Testes morphology and the identification of transcripts of the hormonal pathways of the velvetbean caterpillar Anticarsia gemmatalis Hübner, 1818 (Lepidoptera: Erebidae)

Reproduction results from a series of interdependent steps that occur throughout the life cycle. This process begins during embryonic development, with sex determination, and goes through the differentiation of primary and secondary structures, development of accessory glands, gametogenesis, and vitellogenesis, among others.

In the case of insects, they exhibit great morphological divergence, so that the structural, physiological, and biochemical nature of the reproductive system is highly variable (Gillott, 1995). Specifically in Lepidoptera, the male reproductive system is formed by the testes, vas deferens, seminal vesicles, accessory glands, and ejaculatory ducts (Alves et al., 2006; Zhang et al., 2017b). Interestingly, in some previously characterized lepidopterans, during metamorphosis, the testes fusion process occurs. Thus, in the larval period, there is a pair of paired testicles in the abdomen, which later fuse to form a single organ in adults (Bebas et al., 2018; Brilha et al., 2012; Mari et al., 2018).

In addition to morphological characters, other components are involved in lepidopteran reproduction, including hormonal factors. Sesquiterpenoid hormones (juvenile hormones, JHs) and ecdysteroids (the active form 20-hydroxyecdysone, 20E) play key roles in insect development, including molting and metamorphosis events, and play an important role in reproduction (Palli, 2016). Different processes that occur in the male reproductive system are strongly regulated by these hormones, such as testicular development, regulation of accessory gland secretion, and spermatogenesis, among others (Friedländer, 1997; Parthasarathy et al., 2009). Recently, Chen et al. (2020) investigated the molecular mechanisms of testicular fusion in Spodoptera litura using RNA-seq, to identify the differential expression of genes related to the 20E and JH signaling pathways during the testicular fusion process.

The development of sequencing technologies has allowed genomic and transcriptomic studies to identify numerous candidate genes involved in the regulation of hormonal pathways and in different aspects of reproduction in insects (Chauhan et al., 2016; Wei et al., 2015; Wu et al., 2017). In fact, enzymes that participate in the biosynthesis, degradation, and signaling pathways of JHs and ecdysteroids have already been characterized in several species (Cheng et al., 2014; Huang et al., 2015; Iga and Smagghe, 2010; Zhang et al., 2017a).

Although morphological descriptions, histological analyses, and molecular studies of processes related to reproduction and hormonal pathways have been conducted in different Lepidoptera, information on these processes is limited in Anticarsia gemmatalis Hübner, 1818 (Lepidoptera: Erebidae). This species, popularly known as the velvetbean caterpillar, is considered one of the main defoliating pests of soybeans in Brazil, causing significant economic losses in the regions that produce this legume (O'Neal and Johnson, 2010; Peterson et al., 2017; Roe et al., 2009; Sosa-Gómez et al., 2014; Van Nieukerken et al., 2011).

Knowledge of the histology and ultrastructure of the reproductive system, as well as the study of spermatogenesis, is important for understanding the reproductive biology of this important agricultural pest. Furthermore, identification of genes that encode proteins involved in the main hormonal pathways can be used as a basis for further research in other economically important lepidopterans, and may even help in programs related to pest control. In the current study, we characterized the morphology of the testes of the velvetbean caterpillar at different stages of life, as well as the spermatogenesis process. We also performed the identification of transcripts that encode proteins and/or enzymes involved in the biosynthesis, metabolism, and signaling of JHs and ecdysteroids using information generated from the sequencing and assembly of the transcriptome of A. gemmatalis, thus allowing an overview of potential mechanisms of regulation of hormonal pathways.