Expression analysis of the FoxP homologue in the brain of the honeybee, Apis mellifera

A comprehensive study of arthropod and onychophoran Fox gene expression patterns

Fox genes represent an evolutionary old class of transcription factor encoding genes that evolved in the last common ancestor of fungi and animals. They represent key-components of multiple gene regulatory networks (GRNs) that are essential for embryonic development. Most of our knowledge about the function of Fox genes comes from vertebrate research, and for arthropods the only comprehensive gene expression analysis is that of the fly Drosophila melanogaster. For other arthropods, only selected Fox genes have been investigated. In this study, we provide the first comprehensive gene expression analysis of arthropod Fox genes including representative species of all main groups of arthropods, Pancrustacea, Myriapoda and Chelicerata. We also provide the first comprehensive analysis of Fox gene expression in an onychophoran species. Our data show that many of the Fox genes likely retained their function during panarthropod evolution highlighting their importance in development. Comparison with published data from other groups of animals shows that this high degree of evolutionary conservation often dates back beyond the last common ancestor of Panarthropoda.

Conserved regulation of neurodevelopmental processes and behavior by FoxP in Drosophila

FOXP proteins form a subfamily of evolutionarily conserved transcription factors involved in the development and functioning of several tissues, including the central nervous system. In humans, mutations in FOXP1 and FOXP2 have been implicated in cognitive deficits including intellectual disability and speech disorders. Drosophila exhibits a single ortholog, called FoxP, but due to a lack of characterized mutants, our understanding of the gene remains poor. Here we show that the dimerization property required for mammalian FOXP function is conserved in Drosophila. In flies, FoxP is enriched in the adult brain, showing strong expression in ~1000 neurons of cholinergic, glutamatergic and GABAergic nature. We generate Drosophila loss-of-function mutants and UAS-FoxP transgenic lines for ectopic expression, and use them to characterize FoxP function in the nervous system. At the cellular level, we demonstrate that Drosophila FoxP is required in larvae for synaptic morphogenesis at axonal terminals of the neuromuscular junction and for dendrite development of dorsal multidendritic sensory neurons. In the developing brain, we find that FoxP plays important roles in α-lobe mushroom body formation. Finally, at a behavioral level, we show that Drosophila FoxP is important for locomotion, habituation learning and social space behavior of adult flies. Our work shows that Drosophila FoxP is important for regulating several neurodevelopmental processes and behaviors that are related to human disease or vertebrate disease model phenotypes. This suggests a high degree of functional conservation with vertebrate FOXP orthologues and established flies as a model system for understanding FOXP related pathologies.

Identification of the neurotransmitter profile of AmFoxP expressing neurons in the honeybee brain using double-label in situ hybridization

BACKGROUND

FoxP transcription factors play crucial roles for the development and function of vertebrate brains. In humans the neurally expressed FOXPs, FOXP1, FOXP2, and FOXP4 are implicated in cognition, including language. Neural FoxP expression is specific to particular brain regions but FoxP1, FoxP2 and FoxP4 are not limited to a particular neuron or neurotransmitter type. Motor- or sensory activity can regulate FoxP2 expression, e.g. in the striatal nucleus Area X of songbirds and in the auditory thalamus of mice. The DNA-binding domain of FoxP proteins is highly conserved within metazoa, raising the possibility that cellular functions were preserved across deep evolutionary time. We have previously shown in bee brains that FoxP is expressed in eleven specific neuron populations, seven tightly packed clusters and four loosely arranged groups.

RESULTS

The present study examined the co-expression of honeybee FoxP (AmFoxP) with markers for glutamatergic, GABAergic, cholinergic and monoaminergic transmission. We found that AmFoxP could co-occur with any one of those markers. Interestingly, AmFoxP clusters and AmFoxP groups differed with respect to homogeneity of marker co-expression; within a cluster, all neurons co-expressed the same neurotransmitter marker, within a group co-expression varied. We also assessed qualitatively whether age or housing conditions providing different sensory and motor experiences affected the AmFoxP neuron populations, but found no differences.

CONCLUSIONS

Based on the neurotransmitter homogeneity we conclude that AmFoxP neurons within the clusters might have a common projection and function whereas the AmFoxP groups are more diverse and could be further sub-divided. The obtained information about the neurotransmitters co-expressed in the AmFoxP neuron populations facilitated the search of similar neurons described in the literature. These comparisons revealed e.g. a possible function of AmFoxP neurons in the central complex. Our findings provide opportunities to focus future functional studies on invertebrate FoxP expressing neurons. In a broader context, our data will contribute to the ongoing efforts to discern in which cases relationships between molecular and phenotypic signatures are linked evolutionary.

