Molecular cloning of linotte in Drosophila: a novel gene that functions in adults during associative learning

Receptor tyrosine kinases in Drosophila development

Tyrosine phosphorylation plays a significant role in a wide range of cellular processes. The Drosophila genome encodes more than 20 receptor tyrosine kinases and extensive studies in the past 20 years have illustrated their diverse roles and complex signaling mechanisms. Although some receptor tyrosine kinases have highly specific functions, others strikingly are used in rather ubiquitous manners. Receptor tyrosine kinases regulate a broad expanse of processes, ranging from cell survival and proliferation to differentiation and patterning. Remarkably, different receptor tyrosine kinases share many of the same effectors and their hierarchical organization is retained in disparate biological contexts. In this comprehensive review, we summarize what is known regarding each receptor tyrosine kinase during Drosophila development. Astonishingly, very little is known for approximately half of all Drosophila receptor tyrosine kinases.

Learning and memory in Drosophila: behavior, genetics, and neural systems

The rich behavioral repertoire that Drosophila use to navigate in their natural environment suggests that flies can use memories to inform decisions. Development of paradigms to examine memories that restrict behavioral choice was essential in furthering our understanding of the genetics and neural systems of memory formation in the fly. Olfactory, visual, and place memory paradigms have proven influential in determining principles for the mechanisms of memory formation. Several parts of the nervous system have been shown to be important for different types of memories, including the mushroom bodies and the central complex. Thus far, about 40 genes have been linked to normal olfactory short-term memory. A subset of these genes have also been tested for a role in visual and place memory. Some genes have a common function in memory formation, specificity of action comes from where in the nervous system these genes act. Alternatively, some genes have a more restricted role in different types of memories.

Overexpression screen in Drosophila identifies neuronal roles of GSK-3 beta/shaggy as a regulator of AP-1-dependent developmental plasticity

AP-1, an immediate-early transcription factor comprising heterodimers of the Fos and Jun proteins, has been shown in several animal models, including Drosophila, to control neuronal development and plasticity. In spite of this important role, very little is known about additional proteins that regulate, cooperate with, or are downstream targets of AP-1 in neurons. Here, we outline results from an overexpression/misexpression screen in Drosophila to identify potential regulators of AP-1 function at third instar larval neuromuscular junction (NMJ) synapses. First, we utilize >4000 enhancer and promoter (EP) and EPgy2 lines to screen a large subset of Drosophila genes for their ability to modify an AP-1-dependent eye-growth phenotype. Of 303 initially identified genes, we use a set of selection criteria to arrive at 25 prioritized genes from the resulting collection of putative interactors. Of these, perturbations in 13 genes result in synaptic phenotypes. Finally, we show that one candidate, the GSK-3beta-kinase homolog, shaggy, negatively influences AP-1-dependent synaptic growth, by modulating the Jun-N-terminal kinase pathway, and also regulates presynaptic neurotransmitter release at the larval neuromuscular junction. Other candidates identified in this screen provide a useful starting point to investigate genes that interact with AP-1 in vivo to regulate neuronal development and plasticity.

Quantitative trait loci for locomotor behavior in Drosophila melanogaster

Locomotion is an integral component of most animal behaviors and many human diseases and disorders are associated with locomotor deficits, but little is known about the genetic basis of natural variation in locomotor behavior. Locomotion is a complex trait, with variation attributable to the joint segregation of multiple interacting quantitative trait loci (QTL), with effects that are sensitive to the environment. We assessed variation in a component of locomotor behavior (locomotor reactivity) in a population of 98 recombinant inbred lines of Drosophila melanogaster and mapped four QTL affecting locomotor reactivity by linkage to polymorphic roo transposable element insertion sites. We used complementation tests of deficiencies to fine map these QTL to 12 chromosomal regions and complementation tests of mutations to identify 13 positional candidate genes affecting locomotor reactivity, including Dopa decarboxylase (Ddc), which catalyzes the final step in the synthesis of serotonin and dopamine. Linkage disequilibrium mapping in a population of 164 second chromosome substitution lines derived from a single natural population showed that polymorphisms at Ddc were associated with naturally occurring genetic variation in locomotor behavior. These data implicate variation in the synthesis of bioamines as a factor contributing to natural variation in locomotor reactivity.

