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.

lKCs Honeybee MBs express more than 20 genes in a lKC subtypepreferential manner (for more comprehensive reviews, see Kubo, 2012;Kaneko et al., 2016). Among these genes, nine are expressed preferentially in the lKCs. Five of these 9 genes encode proteins involved in the intracellular Ca 2+ -signaling pathway, such as Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) (Kamikouchi et al., 1998(Kamikouchi et al., , 2000Sen Sarma et al., 2009;Uno et al., 2012), which has an important role in the synaptic plasticity that underlies learning and memory abilities in various animals (Colbran and Brown, 2004;Elgersma et al., 2004;Pasch et al., 2011). Furthermore, Pasch et al. (2011) reported that phosphorylated (activated) CaMKII protein (pCaMKII) is present in lKCs, but not in sKCs or class II KCs (Pasch et al., 2011). These findings suggest that the lKCs are related to Ca 2+signaling-based learning and memory functions ( Figure 1D; Ghosh and Greenberg, 1995;Rose and Konnerth, 2001;Perisse et al., 2009;Shonesy et al., 2014). Matsumoto et al. (2014) used pharmacologic inhibition to indicate that CaMKII is involved in late long-term memory (LTM), but not in mid-term memory (MTM) or early LTM formation (Matsumoto et al., 2014). In addition, Scholl et al. (2015) used RNA interference (RNAi) and pharmacologic inhibition to indicate that CaMKII is necessary for both early and late LTM, but not for MTM (Scholl et al., 2015). Although the two studies reported different effects of CaMKII inhibition on early LTM, they consistently suggest that the lKCs play a role at least in late LTM formation in the honeybee.
Genes encoding for two transcription factors, Mushroom body/large-type Kenyon cell-preferential gene-1 [(Mblk-1)/E93] (Takeuchi et al., 2001) and Broad-Complex (BR-C) , are also expressed preferentially in the lKCs in the honeybee MBs. The MBR-1, a nematode homolog of Mblk-1, is necessary for both pruning excessive neurites during development and learning ability (Kage et al., 2005;Hayashi et al., 2009). Thus, selective expression of Mbk-1 in the lKCs is consistent with the speculation that synaptic plasticity is enhanced in the lKCs. It is also plausible that Mblk-1 and BR-C are involved in transactivation of genes expressed in an lKCpreferential manner in the honeybee brain. Suenami et al. (2016) recently identified three genes, synaptotagmin 14 (Syt14), discs large 5 (dlg5), and phospholipase C epsilon (PLCe), whose expression is more highly enriched in the MBs of the honeybee brain than the previously identified KC subtype-preferential genes . While, Syt14 and dlg5 are highly selectively expressed in the "redefined" lKCs in the MBs, PLCe is highly expressed in the whole MBs; i.e., all of the class I lKCs, mKCs, and sKCs and class II KCs ( Figure 1D; Suenami et al., 2016). Syt14 and dlg5 are involved in membrane trafficking and spine formation, respectively (Fukuda, 2003;Hayashi et al., 2009;Doi et al., 2011;Wang et al., 2014;Suenami et al., 2016), implying that both synaptic transmission and synaptic plasticity are enhanced in the lKCs. FIGURE 1 | Example of genes and proteins expressed in a KC subtype-preferential manner in worker honeybee MBs. (A) Schematic drawing of