Grace Boekhoff-Falk

Hearing in Drosophila: Development of Johnston's organ and emerging parallels to vertebrate ear development

In this review, I describe recent progress toward understanding the developmental genetics governing formation of the Drosophila auditory apparatus. The Drosophila auditory organ, Johnstons organ, is housed in the antenna. Intriguingly, key genes needed for specification or function of auditory cell types in the Drosophila antenna also are required for normal development or function of the vertebrate ear. These genes include distal‐less, spalt and spalt‐related, atonal, crinkled, nanchung and inactive, and prestin, and their vertebrate counterparts Dlx, spalt‐like (sall), atonal homolog (ath), myosin VIIA, TRPV, and prestin, respectively. In addition, Drosophila auditory neurons recently were shown to serve actuating as well as transducing roles, much like their hair cell counterparts of the vertebrate cochlea. The emerging genetic and physiologic parallels have come as something of a surprise, because conventional wisdom holds that vertebrate and invertebrate hearing organs have separate evolutionary origins. The new findings raise the possibility that auditory organs are more ancient than previously thought and indicate that Drosophila is likely to be a powerful model system in which to gain insights regarding the etiologies of human deafness disorders. Developmental Dynamics 232:550–558, 2005.

The Drosophila auditory system

Development of a functional auditory system in Drosophila requires specification and differentiation of the chordotonal sensilla of Johnstons organ (JO) in the antenna, correct axonal targeting to the antennal mechanosensory and motor center in the brain, and synaptic connections to neurons in the downstream circuit. Chordotonal development in JO is functionally complicated by structural, molecular, and functional diversity that is not yet fully understood, and construction of the auditory neural circuitry is only beginning to unfold. Here, we describe our current understanding of developmental and molecular mechanisms that generate the exquisite functions of the Drosophila auditory system, emphasizing recent progress and highlighting important new questions arising from research on this remarkable sensory system. WIREs Dev Biol 2014, 3:179–191. doi: 10.1002/wdev.128

Linking pattern formation to cell-type specification : Dichaete and ind directly repress achaete gene expression in the Drosophila CNS

Mechanisms regulating CNS pattern formation and neural precursor formation are remarkably conserved between Drosophila and vertebrates. However, to date, few direct connections have been made between genes that pattern the early CNS and those that trigger neural precursor formation. Here, we use Drosophila to link directly the function of two evolutionarily conserved regulators of CNS pattern along the dorsoventral axis, the homeodomain protein Ind and the Sox-domain protein Dichaete, to the spatial regulation of the proneural gene achaete (ac) in the embryonic CNS. We identify a minimal achaete regulatory region that recapitulates half of the wild-type ac expression pattern in the CNS and find multiple putative Dichaete-, Ind-, and Vnd-binding sites within this region. Consensus Dichaete sites are often found adjacent to those for Vnd and Ind, suggesting that Dichaete associates with Ind or Vnd on target promoters. Consistent with this finding, we observe that Dichaete can physically interact with Ind and Vnd. Finally, we demonstrate the in vivo requirement of adjacent Dichaete and Ind sites in the repression of ac gene expression in the CNS. Our data identify a direct link between the molecules that pattern the CNS and those that specify distinct cell-types.

Cut mutant Drosophila auditory organs differentiate abnormally and degenerate.

The Drosophila antenna is a sophisticated structure that functions in both olfaction and audition. Previous studies have identified Homothorax, Extradenticle, and Distal-less, three homeodomain transcription factors, as required for specification of antennal identity. Antennal expression of cut is activated by Homothorax and Extradenticle, and repressed by Distal-less. cut encodes the Drosophila homolog of human CAAT-displacement protein, a cell cycle-regulated homeodomain transcription factor. Cut is required for normal development of external mechanosensory structures and Malphigian tubules (kidney analogs). The role of cut in the Drosophila auditory organ, Johnstons organ, has not been characterized. We have employed the FLP/FRT system to generate cut null clones in developing Johnstons organ. In cut mutants, the scolopidial subunits that constitute Johnstons organ differentiate abnormally and subsequently degenerate. Electrophysiological experiments confirm that adult Drosophila with cut null antennae are deaf. We find that cut acts in parallel to atonal, spalt-major, and spalt-related, which encode other transcription factors required for Johnstons organ differentiation. We speculate that Cut functions in conjunction with these factors to regulate transcription of as yet unidentified targets.

Distal-less functions in subdividing the Drosophila thoracic limb primordium

The thoracic limb primordium of Drosophila melanogaster is a useful experimental model in which to study how unique tissue types are specified from multipotent founder cell populations. The second thoracic segment limb primordium gives rise to three structures: the wing imaginal disc, the leg imaginal disc, and a larval mechanosensory structure called Keilins organ. We report that most of the limb primordium arises within neurogenic ectoderm and demonstrate that the neural and imaginal components of the primordium have distinct developmental potentials. We also provide the first analysis of the genetic pathways that subdivide the progenitor cell population into uniquely imaginal and neural identities. In particular, we demonstrate that the imaginal gene escargot represses Keilins organ fate and that Keilins organ is specified by Distal‐less in conjunction with the downstream achaete‐scute complex. This specification involves both the activation of the neural genes cut and couch potato and the repression of escargot. In the absence of achaete‐scute complex function, cells adopt mixed identities and subsequently die. We propose that central cells of the primordium previously thought to contribute to the distal leg are Keilins organ precursors, while both proximal and distal leg precursors are located more peripherally and within the escargot domain. Developmental Dynamics 232:801‐816, 2005.