NMR structure of the WIF domain of the human Wnt-inhibitory factor-1

The human Wnt-binding protein Wnt-inhibitory factor-1 (WIF-1) comprises an N-terminal WIF module followed by five EGF-like repeats. Here we report the three-dimensional structure of the WIF domain of WIF-1 determined by NMR spectroscopy. The fold consists of an eight-stranded beta-sandwich reminiscent of the immunoglobulin fold. Residual detergent (Brij-35) used in the refolding protocol was found to bind tightly to the WIF domain. The binding site was identified by intermolecular nuclear Overhauser effects observed between the WIF domain and the alkyl chain of the detergent. The results point to a possible role of WIF domains as a recognition motif of Wnt and Drosophila Hedgehog proteins that are activated by palmitoylation.

Thirty years of olfactory learning and memory research in Drosophila melanogaster

The last 30 years have witnessed tremendous progress in elucidating the basic mechanisms underlying a simple form of olfactory learning and memory in Drosophila. The application of the mutagenic approach to the study of olfactory learning and memory in Drosophila has yielded insights into the participation of a large number of genes in both the development of critical brain regions as well as in the physiology underlying the acquisition, storage, and retrieval of memory. Newer sophisticated molecular-genetic tools have further allowed for the specification and functional dissection of the neuronal circuitry involved in these processes at a systems level. With these advances in our understanding of the genes, neurons, and circuits involved in learning and memory, the field of Drosophila memory research is nearing a state of integration of the bottom up and top down approaches to understanding this form of behavioral plasticity.

Olfactory memory formation in Drosophila: from molecular to systems neuroscience

The olfactory nervous system of insects and mammals exhibits many similarities, which suggests that the mechanisms for olfactory learning may be shared. Molecular genetic investigations of Drosophila learning have uncovered numerous genes whose gene products are essential for olfactory memory formation. Recent studies of the products of these genes have continued to expand the range of molecular processes known to underlie memory formation. Recent research has also broadened the neuroanatomical areas thought to mediate olfactory learning to include the antennal lobes in addition to a previously accepted and central role for the mushroom bodies. The roles for neurons extrinsic to the mushroom body neurons are becoming better defined. Finally, the genes identified to participate in Drosophila olfactory learning have conserved roles in mammalian organisms, highlighting the value of Drosophila for gene discovery.

The Drosophila ortholog of the human Wnt inhibitor factor Shifted controls the diffusion of lipid-modified Hedgehog

The Hedgehog (Hh) family of morphogenetic proteins has important instructional roles in metazoan development and human diseases. Lipid modified Hh is able to migrate to and program cells far away from its site of production despite being associated with membranes. To investigate the Hh spreading mechanism, we characterized Shifted (Shf) as a component in the Drosophila Hh pathway. We show that Shf is the ortholog of the human Wnt inhibitory factor (WIF), a secreted antagonist of the Wingless pathway. In contrast, Shf is required for Hh stability and for lipid-modified Hh diffusion. Shf colocalizes with Hh in the extracellular matrix and interacts with the heparan sulfate proteoglycans (HSPG), leading us to suggest that Shf could provide HSPG specificity for Hh. We also show that human WIF inhibits Wg signaling in Drosophila without affecting the Hh pathway, indicating that different WIF family members might have divergent functions in each pathway.

Defective neuronal development in the mushroom bodies of Drosophila fragile X mental retardation 1 mutants

