. Unbiased sequencing of several balancers allowed us to identify stocks with incorrectly identified balancers as well as single and double crossover events that had occurred between 2nd chromosome balancers and their homologs. The confirmed crossover events that we recovered were at least 2 Mb from the closest inversion breakpoint, consistent with observations from other balancer chromosomes. Balancer chromosomes differ from one another both by large tracts of sequence diversity generated by recombination and by small differences, such as single nucleotide polymorphisms (SNPs). Therefore, we also report loss-of-function mutations carried by these chromosomes and unique SNP and InDel polymorphisms present on only single balancers. These findings provide valuable information about the structure of commonly used 2nd chromosome balancers and extend recent work examining the structure of X and 3rd chromosome balancers. Finally, these observations provide new insights into how the sequences of individual balancers have diverged over time.

Denying Genetic Causality

This edited volume of essays presents a countermainstream view against genetic underpinnings for cancer, behavior, and psychiatric conditions.