the head and brain of a worker honeybee. MB, mushroom body; OL, optic lobe; AL, antennal lobe; CE, compound eyes. (B) Hematoxylin-eosin staining of a section of the left MB, which corresponds to the boxed region in (A). Ca, calyx; Pe, pedunculus. Class I "classic" lKCs, "classic" sKCs, and class II KCs are indicated by arrows. (C) Double in situ hybridization of CaMKII (green), which is preferentially expressed in "redefined" lKCs, and mKast (magenta), which is preferentially expressed in mKCs in a single MB calyx. Redefined sKCs are stained with nuclear staining and colored blue. This picture well represents the presence of the three class I "redefined" KC subtypes: "redefined" lKCs, mKCs, and "redefined" sKCs. (D) Schematic drawing of five KC subtype-preferential gene expression patterns. Each box contains a schematic drawing of a single MB calyx, in which KC subtypes/populations with strong gene/protein expression are colored green (for class I "redefined" lKCs), magenta (for class I mKCs), blue (for class I "redefined" sKCs), yellow (for the whole MB = class I + II lKCs), gray (for class I "redefined" lKCs + "redefined" sKCs + class II KCs), and red (for KC population expressing FoxP). Genes with a KC subtype-preferential expression pattern discussed in this mini-review are listed below each box. Note that the genes whose expression in the "redefined" lKCs/ "redefined" sKCs was confirmed by double in situ hybridization with mKast are indicated by bold letters. These figures are cited from Kubo (2012) and Kaneko et al. (2016) with some modifications.
sKCs Three genes, ecdysone receptor (EcR), hormone receptor-like 38 (HR38), and E74, are expressed preferentially in the sKCs, and all of them encode transcription factors involved in the ecdysteroidsignaling pathway ( Figure 1D; Paul et al., 2005;Yamazaki et al., 2006;Takeuchi et al., 2007). Expression of HR38 is higher in the brains of foragers than in the brains of nurse bees, suggesting its possible association with the division of labor of workers (Yamazaki et al., 2006). The EcR/ultraspiracle (Usp) heterodimer binds to ecdysteroids to orchestrate transcriptional regulation during metamorphosis . In contrast, HR38 competes with EcR for Usp, and the HR38/Usp heterodimer activates the transcription of target genes distinct from those of the EcR/Usp heterodimer (Zhu et al., 2000;Baker et al., 2003). Thus, Yamazaki et al. (2006) previously proposed that the enhanced expression of HR38 in the forager brain might contribute to switching the mode of ecdysteroid-signaling in the MBs from the EcR-to the HR38-mediated pathway in association with the division of labor of workers (Yamazaki et al., 2006).
Recent studies, however, reported that, in the silk moth and fruit fly, HR38 is an immediate early gene, whose neuronal expression is activated by neuronal excitation (Fujita et al., 2013), and that HR38 expression in the honeybee brain is induced by foraging behavior (Ugajin et al., 2018). These results suggest an alternative possibility that HR38 expression in the sKCs of the honeybee brain is a consequence of the foraging behavior, and does not necessarily represent a gene expression profile specific to the forager brain. These possibilities need to be investigated further.