Fragile X mental retardation 1 (Fmr1) is a highly conserved gene with major roles in CNS structure and function. Its product, the RNA-binding protein FMRP, is believed to regulate translation of specific transcripts at postsynaptic sites in an activity-dependent manner. Hence, Fmr1 is central to the molecular mechanisms of synaptic plasticity required for normal neuronal maturation and cognitive ability. Mutations in its Drosophila ortholog, dfmr1, produce phenotypes of brain interneurons and axon terminals at the neuromuscular junction, as well as behavioral defects of circadian rhythms and courtship. We hypothesized that dfmr1 mutations would disrupt morphology of the mushroom bodies (MBs), highly plastic brain regions essential for many forms of learning and memory. We found developmental defects of MB lobe morphogenesis, of which the most common is a failure of beta lobes to stop at the brain midline. A similar recessive beta-lobe midline-crossing phenotype has been previously reported in the memory mutant linotte. The dfmr1 MB defects are highly sensitive to genetic background, which is reminiscent of mammalian fragile-X phenotypes. Mutations of dfmr1 also interact with one or more third-chromosome loci to promote alpha/beta-lobe maturation. These data further support the use of the Drosophila model system for study of hereditary cognitive disorders of humans.

Egfr/Ras pathway mediates interactions between peripodial and disc proper cells in Drosophila wing discs

All imaginal discs in Drosophila are made up of a layer of columnar epithelium or the disc proper and a layer of squamous epithelium called the peripodial membrane. Although the developmental and molecular events in columnar epithelium or the disc proper are well understood, the peripodial membrane has gained attention only recently. Using the technique of lineage tracing, we show that peripodial and disc proper cells arise from a common set of precursors cells in the embryo, and that these cells diverge in the early larval stages. However, peripodial and disc proper cells maintain a spatial relationship even after the separation of their lineages. The peripodial membrane plays a significant role during the regional subdivision of the wing disc into presumptive wing, notum and hinge. The Egfr/Ras pathway mediates this function of the peripodial membrane. These results on signaling between squamous and columnar epithelia are particularly significant in the context of in vitro studies using human cell lines that suggest a role for the Egfr/Ras pathway in metastasis and tumour progression.

Mutation of linotte causes behavioral defects independently of pigeon in Drosophila

The Drosophila linotte1 mutation was isolated from a genetic screen designed to identify learning and memory genes. For some authors, this mutation affects a novel gene specifically involved in adult learning and memory, whereas for others, it is an allele of the derailed receptor tyrosine kinase gene (the linotte/derailed gene) involved in nervous system development. Here, we show that the original derailed mutation induces a memory phenotype. We also report that a new null mutation, lioexc21, affecting specifically the linotte/derailed gene causes behavioral defects, which can be partially rescued by expression of a lio+/drl+ transgene. The data presented here suggest that the memory phenotype of linotte and derailed mutants is a consequence of abnormal brain development due to loss of function of the linotte/derailed encoded receptor tyrosine kinase.

Cell-type-specific expression of protein tyrosine kinase-related receptor RYK in the central nervous system of the rat

The mammalian RYK is an orphan receptor that contains a catalytically inactive tyrosine-kinase-related domain. Its Drosophila homolog, Lio/Drl, is required for axon pathfinding in developing brain. Our previous study suggested that RYK mRNA is expressed in nestin-positive progenitor cells and neurons. In the present study, immunohistochemistry has been used to further localize RYK in the central nervous system of rats to identify the lineage of the RYK-expressing cells. In the embryonic forebrain, RYK colocalized with nestin in the ventricular zone and with MAP2 in the cortical plate, suggesting that RYK is expressed in neural progenitor cells and neurons. Localization of RYK in embryonic spinal cord also suggested its expression in both cell types. In primary cultures of rat cerebrum, RYK expression was observed in all neurons, as well as in a significant population of oligodendrocytes, O-2A progenitor cells, and type-2 astrocytes. However, no RYK expression was detected in type-1 astrocytes or microglia. Multipotent neural stem cell line MNS-70 was also analyzed for expression of RYK, and most of the cells were positive for both RYK and nestin in the undifferentiated stage. In the differentiated stage, expression of RYK was detected in the neurons, but not in type-1 astrocytes. In conclusion, RYK is expressed in nestin-positive progenitor cells and neurons, and in a certain population of oligodendrocytes, O-2A progenitor cells, and type-2 astrocytes in developing CNS. These findings show that expression of RYK in rat CNS is tightly regulated in a cell-type-specific manner.

What can we teach Drosophila? What can they teach us?