Homeobox gene distal-less is required for neuronal differentiation and neurite outgrowth in the Drosophila olfactory system

Vertebrate Dlx genes have been implicated in the differentiation of multiple neuronal subtypes, including cortical GABAergic interneurons, and mutations in Dlx genes have been linked to clinical conditions such as epilepsy and autism. Here we show that the single Drosophila Dlx homolog, distal-less, is required both to specify chemosensory neurons and to regulate the morphologies of their axons and dendrites. We establish that distal-less is necessary for development of the mushroom body, a brain region that processes olfactory information. These are important examples of distal-less function in an invertebrate nervous system and demonstrate that the Drosophila larval olfactory system is a powerful model in which to understand distal-less functions during neurogenesis.

Expression of the Drosophila homeobox gene, Distal-less, supports an ancestral role in neural development

Background: Distal‐less (Dll) encodes a homeodomain transcription factor expressed in developing appendages of organisms throughout metazoan phylogeny. Based on earlier observations in the limbless nematode Caenorhabditis elegans and the primitive chordate amphioxus, it was proposed that Dll had an ancestral function in nervous system development. Consistent with this hypothesis, Dll is necessary for the development of both peripheral and central components of the Drosophila olfactory system. Furthermore, vertebrate homologs of Dll, the Dlx genes, play critical roles in mammalian brain development. Results: Using fluorescent immunohistochemistry of fixed samples and multiphoton microscopy of living Drosophila embryos, we show that Dll is expressed in the embryonic, larval and adult central nervous system and peripheral nervous system (PNS) in embryonic and larval neurons, brain and ventral nerve cord glia, as well as in PNS structures associated with chemosensation. In adult flies, Dll expression is expressed in the optic lobes, central brain regions and the antennal lobes. Conclusions: Characterization of Dll expression in the developing nervous system supports a role of Dll in neural development and function and establishes an important basis for determining the specific functional roles of Dll in Drosophila development and for comparative studies of Drosophila Dll functions with those of its vertebrate counterparts. Developmental Dynamics 245:87–95, 2016.

John F. Fallon, PhD: fifty years of excellence in limb research and counting.

Passion and thoughtfulness. . . These are an unusual combination of descriptors perhaps, but fitting for the man to whom we dedicate this special issue on limb development, Dr. John F. Fallon. John has approached his work for more than 50 years with a combination of passion and thoughtfulness that has left indelible marks on the field and on the many scientists who have entered or passed through John’s scientific universe. John Fallon was appointed Assistant Professor of Anatomy at the University of Wisconsin-Madison on January 1, 1969, after earning the PhD degree in Biology at Marquette University and serving 2 years on active duty in the US Army Medical Service Corps. He was promoted to Professor of Anatomy in 1981 and had served as a member of the faculty for 41 years upon his retirement on December 21, 2009. During these years, John published 93 peer reviewed original research articles, 20 review articles/book chapters and 4 edited proceedings. Whereas these publications cover a range of topics from the gametes of Nereis limbata to the carapacial ridge of turtles, the majority of John’s contributions are in the field of limb development, a passion he developed in the 1960s as a graduate student in John Saunders’ laboratory at Marquette University. John left his home in Massachusetts when he was thirteen years old and attended high school at Roosevelt Military Academy in Aledo, Illinois. During high school, he became very interested in philosophy and entered Marquette University to pursue undergraduate studies in this subject, also working as a window and awning salesman during college to pay for his education. By his senior year of college, John was completing Bachelor’s degrees in both Philosophy and Zoology, and had applied, and was accepted, to pursue graduate studies in the Philosophy department at Marquette. It was also during his senior year, that John enrolled in a Cell Biology course, which was taught by Dr. John Saunders. He was riveted by Dr. Saunders’ teaching style and approached Dr. Saunders after a class one day and asked whether he might work in his laboratory during his senior year. This turned out to be a career-altering event (Fig. 1). John spent the first several months washing glassware, but he was allowed to watch, and sometimes assist, Mary Gasseling as she was performing some of the very first experiments on the function of the cell death and what would become the zone of polarizing activity in the chick limb. John was hooked! Toward the end of his senior year, he decided that he no longer was interested in pursuing philosophy, but instead wanted to turn his attention to biology. Dr. Saunders personally intervened on John’s behalf to permit a change in departments for his graduate admission in Biology. During his years under Dr. Saunders’ tutelage, John was allowed to spend a few summers at Woods Hole, times that John considers transformative. He published his first paper on collaborative work done there with Dr. C.R. ‘Bunny’ Austin (the Charles Darwin Professor of Animal Embryology, University of Cambridge, UK) using electron microscopy, and recalls his anxiety at the first formal presentation on this work, where he talked about how jelly was released fromNereis limbata eggs during fertilization when the sperm makes contact with the egg. This work tested theories put forth by Drs. Alex Novikoff and Donald Costello who were both present at D ev el op m en ta l D yn am ic s