reduced ocelli encodes the leucine rich repeat protein Pray For Elves in Drosophila melanogaster

Eye development influences horn size but not patterning in horned beetles

Understanding the origin of novel morphological traits is a long-standing objective in evolutionary developmental biology. We explored the developmental genetic mechanisms that underpin the formation of a textbook example of evolutionary novelties, the cephalic horns of beetles. Previous work has implicated the gene regulatory networks associated with compound eye and ocellar development in horn formation and suggested that horns and compound eyes may influence each other's sizes. Therefore, we investigated the functional significance of genes central to visual system formation in the initiation, patterning, and size determination of head horns across three horned beetle species. We find that while the downregulation of canonical eye patterning genes reliably reduces or eliminates compound eye formation, it does not alter the position or shape of head horns yet does result in an increase in relative horn length. We discuss the implications of our results for our understanding of the genesis of cephalic horns in particular and evolutionary novelties in general.

The retrograde IFT dynein is required for normal function of diverse mechanosensory cilia in Drosophila

Introduction

Cilia biogenesis relies on intraflagellar transport (IFT), a conserved transport mechanism which functions bi-directionally to bring protein complexes to the growing ciliary tip and recycle signaling and transport proteins between the cilium and cell body. In Drosophila, anterograde IFT is critical for assembly of sensory cilia in the neurons of both chordotonal (ch) organs, which have relatively long ciliary axonemes, and external sensory (es) organs, which have short axonemal segments with microtubules in distal sensory segments forming non-axonemal bundles. We previously isolated the beethoven (btv) mutant in a mutagenesis screen for auditory mutants. Although many btv mutant flies are deaf, some retain a small residual auditory function as determined both by behavior and by auditory electrophysiology.

Results

Here we molecularly characterize the btv gene and demonstrate that it encodes the IFT-associated dynein-2 heavy chain Dync2h1. We also describe morphological changes in Johnston's organ as flies age to 30 days, and we find that morphological and electrophysiological phenotypes in this ch organ of btv mutants become more severe with age. We show that NompB protein, encoding the conserved IFT88 protein, an IFT complex B component, fails to be cleared from chordotonal cilia in btv mutants, instead accumulating in the distorted cilia. In macrochaete bristles, a class of es organ, btv mutants show a 50% reduction in mechanoreceptor potentials.

Discussion

Thus, the btv-encoded Dync2h1 functions as the retrograde IFT motor in the assembly of long ciliary axonemes in ch organs and is also important for normal function of the short ciliary axonemes in es organs.

Nebulous without white: annotated long-read genome assembly and CRISPR/Cas9 genome engineering in Drosophila nebulosa

The diversity among Drosophila species presents an opportunity to study the molecular mechanisms underlying the evolution of biological phenomena. A challenge to investigating these species is that, unlike the plethora of molecular and genetics tools available for D. melanogaster research, many other species do not have sequenced genomes; a requirement for employing these tools. Selecting transgenic flies through white (w) complementation has been commonly practiced in numerous Drosophila species. While tolerated, the disruption of w is associated with impaired vision, among other effects in D. melanogaster. The D. nebulosa fly has a unique mating behavior which requires vision, and is thus unable to successfully mate in dark conditions. Here, we hypothesized that the disruption of w will impede mating success. As a first step, using PacBio long-read sequencing, we assembled a high-quality annotated genome of D. nebulosa. Using these data, we employed CRISPR/Cas9 to successfully disrupt the w gene. As expected, D. nebulosa males null for w did not court females, unlike several other mutant strains of Drosophila species whose w gene has been disrupted. In the absence of mating, no females became homozygous null for w. We conclude that gene disruption via CRISPR/Cas9 genome engineering is a successful tool in D. nebulosa, and that the w gene is necessary for mating. Thus, an alternative selectable marker unrelated to vision is desirable.

Dynamic sex chromosome expression in Drosophila male germ cells

Given their copy number differences and unique modes of inheritance, the evolved gene content and expression of sex chromosomes is unusual. In many organisms the X and Y chromosomes are inactivated in spermatocytes, possibly as a defense mechanism against insertions into unpaired chromatin. In addition to current sex chromosomes, Drosophila has a small gene-poor X-chromosome relic (4th) that re-acquired autosomal status. Here we use single cell RNA-Seq on fly larvae to demonstrate that the single X and pair of 4th chromosomes are specifically inactivated in primary spermatocytes, based on measuring all genes or a set of broadly expressed genes in testis we identified. In contrast, genes on the single Y chromosome become maximally active in primary spermatocytes. Reduced X transcript levels are due to failed activation of RNA-Polymerase-II by phosphorylation of Serine 2 and 5.