A number of single gene mutations dramatically reduce the ability of fruit flies to learn or to remember. Cloning of the affected genes implicated the adenylyl cyclase second-messenger system as key in learning and memory. The expression patterns of these genes, in combination with other data, indicates that brain structures called mushroom bodies are crucial for olfactory learning. However, the mushroom bodies are not dedicated solely to olfactory processing; they also mediate higher cognitive functions in the fly, such as visual context generalization. Molecular genetic manipulations, coupled with behavioral studies of the fly, will identify rudimentary neural circuits that underly multisensory learning and perhaps also the circuits that mediate more-complex brain functions, such as attention.

Flies, genes, and learning

Flies can learn. For the past 25 years, researchers have isolated mutants, engineered mutants with transgenes, and tested likely suspect mutants from other screens for learning ability. There have been notable surprises-conventional second messenger systems co-opted for intricate associative learning tasks, two entirely separate forms of long-term memory, a cell-adhesion molecule that is necessary for short-term memory. The most recent surprise is the mechanistic kinship revealed between learning and addictive drug response behaviors in flies. The flow of new insight is likely to quicken with the completion of the fly genome and the arrival of more selective methods of gene expression.

Dual-tagging gene trap of novel genes in Drosophila melanogaster

A gene-trap system is established for Drosophila. Unlike the conventional enhancer-trap system, the gene-trap system allows the recovery only of fly lines whose genes are inactivated by a P-element insertion, i.e., mutants. In the gene-trap system, the reporter gene expression reflects precisely the spatial and temporal expression pattern of the trapped gene. Flies in which gene trap occurred are identified by a two-step screening process using two independent markers, mini-w and Gal4, each indicating the integration of the vector downstream of the promoter of a gene (dual tagging). mini-w has its own promoter but lacks a polyadenylation signal. Therefore, mini-w mRNA is transcribed from its own promoter regardless of the vector integration site in the genome. However, the eyes of flies are not orange or red unless the vector is incorporated into a gene enabling mini-w to be spliced to a downstream exon of the host gene and polyadenylated at the 3' end. The promoter-less Gal4 reporter is expressed as a fusion mRNA only when it is integrated downstream of the promoter of a host gene. The exons of trapped genes can be readily cloned by vectorette RT-PCR, followed by RACE and PCR using cDNA libraries. Thus, the dual-tagging gene-trap system provides a means for (i) efficient mutagenesis, (ii) unequivocal identification of genes responsible for mutant phenotypes, (iii) precise detection of expression patterns of trapped genes, and (iv) rapid cloning of trapped genes.

Revelations of the RYK receptor

Significant progress has been made over the last decade in elucidating the mechanisms employed by receptor protein tyrosine kinases (RTKs) in transducing extracellular signals critical for the regulation of diverse cellular activities. Nevertheless, revealing the biological significance of a subset of the RTKs that contain catalytically inactive protein tyrosine kinase domains has proven more elusive. ErbB3 has served as the prototype for models of catalytically inactive RTK function, performing the role of signal diversification in heterodimeric receptor complexes with other ErbB subfamily members. The receptor related to tyrosine kinases (RYK) is unique amongst the catalytically inactive RTKs. Based on structural or functional properties of the extracellular domain, RYK cannot be classified into an existing RTK subfamily. Recent genetic analyses of mouse Ryk and its Drosophila orthologue derailed have defined a role for this novel subfamily of receptors in the control of craniofacial development and neuronal pathway selection, respectively. Recent biochemical data lead us to propose a model that involves RYK in signal crosstalk and scaffold assembly with Eph receptors. This model is consistent with the established roles of Eph receptors and ephrins in craniofacial and nervous system morphogenesis. BioEssays 23:34-45, 2001.