mKCs
So far, only one gene, termed mKast, has been found to be expressed preferentially in the mKCs of the honeybee MBs (Figures 1C,D) (Kaneko et al., 2013). Although mKast belongs to the α-arrestin family, which is involved in downregulation of membrane receptors (Kaneko et al., 2013), the role of mKast in the honeybee is currently obscure. mKast expression in the brain begins at the late pupal stages and is detectable almost exclusively in the adult brain, suggesting its role in regulating adult honeybee behaviors and/or physiology (Yamane et al., 2017).
Since detection of neural activity using immediate early genes revealed that MB KCs (Singh et al., 2018;Ugajin et al., 2018), especially sKCs and some mKCs (Kaneko et al., 2013), are active in the brains of foragers, it is plausible that these KC subtypes are related to sensory information processing during the foraging flights.

Broader Gene Expression Profiles
Three genes, PLCe , protein kinase C (PKC) (Kamikouchi et al., 2000), and E75 , are preferentially expressed in all KC subtypes (=the whole MBs) in the honeybee brain ( Figure 1D). Considering that E75 is expressed preferentially in all KC subtypes (=the whole MBs) , whereas EcR, HR38, and E74 are preferentially expressed in the sKCs (Paul et al., 2005;Yamazaki et al., 2006;Takeuchi et al., 2007), it might be that different ecdysteroidsignaling pathways function in distinct KC subtypes.
With regards to PLC, there are four homologs, including PLCe, in the honeybee. The PLCe is expressed almost selectively in the whole MBs, and expression of the other three homologs is significantly higher in the MBs than in other brain regions (Suenami et al., 2017). Suenami et al. (2017) revealed that pharmacological inhibition of PLC significantly attenuated the memory acquisition, but did not affect memory retention, suggesting that PLCs are involved in early memory formation in the honeybee (Suenami et al., 2017). Thus, although both CaMKII and PLC are involved in Ca 2+ -signaling (Smrcka et al., 2012;Dusaban and Brown, 2015), they play roles at different stages of learning and memory. It will be interesting to test whether their roles at different stages of learning and memory can be attributed to their distinct KC subtype-preferential expression.
Two genes, tachykinin-related peptide (Trp) and juvenile hormone diol kinase (JHDK), are preferentially expressed in both the "redefined" lKCs and "redefined" sKCs, but not in the mKCs (Figure 1D; Takeuchi et al., 2004;Uno et al., 2007;Kaneko et al., 2013). The Trps are multifunctional brain/gut peptides that have important roles in neurotransmission and/or neuromodulation (Van Loy et al., 2010). In Drosophila, tachykinin-expressing neurons control male-specific aggressive behaviors (Asahina et al., 2010). Therefore, it might be that Trp is also involved in the control of aggressive behaviors even in the honeybee. The function of JHDK in insects is not well understood (Uno et al., 2007).
Interestingly, McQuillan et al. (2012) reported that the expression of genes for amine receptors, which are involved in learning and memory, differs across KC subpopulations (McQuillan et al., 2012), which is consistent with the recent notion that different regions of the MBs contribute to learning and memory in Drosophila (Zars et al., 2001;McGuire et al., 2003;Trannoy et al., 2011).

KC Population Expressing FoxP
Recently, Schatton and Scharff (2017); Schatton et al. (2018) identified a novel KC population expressing transcription factor FoxP in the MBs of the honeybee brain ( Figure 1D) (Schatton and Scharff, 2017;Schatton et al., 2018). Although Kiya et al. (2008), who first reported the FoxP expression in the honeybee brain, detected no significant FoxP expression in the honeybee MBs (Kiya et al., 2008), Schatton et al. notified that, in Drosophila, a MB-core subpopulation expresses FoxP, which is related to decision-making (DasGupta et al., 2014). They also reported FoxP expression in the honeybee MBs (Schatton and Scharff, 2017). These findings suggest that neural populations with FoxP expression that are related to reinforcement-based learning abilities are conserved among animal species (Schatton and Scharff, 2017;Schatton et al., 2018).
There seems to be a problem, however: although Schatton et al. indicated that the KC population expressing FoxP does not overlap with mKCs, and speculated that FoxP specifies different subsets of mKC (Schatton and Scharff, 2017), Kaneko et al. (2013) and Suenami et al. (2016) reported that lKCs do not overlap with mKCs, and observed no gaps between the areas where lKC and mKC somata exist (Kaneko et al., 2013;Suenami et al., 2016). Based on the latter findings, the KC population expressing FoxP is assumed to be the lKCs. This point needs to be clarified in future studies.

