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.