Abnormal turning behavior in Drosophila larvae. Identification and molecular analysis of scribbler (sbb)

Our genetic dissection of behavior has isolated scribbler (sbb), a vital gene that encodes a novel protein expressed in the embryonic and larval nervous systems and in the imaginal discs. Larvae with mutations in sbb exhibit abnormally high amounts of turning behavior in the absence of food. sbb is a large gene spanning >50 kb of genomic DNA with four major developmentally regulated transcripts. Transgenic rescue of scribbler behavior was demonstrated by targeting expression of a normal sbb transgene (sbb(+)) expressing one of the major transcripts to the nervous system. The vital function of sbb was restored by ubiquitous expression of this transgene throughout development.

nalyot, a mutation of the Drosophila myb-related Adf1 transcription factor, disrupts synapse formation and olfactory memory

nalyot (nal) is a novel olfactory memory mutant of Drosophila, encoding Adf1, a myb-related transcription factor. Following extended training sessions, Adf1 mutants show normal early memory but defective longterm memory. Adf1 shows widespread spatiotemporal expression, yet mutant alleles reveal no discernible disruptions in gross morphology of the nervous system. Studies at the larval neuromuscular junction, however, reveal a role for Adf1 in the modulation of synaptic growth-in contrast to the role established for dCREB2 in the control of synaptic function (Davis et al., 1996). These findings suggest that Adf1 and dCREB2 regulate distinct transcriptional cascades involved in terminal stages of synapse maturation. More generally, Adf1 provides a novel link between molecular mechanisms of developmental and behavioral plasticity.

Developmental expression of an amn(+) transgene rescues the mutant memory defect of amnesiac adults

The Drosophila memory gene amnesiac (amn) has been proposed to encode a neuropeptide protein, which includes regions homologous to vertebrate pituitary adenylyl cyclase-activating peptide (PACAP; Feany and Quinn, 1995). Definitive experiments to link this gene to memory formation, however, have not yet been accomplished (Kandel and Abel, 1995). The experiments described here demonstrate that the putative amn transcript is involved in adult memory formation. With the use of a UAS-amn(+) transgene, we show complete rescue of memory defects in amn(28A), a mutant allele caused by the insertion of a GAL4 enhancer trap transposon (Moore et al., 1998). Study of the amn(28A) reporter reveals widespread expression in the adult brain but also enriched expression in the embryonic and larval nervous systems. To begin addressing the temporal requirement of amn in memory, we asked whether the memory defects could be rescued by restricting transgenic expression to the adult stage. A heat-shock regimen shown previously to rescue fully the amn ethanol sensitivity defect (Moore et al., 1998) failed to rescue the memory defect. These results, coupled with previous genetic and anatomical studies, suggest that adult memory formation and ethanol sensitivity have different temporal and spatial requirements for amn.

latheo encodes a subunit of the origin recognition complex and disrupts neuronal proliferation and adult olfactory memory when mutant

The Drosophila latheo (lat) gene was identified in a behavioral screen for olfactory memory mutants. The original hypomorphic latP1 mutant (Boynton and Tully, 1992) shows a structural defect in adult brain. Homozygous lethal lat mutants lack imaginal discs, show little cell proliferation in the CNS of third instar larvae, and die as early pupae. latP1 was cloned, and all of the above mentioned defects of hypomorphic or homozygous lethal lat mutants were rescued with a lat+ transgene. lat encodes a novel protein with homology to a subunit of the origin recognition complex (ORC). Human and Drosophila LAT both associate with ORC2 and are related to yeast ORC3, suggesting that LAT functions in DNA replication during cell proliferation.

latheo, a Drosophila gene involved in learning, regulates functional synaptic plasticity

Mutations in the latheo (lat) gene disrupt associative learning in Drosophila , but a role for LAT in regulating neuronal function has not been demonstrated. Here, we report that LAT plays a central role in regulating Ca2(+)- and activity-dependent synaptic plasticity. Immunological localization of the LAT protein indicates it is present at synaptic connections of the larval neuromuscular junction (NMJ) and is enriched in presynaptic boutons. Basal synaptic transmission amplitude at the lat mutant NMJ is elevated 3- to 4-fold, and Ca2+ dependence of transmission is significantly reduced. Multiple forms of synaptic facilitation and posttetanic potentiation (PTP) are strongly depressed or absent at the mutant synapse. Our results suggest that LAT is a novel presynaptic protein with a role in the Ca2(+)-dependent synaptic modulation mechanisms necessary for behavioral plasticity.

