A Qualitative Model of the Interaction of Sexual Behavior and Hormone Gene Transcription in Male Blue Gourami during Reproduction

In the present study, a model is suggested to describe hormone control in male blue gourami (Trichogaster trichopterus) along the gonadotropic brain-pituitary-gonad axis (BPG axis) and the hypothalamic-pituitary-somatotropic axis (HPS axis). This model is based on the cloning and transcription of genes encoding hormones of the two axes involved in spermatogenesis during blue gourami reproduction. Gene transcription is affected by environmental, biological, and behavioral factors. Mature males were examined in two different stag-es—nonreproductive in high-density habitats and reproductive in low-density habitats. Based on gene transcription, gonadotropin-releasing hormone 1 (GnRH1) was involved in controlling spermatogenesis (spermatogonia to spermatids) via the BPG axis in nonreproductive and reproductive stages by controlling follicle-stimulating hormone (FSH), 11-ketotestosterone (11KT) and 17β-estradiol (E 2 ). However, GnRH3 had a larger effect during the reproductive stage via the BPG axis (spermatids to sperm) on luteinizing hormone (LH), 11KT, and 17α-hydroxyprogesterone (17P). At the same time, the HPS axis was involved in spermatogenesis via pituitary adenylate cyc-lase-activating polypeptide (PACAP) and its related peptide PRP (formerly known as GHRH-like peptide) in the brain, and growth hormone (GH) in the pituitary affected synthesis of insulin-like growth factor 1 (IGF1) in the liver.


Introduction
Blue gourami (Trichopodus trichopterus) belongs to the Labyrinithici fish, order Anabantiformes. It is characterized by a specialized organ termed labyrinth, located above its gills, for the respiration of atmospheric oxygen [1]. The geographical distribution of suborder Anabantoidei fish is Central Africa, India, and southern Asia [2]. In its natural habitat, the Anabantoidei fish adapts to an unpredictable environment in which water oxygen concentration varies throughout the year, and can reach a very low percentage, prompting development of the labyrinth organ [1].
Labyrinthici fish undergo two different periods during their life cycle: 1) before labyrinth organ development, from eggs to juveniles, when oxygen is retained over the entire surface by diffusion; 2) after labyrinth development, when the organ becomes important for breathing [1] [3] [4]. The adaptation for the development of eggs and fry in water with low oxygen concentrations involves laying eggs in a bubble nest. The complex sexual and nest-building behaviors of fish belonging to the Anabantoidei species have been described in detail [2] [5] [6] [7]. In natural habitats, when there is a low density of mature males, they become territorial, build a bubble nest and protect it from other males ( Figure   1). The function of the bubble nest is to supply oxygen to the eggs and larvae in water with very low oxygen concentrations [1].
In blue gourami, as a model Labyrinithici fish for hormonal control of reproduction, the male's sexual behavior [10] and pheromones [11] [12] affect the gonadotropic axis-the brain-pituitary-gonadal (BPG) axis [9] [13], and the somatotropic axis-the brain-pituitary-liver and body (BPLB) axis. These axes are very complex and they control gametogenesis [1] [10] [14]. In the present study, a model is suggested to describe hormone control in male blue gourami.  [9]. After courting and fertilization, the female swims under the bubble nest and spawns eggs into it. The male guards the nest with the eggs. If an egg falls out, the male returns it to the nest. It also protects the fish in the first period, immediately after hatch ( Figure 2).

Hormones Involved in the Control of Male Reproduction
Many factors have differential effects on the brains of mature males vs. females  Vitellogenin (VTG) Liver [22] Insulin-like growth factor 1 (IGF1) Liver [22] Journal of Biophysical Chemistry as fish density and pheromones [1] [5]. It has been suggested that the male blue gourami [28] is found in one of two stages after maturation: nonreproductive and reproductive, dependent on conditions that affect the brain and are con-  Figure 3). In the first stage, males are present at high densities, and no sexual behavior or nest building occurs. In the second stage, males are present at low densities and they build nests and become territorial. This situation gives us the opportunity to examine the changes in genes and hormones occurring in the blue gourami brain and BPG axis [28] under these two distinct conditions, mainly GH [22] and PRL [23]; in the plasma and testis, E 2 , T and 17, 20P [24] [25] [26]; and in the liver, VTG and IGF1 [22].  [18] and its related peptide (PRP) [18], growth hormone (GH) [22], and prolactin (PRL) [18]. FSH and LH act on the ovary to synthesize the steroids 17β-estradiol (E 2 ), testosterone (T) and 17α, 20β-dihydroxy-4-pregnen-3-one (17, 20P) [24] [25] [26]; and on the liver to synthesize vitellogenin (VTG) and insulin-like growth factor 1 (IGF1) [22].
the BPG axis at both stages: before sexual behavior and during sexual behavior [17]. It controls the change from spermatogonia to spermatids through FSH, 11KT and E 2 . GnRH3 has a stronger effect via the BPG axis on the change from spermatids to sperm through LH, 11KT and 17P. However, it is very difficult to separate the functions of the hormones controlling the various stages of spermatogenesis that occur continually in blue gourami, as in many Labyrinithici fishes. Testis histology clearly shows the different stages of the sperm cells: spermatogonia, primary spermatocytes, spermatids and sperm [17] [20] in the Journal of Biophysical Chemistry mature males before and during sexual behavior. The only difference was found in the amount of sperm, which increased during sexual behavior. In the model (Figure 3), we suggest that at this stage, GnRH3 affects LH. The model suggests that in addition to the BPG axis, the BPLB axis also affects spermatogenesis. PACAP transcription is significantly higher in mature vs. juvenile males [20] [30]. It is suggested that this is because the BPLB axis involvement in spermatogenesis (from spermatogonia to sperm) affects GH and IGF1, but not sperm release into the water during sexual behavior. In male-derived cells from the pituitary gland, the GHRH-like peptide PRP significantly increases GH transcription [30]. PACAP transcription was found to be higher in reproducing males in vivo, and GH mRNA level was the same in juvenile, mature and reproducing males [31]. Many published studies support the notion that the somatotropic axis and liver, via GH and IGF1, also modulate reproduction directly and indirectly, along with the BPG axis in both males and females [32]. This situation was found with the hormones involved in spermatogenesis as described in the qualitative model suggested in the present study ( Figure 3).

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.