Evolution of the neural sex-determination system in insects: does fruitless homologue regulate neural sexual dimorphism in basal insects?

The role of fruitless in specifying courtship behaviors across divergent Drosophila species

Sex-specific behaviors are critical for reproduction and species survival. The sex-specifically spliced transcription factor fruitless (fru) helps establish male courtship behaviors in invertebrates. Forcing male-specific fru (fruM) splicing in Drosophila melanogaster females produces male-typical behaviors while disrupting female-specific behaviors. However, whether fru's joint role in specifying male and inhibiting female behaviors is conserved across species is unknown. We used CRISPR-Cas9 to force FruM expression in female Drosophila virilis, a species in which males and females produce sex-specific songs. In contrast to D. melanogaster, in which one fruM allele is sufficient to generate male behaviors in females, two alleles are needed in D. virilis females. D. virilis females expressing FruM maintain the ability to sing female-typical song as well as lay eggs, whereas D. melanogaster FruM females cannot lay eggs. These results reveal potential differences in fru function between divergent species and underscore the importance of studying diverse behaviors and species for understanding the genetic basis of sex differences.

fruitless is sex-differentially spliced and is important for the courtship behavior and development of silkmoth Bombyx mori

Sexual dimorphisms of the brain play essential roles in successful reproduction. Silkmoth Bombyx mori exhibits extensive sexual differences in sexual behavior, as well as their morphology. Although the neural circuits that transmit information about sex pheromone in the male brain are extensively analyzed, the molecular mechanisms that regulate their development are still elusive. In the present study, we focused on the silkmoth ortholog of fruitless (fru) as a candidate gene that regulates sexual dimorphisms of the brain. fru transcripts were expressed from multiple promoters in various tissues, and brain-specific transcripts were sex-specifically spliced, in a manner similar to Drosophila. Interestingly, fru was highly expressed in the adult female brain and the male larval testis. Analysis of CRISPR/Cas9-mediated fru knockout strains revealed that fru plays important roles in survival during late larval and pupal stages, testis development, and adult sexual behavior. fru mutant males exhibited highly reduced levels of courtship and low copulation rate, indicating that fru plays significant roles in the sexual behavior of silkmoths, although it is not absolutely necessary for copulation. In the fru mutant males, sexually dimorphic pattern of the odorant receptor expression was impaired, possibly causing the defects in courtship behavior. These results provide important clues to elucidate the development of sexual dimorphisms of silkmoth brains, as well as the evolution of fruitless gene in insects.

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.

Molecular characterization and expression profiling of transformer 2 and fruitless-like homologs in the black tiger shrimp, Penaeus monodon

Transformer 2 (tra 2) and fruitless (fru) genes have been proven to play a key role in sex determination pathways in many Arthropods, including insects and crustaceans. In this study, a paralog of P. monodon tra 2 (Pmtra 2), P. monodon ovarian associated transformer 2 (PmOvtra 2) and 2 isoforms of P. monodon fruitless-like gene (Pmfru-1 and Pmfru-2) were identified and characterized. The full cDNA sequence of PmOvtra 2 consisted of 1,774 bp with the longest open reading frame (ORF) of 744 bp encoding for 247 amino acids. The PmOvtra 2 exhibited a predicted RNA-recognition motif (RRM) domain and two arginine-serine (RS) regions, suggesting its function in RNA splicing. The full cDNA sequence of Pmfru-1 consisted of 1,306 bp with 1,182 bp ORF encoding for 393 amino acids, whereas the full cDNA sequence of Pmfru-2 consisted of 1,858 bp with 1,437 bp ORF encoding 478 amino acids. The deduced amino acid sequences of Pmfru-1 and Pmfru-2 exhibited highly conserved domains of Fru proteins, including Broad-complex, Tramtrack and Bric-a-brac (BTB), and zinc finger (ZF) domains. In addition, Pmfru-1 and Pmfru-2 were suggestively originated from the same single genomic locus by genomic sequence analysis. Specifically, Pmfru pre-mRNA was alternatively spliced for Pmfru-1 and Pmfru-2 to include mutually exclusive exon 7 and exon 6, respectively. Temporal and spatial expression of PmOvtra 2, Pmfru-1, and Pmfru-2 were also investigated by qPCR. The results showed that all were expressed in early developmental stages with undifferentiated gonads starting from nauplius until postlarvae. The expression of PmOvtra 2 started at nauplius stage and gradually increased from mysis to postlarvae (PL) 1. However, the expression of Pmfru-1 was low at the nauplii stage and slightly increased from protozoea to PL5, whereas the expression of Pmfru-2 maintained a low level from nauplius to mysis and then gradually increased at the PL stages. Expressions of PmOvtra 2, Pmfru-1, and Pmfru-2 were detected in various tissues including nervous tissue, gill, heart, hepatopancreas, gut, and gonads. Interestingly, the sexually dimorphic expression of PmOvtra 2, Pmfru-1, and Pmfru-2 was demonstrated in fully developed gonads in which the ovary showed significantly higher expressions than the testis. The great difference in the expression pattern of PmOvtra 2, Pmfru-1, and Pmfru-2 in the ovary and testis suggested their roles in the female sex determination in P. monodon.

Identification and expression pattern of the sex determination gene fruitless-like in Cherax quadricarinatus

The fruitless (fru) gene has an important function in the courtship behavior and sex determination pathway of Drosophila melanogaster; however, the fru gene has never been reported in shrimps. In this study, the fruitless-like gene was identified in Cherax quadricarinatus (Cqfru) and is reported here for the first time. A sequence analysis revealed a conserved BTB domain in Cqfru which is the same as fru in D. melanogaster. An analysis of the expression level of Cqfru showed that it was highly expressed in the gastrula stage during embryonic development. Furthermore, in situ hybridization and expression distribution in tissues showed that its sexually dimorphic expression may be focused on the hepatopancreas, brains, and gonads. The gonads, brains, and hepatopancreas of males had a higher expression level of Cqfru than those of females; however, the expression level of the abdominal ganglion was found to be higher in females than in males in this study. The results of an RNA interference treatment showed that a knockdown of Cqfru reduced the expression of the insulin-like androgenic gland hormone (IAG) and tumor necrosis factor (TNF). The characteristic fru gene in shrimps is reported here for the first time, with the results providing basic information for research into the sex-determination mechanism in C. quadricarinatus.

Recent neurogenetic findings in insect courtship behaviour

Insect courtship parades consist of series of innate and stereotyped behaviours that become hardwired-in during the development of the nervous system. As such, insect courtship behaviour provides an excellent model for probing the principles of neuronal assembly, which underlie patterns of behaviour. Here, we present the main advances of recent studies - in species all the way from flies to planthoppers - and we envisage how these could lead to further propitious findings.