Carotenoids act on coloration and increase immunity and antioxidant activity in the novel “Yongzhang Golden turtle” strain of Pelodiscus sinensis
Introduction
Body color, which can influence colony formation and sexual selection and help escape from predators, is an important characteristic in animals (Svensson and Wong, 2011; Toews et al., 2017). In aquaculture, body color reflects the quality of ornamental aquatic products and affects the economic and commercial value of the animals (Tan et al., 2020). In aquatic products, it is a key indicator that aquaculture practitioners use to evaluate health status and consumers use to judge quality. Chinese soft-shelled turtle (Pelodiscus sinensis), an important commercial freshwater reptile, has been cultured in many Asian countries for decades, including China, Japan, Korea and Vietnam, due to its high nutritional value and high economic profits (Fritz et al., 2010; Liang et al., 2018; Zhang et al., 2015). Soft-shelled turtles with yellow-like color have been associated with higher quality, healthier meat, and higher commercial value in the market (Liu et al., 2013; Wang et al., 2020).
Recently, a novel strain of P. sinensis with bright golden-yellow color, named Yongzhang golden turtle (YGT), was obtained through selective breeding over 16 years in Fuping Baoding, Hebei Province, China (Wang et al., 2021a). Before applying for the approval of the new YGT strain from Chinese Commission for Examination and Approval of Original and Improved Aquatic Species, we performed a test on the growth and survival rates of the YGT and AWT during the first 2 years after hatching. Under intensive culture, the growth rate of the YGT increased by 16.7%–28.2% and its survival rate increased by 8.9%–11.8% compared to those of the AWT in the first year. The growth rate of the YGT was 21.4%–29.7% higher than that of the AWT and the survival rate increased by 8.0–13.3% in the second year. In a fish and P. sinensis polyculture, the growth rate of the YGT was 26.5%–33.1% higher and the survival rate increased by 4.3%–10.9% compared to those of the AWT in the first year, while the growth rate of the YGT was 20.6%–33.6% higher and its survival rate was increased by 2.6%–8.2% compared to those of the AWT in the second year (Variety registration number: GS-01-011-2018). Therefore, in addition to higher growth and survival rates compared to the AWT, the YGT presented higher commercial value because of the ornamental value of its golden-yellow body coloration, with a 10–20 times higher price of hatching YGT than instead of AWT. However, the mechanisms causing the formation of ornamental integumentary color and health status assessment have not been investigated in the YGT.
In vertebrates, coloration depends mainly on various pigments synthesized by pigment cells, such as melanocytes, iridophores and xanthophores (Cuthill et al., 2017; Eliason, 2018; Eliason et al., 2015; Grether et al., 2004; Svensson and Wong, 2011). Carotenoids are considered natural pigments, most of which are widely distributed in bacteria, fungi and plants and are used to acquire conspicuous and ornamental red, yellow and orange coloration in animals (Maoka, 2020; Weaver et al., 2018a). Carotenoids, which are essential nutrients for the physiological functions of animals, are sources of vitamin A and play an important role in other biological activities, such as reducing oxidative stress (Krinsky, 1998; Stahl and Sies, 2003; Kiokias and Gordon, 2004; Weaver et al., 2018b; Pérez-Gálvez et al., 2020), enhancing immune function (Blount et al., 2003; Chew and Park, 2004; Tan et al., 2019; Wang et al., 2019), improving stress tolerance (Babin et al., 2010; Cheng et al., 2020; Zuluaga et al., 2017), and enhancing embryonic development, growth, and gonadal maturation in many species (Giraudeau et al., 2016; Lehnert et al., 2018; Schubert and Gibert, 2020). Studies have found the association between carotenoid coloration and various measures of individual quality in fish, reptiles, birds and domestic mammals (Huang et al., 2021; Maoka, 2020; Olsson et al., 2013; Plaza et al., 2020; Weaver et al., 2018b).