Expression of receptor tyrosine kinase RYK in developing rat central nervous system

Receptor tyrosine kinase RYK is a mammalian homologue of Drosophila Lio, which is involved in learning and memory and in axon guidance. We cloned a rat ryk gene and characterized its expression pattern in the central nervous system. Northern blot analysis of the whole brain revealed that the RYK mRNA was abundant during the period from 13 to 18 embryonic days (E13-18) and it decreased by E20. In the postnatal brain, the RYK signal was higher in postnatal one week (P1W) cerebrum and in P2W cerebellum than in later stages. In situ hybridization revealed that RYK was expressed throughout the central nervous system, mainly in the ventricular zone on E11 and E13. On E18 and E20, the remarkable level of RYK mRNA was detected in the ventricular zone as well as in the cortical plate of the forebrain. These two regions overlapped the immunoreactive areas of nestin and MAP2, a neural stem cell marker and a mature neural marker, respectively. Moreover, the double-labeling analysis showed that the same cells expressed both RYK and nestin in the ventricular zone. In the postnatal brain, RYK was predominantly expressed in neurons of various regions. These observations suggest that RYK plays a contributory role as a multifunctional molecule in the differentiation and maturation of neuronal cells in the central nervous system.

The receptor tyrosine kinase gene linotte is required for neuronal pathway selection in the Drosophila mushroom bodies

The linotte (lio) mutant was first isolated as a memory mutant. The lio gene encodes a putative receptor tyrosine kinase (RTK), homologous to the human protein RYK. This gene has been independently identified in a screen for embryonic nervous system axonal guidance defects and called derailed (drl). Here, we report that linotte mutants present structural brain defects in the adult central complex (CX) and mushroom bodies (MB). linotte and derailed are allelic for this phenotype, which can be rescued by a drl+ transgene. The Lio RTK is expressed preferentially in the adult CX and MB. Our results suggest that, analogous to its role within the embryonic nervous system, the Lio RTK is involved in neuronal pathway selection during adult brain development.

The Drosophila putative kinase linotte (derailed) prevents central brain axons from converging on a newly described interhemispheric ring

The lio gene encodes a putative receptor tyrosine kinase, with unique motifs both in the extracellular and catalytic domains (Dura, J.-M., Préat, T., Tully, T., 1993. Identification of linotte, a new gene affecting learning and memory in Drosophila melanogaster. J. Neurogenet. 9, 1-14). We show here that a complete deletion of lio activity causes specific structural defects in the adult brain. Gal4 enhancer-trap lines used as cell markers revealed that in lio mutants central brain axons behave as if they were abnormally attracted by the midbrain area. The Lio protein is expressed in third instar larvae in a few cells at the junction of the cerebral hemispheres. These glial cells form a newly described ring structure, showing an invariable fibrous organization. In the wild-type this ring disappears at midpupation. Our results indicate that the Lio putative kinase plays a major role in the modeling of the adult brain by controlling the fate of the transient interhemispheric ring.

Integrins: a role for adhesion molecules in olfactory memory

A gene required for short-term memory in Drosophila, Volado, encodes an alpha integrin and is preferentially expressed in the mushroom bodies of the adult brain. Adhesion molecules of this kind may play a role in olfactory memory by altering the strength of synaptic connections in an experience-dependent manner.

Gene discovery in Drosophila: new insights for learning and memory

Genetic approaches have been used to investigate increasingly complex biological systems. Here we review the current state of genetic analysis of learning and memory in the fruitfly, Drosophila melanogaster. Emerging findings support two main themes. First, discovery and manipulation of genes involved with behavioral plasticity in genetically accessible systems such as D. melanogaster enables dissection of the biochemical, cellular, anatomical, and behavioral pathways of learning and memory. Second, because core cellular mechanisms of simple forms of learning are evolutionarily conserved, biological pathways discovered in invertebrates are likely to be conserved in vertebrate systems as well.

