The Journal of Neurogenetics presents this special issue to honor the lifelong contributions of Karl-Friedrich Fischbach on the occasion of his retirement from university duties in March 2014. Karl-Friedrich Fischbach started his scientific career in the 1970s and since then has continuously contributed to the field of Drosophila neurogenetics. His more recent work in the Irre cell recognition module defines our current understanding of the role of these important proteins in the development of the Drosophila nervous system. Most people might associate the scientist Karl-Friedrich Fischbach with his exquisite and elegant description of the precise anatomy of several different cell types of the fly visual system. This account was first published with Andreas Dittrich in 1989 and, together with his subsequent papers within 3 years, set the stage for numerous Drosophila neurobiology projects around the world. Influenced by his early mentor—Martin Heisenberg, Karl-Friedrich furthermore aimed at establishing causal links between cell types and behavior in wild-type and mutant flies, a forerunner to the highly active field of neural circuit analysis. In addition, Karl-Friedrich took another step and identified neurons of the Drosophila brain, which participate in a specific task or computation. Together with Bernhard Bausenwein, he managed this by employing activity-dependent labeling using radioactive 2-deoxyglucose. At that time, this approach provided the most direct access to active neuronal circuitries of the Drosophila brain—a prelude to the current direct electrical or optical recordings of cellular activity in the central nervous system (CNS) of adult Drosophila.
Today, techniques for the functional analysis of neural circuits are being developed at a breathtaking speed. This transition occurred half a century after the early pioneers, for example, Seymour Benzer in the US and Martin Heisenberg in Germany, introduced Drosophila as a genetic model organism for studies on the function of the brain and control of behavior. Combined with detailed studies on the development and structure of the Drosophila nervous system, these new tools in Drosophila neurogenetics provide unprecedented power in neuroscience research. Speculations about functional implications of neural structures can now be directly verified by experimentation to obtain the precise understanding of development and function at large.
To set priorities to this transition, Karl-Friedrich Fischbach, who was facing his retirement, and Dierk Reiff, who was just starting his professorship at the Freiburg University, organized a meeting on the fruit fly as a neurogenetic model organism called “Drosophila melanogaster: from Development to Function.” Scientists from around 20 countries and different research areas gathered during the meeting in Freiburg (Germany) from September 26 to 29, 2013. Leading experts in their field agreed to participate in the Freiburg meeting and many of them have contributed to this special issue.
Karl-Friedrich's former teachers frame the contents of this special issue. In the first article, Hanns-Christof Spatz calls attention to Karl-Friedrich's very first scientific publications which he performed as undergraduate and PhD student in Spatz's lab. The final essay is by Martin Heisenberg. In “The Beauty of the Network in the Brain and the Origin of the Mind in the Control of Behavior” Heisenberg is asking fundamental questions and discusses concepts like “initiative activity” and “the probabilistic nature of action selection”, questions that fueled vivid disputes with Karl-Friedrich in the past.
We must note that it is Karl-Friedrich Fischbach's extraordinary efforts in the assembly of a rich body of contributors from the meeting attendees and his unfailing help in the editorial process, which have made this special issue possible. The accomplishment of this unique collection of papers reflects Karl's diligence and concentration, personal traits well known to his colleagues and friends. These articles can be grouped into three sets, all of which contain reviews as well as original research contributions.
The first set contains articles on “General genetic and developmental mechanisms.” A review by Gerhard M. Technau, Ana Rogulja-Ortmann, Christian Berger, Oliver Birkholz, and Christof Rickert is about hox genes in the embryonic CNS of Drosophila. It summarizes the formation and specialization of segmental units along the anterior–posterior axes. Holger Apitz and Iris Salecker analyze neurogenesis in the postembryonic Drosophila optic lobe, where neurogenesis results in the abundance of cell types in fixed numeral relations. Yanrui Jiang and Heinrich Reichert subsequently discuss the recent advances in stem cell and cancer biology using Drosophila as a model system. In their original research paper on “Optomotor-Blind”, Aditya Sen, Stefan Grimm, Kerstin Hofmeyer, and Gert O. Pflugfelder identify the region from which the HS/VS lobular plate tangential cells are likely to develop. Tripti Gupta and Angela Giangrande review cellular and molecular mechanisms underlying collective cell migration in animal models and advances in the field based on Drosophila wing glia research. Carmen Mohr and Britta Hartmann give a timely overview about how alternative splicing greatly enhances the diversity in gene expression in the nervous system. To conclude this set, Egemen Agi, Marion Langen, Steven Altschuler, Lani Wu, Timo Zimmermann, and Peter Robin Hiesinger review the evolution and development of neural superposition, a long-standing question in dipteran neurobiology.
The second set of articles covers the functions of the proteins related to the Irre Cell Recognition Module. Martin Helmstäder, Martin Höhne, and Tobias B. Huber provide an overall review of this interesting class of cell adhesion proteins. In their original research paper, Mara Silvia A. Costa, Maiaro Cabral R. Machado, Felipe M. Vieceli, Luana Amistá, Jose Eduardo Baroneza, C.Y. Irene Yan, and Ricardo Guelerman P. Ramos describe the expression pattern of the Neph-like subfamily of IRM proteins in an avian system. Sujin Bao reviews the role of Rst and its partners in the assembly of the Drosophila compound eye. Kevin Lüthy, Birgit Ahrens, Shilpa Rawal, Zhiyuan Lu, Dorota Tarnogorska, Ian A. Meinertzhagen, and Karl-Friedrich Fischbach show in their original research paper that the IRM protein Kirre is required to form the reciprocal synaptic network of L4 neurons in the Drosophila lamina. The last paper of this section by Susanne Önel, Marco Rust, Raif Jacob, and Renate Renkawitz-Pohl speculates on common molecular and cellular mechanisms in myoblast adhesion/fusion and trans-synaptic adhesion/synaptic vesicle exocytosis.
The third and last set covers “Functional Aspects of the Drosophila Nervous System.” In their original research paper, Atulya Iyengar and Chun-Fang Wu describe different flight and seizure motor patterns in Drosophila mutants by performing simultaneous acoustic and electrophysiological recordings of wing beat and flight muscle activity. Charlotte Helfrich-Förster gives a very personal review of her contributions to the elucidation of the neural implementation of the circadian clock in Drosophila. Mariel M. Velez, Daryl Gohl, Thomas R. Clandinin, and Mathias F. Wernet contribute an original paper about separate neural circuits for the guidance of behavioral responses to polarized light presented to either the dorsal or ventral retina in Drosophila. Alexander Borst reviews the recent work of his group on the mission to elucidate the neural circuits for elementary motion detection in the fly, a problem that is awaiting ultimate solution in near future at the level of all neurons, synapses, and computations in the circuitry. Krishna V. Melnattur, Randal Pursley, Tsu-Yang Lin, Chun-Yuan Ting, Paul D. Smith, Thomas Pohida, and Chi-Hon Lee perform a similar task on the circuitry underlying color vision. In their original paper, they show that multiple redundant medulla projection neurons mediate color vision in Drosophila.
Taken together, these articles embrace traditional and most advanced approaches in the field, mark important contributions over a period of about 40 years, and demonstrate the power of Drosophila neurobiology. Furthermore, the special issue aims to demonstrate the substantial changes in the field as well as the impact Karl-Friedrich Fischbach had on this development. Based on these changes, future neurogenetics studies in Drosophila will increasingly encompass both developmental and functional aspects and live up to Karl-Friedrich's notion that “methodologically and conceptionally behavioral analysis strongly profits from a developmental viewpoint.”