ANALYSIS OF KC SUBTYPE DIFFERENTIATION DURING METAMORPHOSIS
Genes expressed in a KC subtype-preferential manner can be used as markers to trace the differentiation of KC subtypes or their evolution in hymenopteran insects.
In honeybees, larval MBs comprise only class II KCs. Class I "classical" lKCs and sKCs are newly produced from proliferating neuroblasts whose somata are located in the inner core inside of the MB calyces during the pupal stages (Farris et al., 1999) and cease their proliferation at the P2 and P5 stages, respectively. Suenami et al. (2016) recently used three genes, Syt14, dlg5, and PLCe, as markers to trace the differentiation of the "redefined"  (Oya et al., 2017). (Lower panel) Schematic illustrations of KC subtypes in a single MB calyx of each hymenopteran species. Class I "redefined" lKCs, mKCs, and "redefined" sKCs in the MB are colored green, magenta, and blue for the honeybee, scoliid wasp, and hornet, respectively. Two putative class I KC subtypes in the parasitic wasp MB are shown in yellow and light blue, respectively. The single putative class I KC subtype in the sawfly MB is shown in pale green. (B) Flowchart to produce homo-/heterozygous mutant workers by genome editing using CRISPR/Cas9. Mosaic queens (F0) with genome-edited germline cells are first produced by inducing fertilized embryos injected with sgRNA and Cas9 mRNA into queens. Subsequently, the mosaic queens are induced by transiently anesthetizing them with CO 2 to lay unfertilized eggs, which grow into drones. Mutant drones (F1) derived from the mosaic queens are reared to adulthood, and the sperm collected from the sexually matured mutant drones is used to artificially inseminate a wild-type queen to produce a heterozygous queen (F2). Hetero-and homozygous mutant workers (F3) are produced by again artificially inseminating the heterozygous queen with sperm from the genome-edited drones. Figures for (A,B) are cited from Oya et al. (2017) and Kohno et al. (2016) with some modifications. lKC (Syt14, and dlg5) and all KC subtypes (PLCe) . The PLCe is already expressed in larval MBs and continues to be expressed in the whole MBs during the pupal stages, suggesting that Ca 2+ -signaling is enhanced in the whole MBs during the entire honeybee lifespan. The expression of Syt14 and dlg5 becomes detectable at the middle pupal stages (around P3), and is restricted to the lKCs at the adult stage, suggesting that expression of Syt14 and dlg5 is characteristic of differentiated lKCs . The FoxP expression is also not detected in larval MBs, but becomes detectable in the MBs at the middle-to-late pupal stages (P4-5) (Schatton et al., 2018), suggesting that FoxP expression is also characteristic of differentiated KCs. In contrast, KCs expressing mKast become detectable at the late pupal stages (P7 and P8) (Kaneko et al., 2013), suggesting that mKCs develop after the lKCs begin to differentiate or mKast is expressed at the late stage of mKC differentiation. Farris and Schulmeister (2011) indicated that both aculeate insects and parasitic wasps, which are hymenopteran insects that appeared later in the course of evolution, have more morphologically elaborate MB calyces than sawflies, which are primitive hymenopteran insects, and proposed that the elaborate MB calyces are associated with the higher learning ability of parasitic wasps (Farris and Schulmeister, 2011). This leads to the question of when during the evolution of hymenopteran insects were KC subtypes acquired? To address this question, Oya et al. (2017) performed in situ hybridization of Trp homologs to compare KC subtypes among the brains of four hymenopteran insect species: (1) a phytophagous and solitary sawfly (Symphyta; Arge similis), (2) a solitary parasitic wasp (Apocrita; Ascogaster reticulata), (3) an eusocial hornet (Aculeata; Vespa mandarinia), and (4) a nidificating and solitary scoliid wasp (Aculeata; Campsomeris prismatica) (Oya et al., 2017). As Trp is expressed in both "redefined" lKCs and "redefined" sKCs, but not in mKCs; the presence of all three KC subtypes can be visualized in a certain hymenopteran insect brain by performing in situ hybridization of a single Trp homolog (Takeuchi et al., 2004).