Kenyon Cell Subtypes/Populations in the Honeybee Mushroom Bodies: Possible Function Based on Their Gene Expression Profiles, Differentiation, Possible Evolution, and Application of Genome Editing

Mushroom bodies (MBs), a higher-order center in the honeybee brain, comprise some subtypes/populations of interneurons termed as Kenyon cells (KCs), which are distinguished by their cell body size and location in the MBs, as well as their gene expression profiles. Although the role of MBs in learning ability has been studied extensively in the honeybee, the roles of each KC subtype and their evolution in hymenopteran insects remain mostly unknown. This mini-review describes recent progress in the analysis of gene/protein expression profiles and possible functions of KC subtypes/populations in the honeybee. Especially, the discovery of novel KC subtypes/populations, the “middle-type KCs” and “KC population expressing FoxP,” necessitated a redefinition of the KC subtype/population. Analysis of the effects of inhibiting gene function in a KC subtype-preferential manner revealed the function of the gene product as well as of the KC subtype where it is expressed. Genes expressed in a KC subtype/population-preferential manner can be used to trace the differentiation of KC subtypes during the honeybee ontogeny and the possible evolution of KC subtypes in hymenopteran insects. Current findings suggest that the three KC subtypes are unique characteristics to the aculeate hymenopteran insects. Finally, prospects regarding future application of genome editing for the study of KC subtype functions in the honeybee are described. Genes expressed in a KC subtype-preferential manner can be good candidate target genes for genome editing, because they are likely related to highly advanced brain functions and some of them are dispensable for normal development and sexual maturation in honeybees.

FoxP expression identifies a Kenyon cell subtype in the honeybee mushroom bodies linking them to fruit fly αβc neurons

The arthropod mushroom bodies (MB) are a higher order sensory integration centre. In insects, they play a central role in associative olfactory learning and memory. In Drosophila melanogaster (Dm), the highly ordered connectivity of heterogeneous MB neuron populations has been mapped using sophisticated molecular genetic and anatomical techniques. The MB-core subpopulation was recently shown to express the transcription factor FoxP with relevance for decision-making. Here, we report the development and adult distribution of a FoxP-expressing neuron population in the MB of honeybees (Apis mellifera, Am) using in situ hybridisation and a custom-made antiserum. We found the same expression pattern in adult bumblebees (Bombus terrestris, Bt). We also designed a new Dm transgenic line that reports FoxP transcriptional activity in the MB-core region, clarifying previously conflicting data of two other reporter lines. Considering developmental, anatomical and molecular similarities, our data are consistent with the concept of deep homology of FoxP expression in neuron populations coding reinforcement-based learning and habit formation.

Genome-Wide Identification and Characterization of Fox Genes in the Honeybee, Apis cerana, and Comparative Analysis with Other Bee Fox Genes

The forkhead box (Fox) gene family, one of the most important families of transcription factors, participates in various biological processes. However, Fox genes in Hymenoptera are still poorly known. In this study, 14 Fox genes were identified in the genome of Apis cerana. In addition, 16 (Apis mellifera), 13 (Apis dorsata), 16 (Apis florea), 17 (Bombus terrestris), 16 (Bombus impatiens), and 18 (Megachile rotundata) Fox genes were identified in their genomes, respectively. Phylogenetic analyses suggest that FoxA is absent in the genome of A. dorsata genome. Similarly, FoxG is missing in the genomes A. cerana and A. dorsata. Temporal expression profiles obtained by quantitative real-time PCR revealed that Fox genes have distinct expression patterns in A. cerana, especially for three genes ACSNU03719T0 (AcFoxN4), ACSNU05765T0 (AcFoxB), and ACSNU07465T0 (AcFoxL2), which displayed high expression at the egg stage. Tissue expression patterns showed that FoxJ1 is significantly higher in the antennae of A. cerana and A. mellifera compared to other tissues. These results may facilitate a better understanding of the potential physiological functions of the Fox gene family in A. cerana and provide valuable information for a comprehensive functional analysis of the Fox gene family in Hymenopterans.

