1 Introduction
The giant river prawn Macrobrachium rosenbergii lives in fresh water in the tropical regions of northern Australia and Southeast Asia. This species is the largest Macrobrachium species with complex traits including an omnivorous diet, faster growth, and a short reproductive cycle. It has become a favored species in prawn farms and is commercially cultured in many countries and regions (Chen et al. 2012). M. rosenbergii was first introduced to China (from Japan) in 1976, raised rapidly in large scale. Its cultivation has maintained a increasing trend, reaching 139.6 thousand tons of live weight aquaculture production in 2020, which accounted for 47.48% of the worldwide total production (294 thousand tons) (FAO 2022; Ministry of Agriculture and Rural Affairset al. 2020).
One striking feature of the giant river prawn is that males grow faster and distinctly larger than mature females. Additionally, the male’s second pair of chelipeds is larger and thicker than that of the female, exhibiting sexual dimorphism (Tan et al.2020). Genetic sex-determination in M. rosenbergii follows the ZW mode, and WW females can sex reverse into functional males via androgenic gland cell transplantation (Levy et al. 2019). The sex-linked genes on the ZW chromosomes were observed (Ma et al. 2019). The giant river prawn’s physiological molting is a basic process of the post-larvae prawn, through which they realize metamorphosis, growth, and development. Successful molting is regulated by both the nervous system and the neuroendocrine system, exhibiting an initial rise and a coordinated decline in the circulating concentration of molt hormone and molt-inhibiting hormone (Yan et al.2016). Those genes that present in the ecdysone signal pathway, underlying hormonal regulation, have been identified as being involved in molting and epidermis reconstruction. These include chitin binding proteins, crustacean hyperglycemic hormone, calcification-related cuticular proteins, and chitinase in Penaeid white shrimp (Litopenaeus vannamei ) (Zhanget al. 2019). In fact, a comparison between higher growth and lower growth transcriptomes revealed genes potentially involved in superior growth performance based on family selected stocks ofLitopenaeus vannamei (Santoset al. 2021). However, little attention has been given, so far, to the regulation of sexual dimorphism in growth by the nervous system, which could be useful in understand nervous regulation of life processes (Bobkova et al. 2020).
In evolutionary biology, the nervous system tends to concentrate, a progress from scattered and simple nervous net to a complex centralized nervous system. The evolution of the nervous system allows animals to regulate body functions by generating, modulating, and transmitting information (Arendt et al. 2016). The brain of M. rosenbergii is located in a mass of spongy tissue within the base of its eyestalks. It comprises the protocerebrum, deutocerebrum, tritocerebrum, nerve roots, commissures, and clusters of cell bodies working in concert. Its chained neural system is composed of cerebral ganglia, thoracic ganglia and abdominal ganglia (Liao 2001). Extracts from the thoracic ganglia stimulate oocyte development in the giant river prawn (Liao et al. 2001), which acts indirectly on the gonads by triggering the release of a putative gonad stimulating factor from the thoracic ganglion (Jayasankar et al. 2020). Secretions from the eyestalk, brain, and thoracic ganglia of decapod crustaceans have a stimulatory effect on ovarian maturation (Mohamed & Diwan 1991). The eyestalk’s regulation of ovarian maturation has been well-studied by Illumina sequencing in pacific white shrimp Litopenaeus vannamei(Wang et al. 2019). Due to a lack of completely related-gene analysis, the possible molecular mechanisms by which the brain and thoracic ganglion regulate gonad development and growth remain unclear. Nanopore-based full-length transcriptome sequencing methods are capable of sequencing transcripts from end to end at a single molecule level and can be used to annotate transcriptome structures in a variety of organisms (Roach et al. 2020). However, comparative studies of the transcriptomes for the brain and thoracic ganglia between sexes of the giant river prawn have received little attention outside of miRNA (Xia et al. 2022).
To investigate and gain a better understanding of sex-linked growth dimorphism through regulation of the brain and thoracic ganglia inM. rosenbergii , we analyzed the full-length transcriptomic characterization of the brain and thoracic ganglion of female and male prawns using SMART single molecule sequencing and compared gene expression differences between the sexes. This allowed us to discover that differentially expressed genes were significantly enriched in the endoplasmic reticulum pathway’s protein processing, a unique protein-synthesizing mechanism that plays a crucial role in efficient neural communication and is responsible to growth dimorphism in the giant river prawn. To our knowledge, this is the first full-length transcriptome wide gene expression profiling of the brain and thoracic ganglion in M. rosenbergii . This study will serve as a basic resource for further studies of growth regulation mechanisms, and be of special interest to exploring molecular breeding for targeting growth genes.