POSSIBLE KC SUBTYPE EVOLUTION IN HYMENOPTERAN INSECTS
The brains of V. mandarinia and C. prismatica have three class I KC subtypes (lKCs, mKCs, and sKCs), as observed in the honeybee. In contrast, the brain of A. reticulata has only two KC subtypes; "large" KCs with significant Trp-expression and "small" KCs with no significant Trp-expression, and the brain of the sawfly A. similis has no discriminable KC subtypes (Farris and Schulmeister, 2011) (Figure 2A). Discrimination of class I and II KCs is difficult in A. reticulata and A. similis, because the MB calyces are shallow and Class I and II KCs seem to be merged in these species.
It is plausible that the advanced learning abilities of parasitic wasps to search for their host insects require MBs with elaborate calyces and both ancestral (original) and second KC subtypes, whereas the highly advanced learning abilities of aculeate insects to return to their nests require MBs with all of the class I KC subtypes, in addition to the elaborate MB calyces (Whitfield, 2003;Huber, 2009;Johnson et al., 2013). To test this notion, the correspondence between one and two KC subtypes detected in sawfly and parasitic wasps, and three KC subtypes detected in aculeate insects will need to be examined by in situ hybridization for homologs of genes expressed in a KC subtype-preferential manner in the honeybee (e.g., Syt14, dlg5, or Mblk-1 for "redefined" lKCs; mKast for mKCs; and Trp or JHDK for "redefined" lKCs/sKCs, respectively. See also Figure 1D) (Kubo, 2012;Kaneko et al., 2013Kaneko et al., , 2016Suenami et al., 2016). The KC subtype/population that expresses FoxP in these hymenopteran insect species is also an intriguing topic for future investigation (Schatton and Scharff, 2017). Such experiments are expected to unveil KC subtype/population of ancestor origin in the hymenopteran insects and those unique to aculeate insects.

APPLICATION OF GENOME EDITING FOR ANALYSIS OF THE ROLE OF KC SUBTYPES IN THE HONEYBEE
While RNAi is effective for analyzing gene function, its efficiency sometimes varies depending on the animal species and target genes and/or organs (Matsumoto et al., 2014). In addition, it is difficult to suppress gene function for a long time (Matsumoto et al., 2014). An alternative method for the analysis of gene function is genome editing. Genome editing has been applied to some hymenopteran insects, including the sawfly Athalia rosae (Hatakeyama et al., 2016), parasitic wasp Nasonia vitripennis (Li et al., 2017), and two social ants, Ooceraea biroi and Harpegnathos saltator (Trible et al., 2017;Yan et al., 2017). A transgenic technique using piggyBac has been applied to honeybees (Schulte et al., 2014). Recently, Kohno et al. (2016) established a basic genome-editing technique in the honeybee to analyze in vivo gene function (Kohno et al., 2016).
To analyze the roles of genes in regulating the behaviors and/or brain functions exhibited by honeybee workers, it is necessary to produce hetero-or homozygous mutant workers (F3) through several steps (Figure 2B; Kohno et al., 2016). For this, it is important that adult mutant honeybees [mutant drones (F1) and homozygous mutant workers (F3)] should be alive; in other words, the target gene(s) must be dispensable for normal development and sexual maturation in honeybees. Kohno et al. selected major royal jelly protein 1 (mrjp1) as a target gene to establish basic honeybee genome-editing techniques. The MRJP1 is the most abundant protein component of the royal jelly, which is produced by the hypopharyngeal glands of young nurse bees and secreted as food for the larvae, drones, and queens (Kubo et al., 1996;Ohashi et al., 1997;Schmitzová et al., 1998). As expected, the results indicated that mrjp1 is dispensable for normal drone development (Kohno et al., 2016).
Genes expressed in a KC subtype-preferential manner can also be good candidate target genes for genome editing, because some of them are assumed to relate to some brain functions and some of them are dispensable for normal honeybee development and sexual maturation. Investigation of the functions of genes involved in development and sexual maturation will require other methods as well, such as the expression of knocked-in genes in a stage-and/or tissue-specific manner by genome-editing.

AUTHOR CONTRIBUTIONS
TK drafted the manuscript and figures. SS, SO, HK, and TK wrote and reviewed the manuscript, and completed figures.

FUNDING
This work was supported by a Grant-in-Aid for JSPS Fellows (17J03716).