FoxP in bees: A comparative study on the developmental and adult expression pattern in three bee species considering isoforms and circuitry

Mutations in the transcription factors FOXP1, FOXP2, and FOXP4 affect human cognition, including language. The FoxP gene locus is evolutionarily ancient and highly conserved in its DNA-binding domain. In Drosophila melanogaster FoxP has been implicated in courtship behavior, decision making, and specific types of motor-learning. Because honeybees (Apis mellifera, Am) excel at navigation and symbolic dance communication, they are a particularly suitable insect species to investigate a potential link between neural FoxP expression and cognition. We characterized two AmFoxP isoforms and mapped their expression in the brain during development and in adult foragers. Using a custom-made antiserum and in situ hybridization, we describe 11 AmFoxP expressing neuron populations. FoxP was expressed in equivalent patterns in two other representatives of Apidae; a closely related dwarf bee and a bumblebee species. Neural tracing revealed that the largest FoxP expressing neuron cluster in honeybees projects into a posterior tract that connects the optic lobe to the posterior lateral protocerebrum, predicting a function in visual processing. Our data provide an entry point for future experiments assessing the function of FoxP in eusocial Hymenoptera.

Drosophila FoxP mutants are deficient in operant self-learning

Intact function of the Forkhead Box P2 (FOXP2) gene is necessary for normal development of speech and language. This important role has recently been extended, first to other forms of vocal learning in animals and then also to other forms of motor learning. The homology in structure and in function among the FoxP gene members raises the possibility that the ancestral FoxP gene may have evolved as a crucial component of the neural circuitry mediating motor learning. Here we report that genetic manipulations of the single Drosophila orthologue, dFoxP, disrupt operant self-learning, a form of motor learning sharing several conceptually analogous features with language acquisition. Structural alterations of the dFoxP locus uncovered the role of dFoxP in operant self-learning and habit formation, as well as the dispensability of dFoxP for operant world-learning, in which no motor learning occurs. These manipulations also led to subtle alterations in the brain anatomy, including a reduced volume of the optic glomeruli. RNAi-mediated interference with dFoxP expression levels copied the behavioral phenotype of the mutant flies, even in the absence of mRNA degradation. Our results provide evidence that motor learning and language acquisition share a common ancestral trait still present in extant invertebrates, manifest in operant self-learning. This 'deep' homology probably traces back to before the split between vertebrate and invertebrate animals.

Conserved role of Drosophila melanogaster FoxP in motor coordination and courtship song

FoxP2 is a highly conserved vertebrate transcription factor known for its importance in human speech and language production. Disruption of FoxP2 in several vertebrate models indicates a conserved functional role for this gene in both sound production and motor coordination. Although FoxP2 is known to be strongly expressed in brain regions important for motor coordination, little is known about FoxP2's role in the nervous system. The recent discovery of the well-conserved Drosophila melanogaster homolog, FoxP, provides an opportunity to study the role of this crucial gene in an invertebrate model. We hypothesized that, like FoxP2, Drosophila FoxP is important for behaviors requiring fine motor coordination. We used targeted RNA interference to reduce expression of FoxP and assayed the effects on a variety of adult behaviors. Male flies with reduced FoxP expression exhibit decreased levels of courtship behavior, altered pulse-song structure, and sex-specific motor impairments in walking and flight. Acute disruption of synaptic activity in FoxP expressing neurons using a temperature-sensitive shibire allele dramatically impaired motor coordination. Utilizing a GFP reporter to visualize FoxP in the fly brain reveals expression in relatively few neurons in distributed clusters within the larval and adult CNS, including distinct labeling of the adult protocerebral bridge - a section of the insect central complex known to be important for motor coordination and thought to be homologous to areas of the vertebrate basal ganglia. Our results establish the necessity of this gene in motor coordination in an invertebrate model and suggest a functional homology with vertebrate FoxP2.

Recent Advances in the Genetics of Vocal Learning

Language is a complex communicative behavior unique to humans, and its genetic basis is poorly understood. Genes associated with human speech and language disorders provide some insights, originating with the FOXP2 transcription factor, a mutation in which is the source of an inherited form of developmental verbal dyspraxia. Subsequently, targets of FOXP2 regulation have been associated with speech and language disorders, along with other genes. Here, we review these recent findings that implicate genetic factors in human speech. Due to the exclusivity of language to humans, no single animal model is sufficient to study the complete behavioral effects of these genes. Fortunately, some animals possess subcomponents of language. One such subcomponent is vocal learning, which though rare in the animal kingdom, is shared with songbirds. We therefore discuss how songbird studies have contributed to the current understanding of genetic factors that impact human speech, and support the continued use of this animal model for such studies in the future.