Drosophila learning and memory: recent progress and new approaches

The processes of learning and memory have traditionally been studied in large experimental organisms (Aplysia, mice, rats and humans), where well-characterized behaviors are easily tested. Although Drosophila is one of the most experimentally tractable organisms, it has only recently joined the others as a model organism for learning and memory. Drosophila behavior has been studied for over 20 years; however, most of the work in the learning and memory field has focused on initial learning, because establishing memory in Drosophila has not been as straightforward as in other organisms. A major recent advance in this field has been the development of a training protocol that induces long-term memory in files. This made possible experiments that implicated the Drosophila CREB gene as a critical component in the consolidation of long-term memory, and paves the way for future experiments utilizing the well developed tools in Drosophila. This review will briefly summarize what is known in the field of Drosophila learning and memory to date, and discuss why the unique aspects of this field make traditional approaches difficult and reward the use of alternative paths of experimentation.

Learning without performance in PKC-deficient Drosophila

The performance of a task is often assumed to be a prerequisite for the learning of many tasks, including the associative conditioning of courtship in the fruit fly, Drosophila melanogaster. Transgenic flies specifically inhibited for the enzyme protein kinase C dissociate the acquisition of learning and memory from performance of the task. They fail to show immediate suppression of courtship but nonetheless develop normal memory of it.

Genetic dissection of sexual behavior in Drosophila melanogaster

Mating of Drosophila melanogaster is a sterotypically patterned behavior consisting of a fixed sequence of actions that are primarily under genetic control. Mutations that disrupt specific aspects of mating activities offer a starting point for exploring the molecular machineries underlying sexual behavior. Several genes, identified as causing aberrant sexual behavior when mutated, have been isolated and cloned, providing molecular probes for expression and mosaic analyses that can be used in specifying the cells responsible for the behavior. This review presents current understandings of mating behavior obtained by such molecular and cellular approaches and provides an overview of future directions of research in behavioral genetics.

Discovery of genes involved with learning and memory: an experimental synthesis of Hirschian and Benzerian perspectives

The biological bases of learning and memory are being revealed today with a wide array of molecular approaches, most of which entail the analysis of dysfunction produced by gene disruptions. This perspective derives both from early "genetic dissections" of learning in mutant Drosophila by Seymour Benzer and colleagues and from earlier behavior-genetic analyses of learning and in Diptera by Jerry Hirsh and coworkers. Three quantitative-genetic insights derived from these latter studies serve as guiding principles for the former. First, interacting polygenes underlie complex traits. Consequently, learning/memory defects associated with single-gene mutants can be quantified accurately only in equilibrated, heterogeneous genetic backgrounds. Second, complex behavioral responses will be composed of genetically distinct functional components. Thus, genetic dissection of complex traits into specific biobehavioral properties is likely. Finally, disruptions of genes involved with learning/memory are likely to have pleiotropic effects. As a result, task-relevant sensorimotor responses required for normal learning must be assessed carefully to interpret performance in learning/memory experiments. In addition, more specific conclusions will be obtained from reverse-genetic experiments, in which gene disruptions are restricted in time and/or space.

Temporally manipulated rescue of visual and courtship abnormalities caused by a nonA mutation in Drosophila

The no-on-transient-A gene was identified by independently isolated mutations causing visual-response abnormalities or courtship song defects. The nonA(diss) mutant is abnormal for all of these phenotypes. The pleiotropic effects of this dissonance allele dovetail with the widespread tissue expression of the nonA products. This gene, which encodes a putative RNA-binding protein, is expressed at essentially all stages of the life cycle. To determine whether the behavioral and physiological abnormalities exhibited by nonA mutant adults have a developmental etiology or are the result of an impaired mature nervous (or perhaps neuro-muscular) system, a conditional form of this gene was constructed. Animals from the resulting hsp-nonA (cDNA) transgenic strain were subjected to heat-shock regimes such that the gene's coding sequences were activated during development only, solely in the imaginal stage, or both. Surprisingly, expression during any of these time periods effected rescue of the visual-response and the courtship-song defects.

CREB and the formation of long-term memory

Cyclic AMP response element binding protein (CREB)-responsive transcription plays a central role in the formation of long-term memory in Drosophila, Aplysia and mice. Agents that disrupt the activity of CREB specifically block the formation of long-term memory, whereas agents that increase the amount or activity of the transcription factor accelerate the process. These results have led to the recent hypothesis that CREB is pivotal in the switch from short-term (protein synthesis independent) to long-term (protein synthesis dependent) memory.