The improvement of antioxidant capability is a useful means to enhance the body conditions of animals (McGraw, 2005). Thus, the mechanisms by which carotenoids have beneficial effects on body conditions are mainly dependent on their effective antioxidant properties. Carotenoids can scavenge two potentially harmful reactive oxygen species (ROS), singlet molecular oxygen and peroxyl radicals, to protect cells, tissues, and the body from oxidative damage and stress (Kiokias and Gordon, 2004; Krinsky, 1998; Stahl and Sies, 2003; Young and Lowe, 2018). The immune system is one of the main contributors to total free radical production in vertebrates, and the administration of antioxidants can improve several immune functions (Chew and Park, 2004; Saino et al., 1999). Therefore, carotenoids, as immunoenhancers, can increase the efficiency of immune responses and stimulate innate immunity components (Chew and Park, 2004).
This study aimed to identify whether carotenoids mainly contribute to the coloration of the YGT and investigate their effect on the YGT's antioxidant activity and immune competence.
Section snippets
Animals and sample collection
Chinese soft-shelled turtle (P. sinensis) at the age of ∼1-year-old, including 10 atrovirens wild type turtle (AWTs) and 10 YGTs, were collected from the Fuping Jingtao Turtle Breeding Farms (Fuping, Baoding, China). All turtles were euthanized and then decapitated to obtain calipash and skin samples to assess carotenoids concentration. We also collected the turtles' liver to assess antioxidant capacity and their spleen and plasma to assess immune competence. The tissue samples were isolated
Carotenoids concentrations in the skin and calipash of the YGT were significantly higher than those of the AWT
YGTs had a special yellow coloration compared with AWTs whose body color was atrovirens with black spots (Fig. 1A). To identify the carotenoids responsible for the color differences between atrovirens and bright yellow morphs, we isolated the skin and calipash and examined their carotenoid concentrations (Fig. 1B and Fig. 1C). The carotenoid concentration of skin and calipash in the YGT was significantly higher than that in the AWT (Supplemental Table 1 and Fig. 1D).
Liver antioxidant capacity in the YGT was higher than that in the AWT
To assess the antioxidant
Discussion
Body color is one of the major criteria for the quality and market price of aquatic products, and it is related to tissues carotenoids concentration (Yu et al., 2021). Because the costs of acquisition and utilization are necessary for elaborating coloration, only strong and healthy individuals are highly used for ornamental purposes (Steffen et al., 2021). Carotenoids coloration may be an indicator of foraging ability (McGraw and Ardia, 2003; Weaver et al., 2018a).
In this study, the results
Conclusions
In this study, we investigated the relationship between carotenoids concentration, body coloration, antioxidant activity, and immune competence. Carotenoids are responsible for body coloration and can enhance the antioxidative status and innate immune competence of YGTs. First, reptilian immune systems are generally characterized by a robust innate immune response and a moderate acquired immune response. Innate immunity is the first line of defense against pathogen invasion, which is activated
Author statement
Ju Zhang, Xifeng Wang, Weiguo Du and Yueqiang Guan conceived and designed the experiments; Ju Zhang and Zihan Ding performed the experiments. Ju Zhang and Xifeng Wang wrote the manuscript. Ju Zhang, Xifeng Wang, Weiguo Du and Yueqiang Guan reviewed and edited the manuscript.
Declaration of Competing Interest
All experimental protocols were complied with the ARRIVE guidelines and followed the National Institutes of Health guide for the care and use of Laboratory animals (NIH Publications No. 8023, revised 1978). The authors declare that they have no competing interests.
Acknowledgments
The authors thank Guoming Cui and Yongzhang Han for providing atrovirens wild-type turtle and Yongzhang golden turtle, respectively. This research was supported by the Project of Application of Hebei Agricultural Scientific and Technological Achievements (Grant number: 20826703D), and the Project of Hebei Agricultural Innovation Team of Freshwater Aquaculture (Grant number: HBCT2018180203).
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