Identification and expression analysis of nervous wreck, which is preferentially expressed in the brain of the male silkworm moth, Bombyx mori

Sexually dimorphic neural circuits are essential for reproductive behaviour. The molecular basis of sexual dimorphism in the silkworm moth (Bombyx mori) brain, however, is unclear. We conducted cDNA subtraction screening and identified nervous wreck (Bmnwk), a synaptic growth regulatory gene, whose expression is higher in the male brain than in the female brain of the silkworm. Bmnwk was preferentially expressed in the brain at the late pupae and adult stages. In situ hybridization revealed that Bmnwk is highly expressed in the optic lobe of the male moth brain. These findings suggest that Bmnwk has a role in the development and/or maintenance of the optic lobe in the male silkworm brain.

Evo-devo, deep homology and FoxP2: implications for the evolution of speech and language

The evolution of novel morphological features, such as feathers, involves the modification of developmental processes regulated by gene networks. The fact that genetic novelty operates within developmental constraints is the central tenet of the 'evo-devo' conceptual framework. It is supported by findings that certain molecular regulatory pathways act in a similar manner in the development of morphological adaptations, which are not directly related by common ancestry but evolved convergently. The Pax6 gene, important for vision in molluscs, insects and vertebrates, and Hox genes, important for tetrapod limbs and fish fins, exemplify this 'deep homology'. Recently, 'evo-devo' has expanded to the molecular analysis of behavioural traits, including social behaviour, learning and memory. Here, we apply this approach to the evolution of human language. Human speech is a form of auditory-guided, learned vocal motor behaviour that also evolved in certain species of birds, bats and ocean mammals. Genes relevant for language, including the transcription factor FOXP2, have been identified. We review evidence that FoxP2 and its regulatory gene network shapes neural plasticity in cortico-basal ganglia circuits underlying the sensory-guided motor learning in animal models. The emerging picture can help us understand how complex cognitive traits can 'descend with modification'.

Alternative splicing and gene duplication in the evolution of the FoxP gene subfamily

The FoxP gene subfamily of transcription factors is defined by its characteristic 110 amino acid long DNA-binding forkhead domain and plays essential roles in vertebrate biology. Its four members, FoxP1–P4, have been extensively characterized functionally. FoxP1, FoxP2, and FoxP4 are involved in lung, heart, gut, and central nervous system (CNS) development. FoxP3 is necessary and sufficient for the specification of regulatory T cells (Tregs) of the adaptive immune system.

In Drosophila melanogaster, in silico predictions identify one unique FoxP subfamily gene member (CG16899) with no described function. We characterized this gene and established that it generates by alternative splicing two isoforms that differ in the forkhead DNA-binding domain. In D. melanogaster, both isoforms are expressed in the embryonic CNS, but in hemocytes, only isoform A is expressed, hinting to a putative modulation through alternative splicing of FoxP1 function in immunity and/or other hemocyte-dependent processes. Furthermore, we show that in vertebrates, this novel alternative splicing pattern is conserved for FoxP1. In mice, this new FoxP1 isoform is expressed in brain, liver, heart, testes, thymus, and macrophages (equivalent in function to hemocytes). This alternative splicing pattern has arisen at the base of the Bilateria, probably through exon tandem duplication. Moreover, our phylogenetic analysis suggests that in vertebrates, FoxP1 is more related to the FoxP gene ancestral form and the other three paralogues, originated through serial duplications, which only retained one of the alternative exons. Also, the newly described isoform differs from the other in amino acids critical for DNA-binding specificity. The integrity of its fold is maintained, but the molecule has lost the direct hydrogen bonding to DNA bases leading to a putatively lower specificity and possibly affinity toward DNA.

With the present comparative study, through the integration of experimental and in silico studies of the FoxP gene subfamily across the animal kingdom, we establish a new model for the FoxP gene in invertebrates and for the vertebrate FoxP1 paralogue. Furthermore, we present a scenario for the structural evolution of this gene class and reveal new previously unsuspected levels of regulation for FoxP1 in the vertebrate system.