Introduction
In mammals, offspring have access to milk, a substance rich in a range of nutritious and non-nutritious factors that are essential for the survival of the developing neonate (Jiang et al., 2020; Lu et al., 2019). Newborn ungulates such as ruminants rely on the ingestion of a sufficient amount of good-quality colostrum to acquire passive immunization and thus protecting against infectious diseases (Ahmad et al., 2007; Kessler, Pistol, Bruckmaier, & Gross, 2020). The first mammalian milk, colostrum, contains non-nutritional substances such as proteins, peptides and steroids needed by the newborn mammal and that are absorbed through the gut of the neonate. These substances play a role in development, in endocrine and in additional physiological functions, and in the immune system of the neonate (Ahmad et al., 2007; Buckley, Val, & Sloman, 2011). Feeding maternal colostrum has been shown to increase absorption of nutrients such as glucose and improve intestinal morphology and absorption of nutrients compared with formula (Ahmad et al., 2007; RH Drent, 1980). Offspring fed by parents grow and mature faster (Beekman, Thompson, & Jusup, 2019). Sometimes parental care, a prevalent behavior in teleost fishes, increases offspring survival. A behavior system for providing food for young is one of the more significant evolutionary traits shared among a wide range of taxa (Ahmad et al., 2007; Holbrook, 2011). Boulengerula taitanus is a direct-developing, oviparous caecilian, the skin of which is transformed in brooding females to provide a rich supply of nutrients for the developing offspring (Kupfer et al., 2006). bi-parental care in a teleost, i.e. mucus-provisioning behaviour in the scale-eating cichlidPerissodus microlepis endemic to Lake Tanganyika (S. Satoh et al., 2019). American cichlids, such as substrate incubators and mouth incubators, have parental behavior (S. Satoh, Tanoue, Ruitton, Mohri, & Komatsu, 2017).
The discus fish Symphysodon spp ., an Amazonian cichlid, also employs an unusual parental care behavior where free-swimming fry feed on parental epidermal mucus after hatching (Ahmad et al., 2007; Buckley et al., 2010; Holbrook, 2011). Some study reports that circumstantial evidences indicate that both sexes of parents care for their offspring and provide their body mucus as food for fry when fry begin to freely swim (Ahmad et al., 2007; Shun Satoh, Tanoue, & Mohri, 2018). In discus fish, mucus provisioning is essential for offspring survival, and offspring depend on parental mucus more than any other species that provide mucus to their offspring (R. DeAngelis, Dodd, Snyder, & Rhodes, 2018). In the past many studies have focused on the behavioral characteristics of mucus feeding in the discus (Buckley et al., 2011). For example, the manner in which the number of fry influences the costs and benefits of mucus provisioning in discus fish, found that fry grew more rapidly when they were raised with parents than when raised without parents (Shun Satoh et al., 2018). Some parts also studied the composition of Midas cichlid (Cichlasoma citrinellum ) mucus and found the amount of rich in protein, prolactin, growth hormone and so on in the mucus (Ahmad et al., 2007; Schutz & Barlow, 1997). However, there are few regulatory mechanisms involved in the secretion of milk by the skin, only test the role of hormone prolactin (PRL) in brood care behavior of the cooperatively breeding cichlid ( Neolamprologus pulcher ) (Ahmad et al., 2007; Bender, Taborsky, & Power, 2008). PRL mRNA was determined in the pituitary glands of breeders of both sexes, helpers that showed brood care behavior and nonbreeding fish as controls, but found no evidence that elevated levels of PRL are directly involved in the regulation of brood care behavior in this species (Ahmad et al., 2007; Bender et al., 2008).
The brain plays a critical role in upstream regulation of processes central to mating effort, parental effort (Bentz, Rusch, Buechlein, & Rosvall, 2019; Tavares et al., 2019). For breeding animals, rapidly shifting social and physical demands may lead to especially critical physio-logical, behavioral, and life history trade-offs (Ahmad et al., 2007; Robert, 1972; Stiver & Alonzo, 2009). Understanding how species and behaviors are coordinated among and within individuals is of keen interest in social neuroscience, especially as behavioral variation corresponds to variation within the brain (R. DeAngelis et al., 2018). Animals shift to parenting while also maintaining their own energy reserves (R Drent & Daan, 1980). The central nervous system plays a critical role in upstream regulation of these processes. The nonapeptides arginine vasopressin (AVP) and oxytocin (OT), as well as their non-mammalian homologs arginine vasotocin (AVT) and isotocin (IT), have been implicated as key neuromodulators in a variety of social behaviors, including parental care. AVP/AVT and OT/IT neuron cell bodies reside primarily within the preoptic area of the hypothalamus (POA), and project widely throughout the brain (R. DeAngelis et al., 2018). In prairie voles, AVP injections into the lateral septum, a brain region known to be involved in mediating behavioral acts related to offspring care (Dulac, O’Connell, & Wu, 2014; Wang, Ferris, & De Vries, 1994). Among teleosts, the anemonefish (Amphiprion ocellaris ) presents an exciting opportunity for exploring neuroendocrine regulation of male parental care. A. ocellaris lives in relatively small and simple social groups, where pair bonds and social hierarchies are established long before mating occurs. Therefore, there is no active courtship, nest building or intraspecific aggressive interactions co-occurring during high levels of parental care, enabling the underlying regulatory mechanisms to be more specifically extricated (R. DeAngelis, Gogola, Dodd, & Rhodes, 2017; R. S. DeAngelis & Rhodes, 2016; Iwata, Nagai, & Sasaki, 2010; Ji, Long, Briody, & Chien, 2011). So, do discus fish have similar brain regulation mechanisms?
The case of parental care in fish is indeed a fascinating model system for answering a variety of questions about the cost of reproduction and the parent–offspring conflict.  In order to further reveal the regulatory mechanism of discus lactation, we performed transcriptome analysis on the skin and brain tissues of parental females and non-parental females. This research through the transcriptome sequencing technology to discus fish skin stage in non-parental females (NP_S) and parental females (P_S), analysis and screening of the skin in NP_S and P_S about differentially expressed genes. The discus fish brain stage as the same as skin, discus fish brain stage in non-parental females (NP_B) and parental females (P_B). For the future of discus tending a special behavior provide theoretical basis for further research. This study will help us to understand the mechanism underlying discus lactation and identify genes related to the lactation of discus fish and finally propose effective strategies to breed child for better control of discus lactation.