The sexual identity of adult intestinal stem cells controls organ size and plasticity

Sex differences in physiology and disease susceptibility are commonly attributed to developmental and/or hormonal factors, but there is increasing realisation that cell-intrinsic mechanisms play important and persistent roles^1,2. Here we use the Drosophila melanogaster intestine to investigate the nature and significance of cellular sex in an adult somatic organ in vivo. We find that the adult intestinal epithelium is a cellular mosaic of different sex differentiation pathways, and displays extensive sex differences in expression of genes with roles in growth and metabolism. Cell-specific reversals of the sexual identity of adult intestinal stem cells uncover its key roles in controlling organ size, its reproductive plasticity and its response to genetically induced tumours. Unlike previous examples of sexually dimorphic somatic stem cell activity, the sex differences in intestinal stem cell behaviour arise from intrinsic mechanisms, which control cell cycle duration and involve a new doublesex- and fruitless-independent branch of the sex differentiation pathway downstream of transformer. Together, our findings indicate that the plasticity of an adult somatic organ is reversibly controlled by its sexual identity, imparted by a new mechanism that may be active in more tissues than previously recognised.

The let-7-Imp axis regulates ageing of the Drosophila testis stem-cell niche

Adult stem cells support tissue homeostasis and repair throughout the life of an individual. During ageing, numerous intrinsic and extrinsic changes occur that result in altered stem-cell behaviour and reduced tissue maintenance and regeneration. In the Drosophila testis, ageing results in a marked decrease in the self-renewal factor Unpaired (Upd), leading to a concomitant loss of germline stem cells. Here we demonstrate that IGF-II messenger RNA binding protein (Imp) counteracts endogenous small interfering RNAs to stabilize upd (also known as os) RNA. However, similar to upd, Imp expression decreases in the hub cells of older males, which is due to the targeting of Imp by the heterochronic microRNA let-7. In the absence of Imp, upd mRNA therefore becomes unprotected and susceptible to degradation. Understanding the mechanistic basis for ageing-related changes in stem-cell behaviour will lead to the development of strategies to treat age-onset diseases and facilitate stem-cell-based therapies in older individuals.

The transmembrane LRR protein DMA-1 promotes dendrite branching and growth in C. elegans

Dendrites often adopt complex branched structures. The development and organization of these arbors fundamentally determine the potential input and connectivity of a given neuron. The cell-surface receptors that control dendritic branching remain poorly understood. Here, we show that in Caenorhabditis elegans, a previously uncharacterized transmembrane protein containing extracellular leucine-rich repeat (LRR) domains, which we name DMA-1 (Dendrite-Morphogenesis-Abnormal), promotes dendrite branching and growth. Sustained expression of dma-1 is found only in the elaborately branched sensory neurons PVD and FLP. Genetic analysis showed that loss of dma-1 causes much reduced dendritic arbors while overexpression of dma-1 results in excessive branching. Forced expression of dma-1 in neurons with simple dendrites was sufficient to promote ectopic branching. Animals lacking dma-1 are defective in sensing harsh touch. DMA-1 is the first transmembrane LRR protein to be implicated in dendritic branching and expands the breadth of roles played by LRR receptors in nervous system development.

The role of the RING-finger protein Elfless in Drosophila spermatogenesis and apoptosis

elfless (CG15150, FBgn0032660) maps to polytene region 36DE 5' (left) of reduced ocelli/Pray for Elves (PFE) on chromosome 2L and is predicted to encode a 187 amino acid RING finger E3 ubiquitin ligase that is putatively involved in programmed cell death (PCD, e.g., apoptosis). Several experimental approaches were used to characterize CG15150/elfless and test whether defects in this gene underlie the male sterile phenotype associated with overlapping chromosomal deficiencies of region 36DE. elfless expression is greatly enhanced in the testes and the expression pattern of UAS-elfless-EGFP driven by elfless-Gal4 is restricted to the tail cyst cell nuclei of the testes. Despite this, elfless transgenes failed to rescue the male sterile phenotype in Df/Df flies. Furthermore, null alleles of elfless, generated either by imprecise excision of an upstream P-element or by FLP-FRT deletion between two flanking piggyBac elements, are fertile. In a gain-of-function setting in the eye, we found that elfless genetically interacts with key members of the apoptotic pathway including the initiator caspase Dronc and the ubiquitin conjugating enzyme UbcD1. DIAP1, but not UbcD1, protein levels are increased in heads of flies expressing Elfless-EGFP in the eye, and in testes of flies expressing elfless-Gal4 driven Elfless-EGFP. Based on these findings, we speculate that Elfless may regulate tail cyst cell degradation to provide an advantageous, though not essential, function in the testis.