Introductory Chapter: Biotechnological Applications in the Aquaculture Industry

1. Introduction

Aquaculture has important roles for human beings, especially in meeting the need for animal protein, and creates important opportunities such as healthy nutrition for people, supply of raw materials to the industrial sector, creation of employment, contribution to rural development, and conservation of biological diversity. As a matter of fact, aquaculture has been identified as the fastest growing food sector in the world by the FAO.

Biotechnology is a technique that uses living organisms and their building blocks, and it is widely employed in different areas such as health, agriculture, food, and industry. The importance of biotechnology, which has great potential in terms of contribution to agricultural production, in aquaculture has been increasing, especially in recent years. Today, biotechnology is a tool that contributes significantly to aquaculture development. The excessive demand for aquatic foods, whose importance as an animal protein source increases daily, causes biotechnology to become widely used in aquaculture.

Modern biotechnology holds great potential not only to meet demand for aquatic food but also to improve aquaculture. In this regard, biotechnology holds tremendous potential to improve the quality and quantity of aquatic organisms reared in aquaculture. Today, biotechnology continues to develop rapidly, and its applications in aquaculture are increasing day by day [1].

2. Application fields of biotechnology in aquaculture

Effective use of biotechnological methods in aquaculture contributes not only to the improvement of this sector but also to the economic development of countries [2]. Biotechnology can be used for many purposes in aquaculture. In this regard, biotechnological applications in this field mainly focus on gender control, chromosome manipulations, gene transfer, and cryopreservation-related issues.

2.1 Gender control

Because of the early maturation of one or both sexes and, as a result, negative changes in growth, feed evaluation rate, behavior, health, body, and meat color, gender control practices are carried out [3]. Various techniques are used in breeding to produce single-breed or sterile populations. Gender control in fish is implemented by three different methods, such as feminization, masculinization, and sterilization.

2.1.1 Feminization

Usually, feminization is implemented by the administration of 17β − estradiol application at the first feeding of offspring. Other estrogens (ethyl-estradiol) are also used for feminization, but estradiol and estrone are not preferred because they are naturally found in fish. In this regard, the critical point in this application is that the fry receives a sufficient level of hormones during the sexual differentiation period, starting from the first feeding.

In many aquacultured species, females grow faster than males and are less aggressive. For this reason, the female population is less stressed, hence healthier and more resistant to diseases. Because of these features, all-female populations are preferred in the breeding of many species. However, the fact that all female populations spend most of the energy they get during sexual maturation on gonad development leads to a decrease in meat yield.

2.1.2 Masculinization

Since functional male (XX) individuals are needed to obtain all-female individuals in the indirect sex change method, androgen hormones such as 17α-methyltestosterone must be introduced into the bloodstream of fish that have hatched and consumed the yolk sac. In this method, the hormone should be added to the feed in certain ratios to ensure that the fish larvae that have consumed the yolk sac consume this feed, or to add this hormone to the aquatic environment to ensure that the larvae are bathed [4].

2.1.3 Sterilization

The production of sterile fish in aquaculture is possible by changing the chromosome numbers. For this purpose, it is necessary to apply one of the environmental shocks to the eggs shortly after fertilization. The aim of sterilization is to ensure that metabolic energy is spent on growth instead of gamete development. As a result, the negative effects of reproductive activity on growth, survival, and meat quality in fish can be prevented [5].

2.2 Chromosome manipulations

Creatures that reproduce with male and female develop from fertilized eggs formed by the union of their parents’ sex cells. Sex cells provide the connection between parents and the next generation. The most important asset of a sex cell is the chromosomes that carry heredity factors from generation to generation. One of the homologous chromosomes, which are the same shape and size, comes from the mother and the other from the father, are separated from each other when gametes are formed, and one from each partner goes to each gamete [6].

With the merger of male and female gametes, the spouses come together again and the chromosome numbers remain constant between generations. There are various manipulations performed for different purposes against meiotic and mitotic events during chromosome division. These manipulation processes gynogenesis, androgenesis, triploidization, and tetraploidization techniques. There are various environmental shocks and chemicals used to alter chromosome numbers. These are heat shock, pressure shock, and chemicals such as colchicine, cytochalasin B, and N₂O [3].

2.2.1 Triploidy

The final division of chromosomes within the egg occurs immediately after sperm penetration during fertilization. In this process, the pole cell in the egg is prevented from being expelled from the egg after fertilization by the environmental shock effect.

In this way, the offspring obtained from a triploid egg, if they carry XXX chromosome, sterile females, if they carry XXY chromosomes, fertile male individuals are formed. Many of the studies on triploidy are based on the rule that the triploid homogametic sex is sterile and grows better [7]. Providing triploidy can be acquired by blocking the second meiosis division and keeping the second pole cell after fertilization [5]. Additionally, hot or cold shock, hydrostatic pressure, and chemicals such as colchicine, cytochalasin B, and N₂O are used for the shock performed during triploid application [3].

2.2.2 Tetraploidy

The aim of tetraploidization is to produce fish with four chromosomes. In this process, individuals with 4 N chromosomes are obtained by applying shock during the first mitotic division after a normal egg is fertilized by an active spermatozoa.

2.2.3 Gynogenesis

Gynogenesis is a technique occurring between inactivated sperm and normal egg. As a result of fertilization, all individuals have XX chromosomes, allowing obtain a female individual. In the process of gynogenesis, the male germ cell nucleus is an inactive structure in the egg plasma and the embryo’s development is controlled only by maternal heredity [8].

Mature eggs of gynogenetic species do not take action to form the embryo when there are no male sex cells in the environment. Therefore, for success in gynogenetic reproduction, there must be male sex cells that will activate mature eggs. Sperm cells, which have destroyed genetic material, are used in the fertilization of eggs during gynogenesis. γ-rays, X-rays, and ultraviolet (UV) are used to neutralize the genetic material of sperm cells [9]. Since the inheritance material of the sperm is destroyed, the sperm taken from different fish species can also be used for fertilization of the eggs. To activate the eggs, artificial reproduction using sperm with UV radiation is required, and then, physical or chemical shocks are required to restore the embryo’s diploid state. These shocks prevent nuclear division by destroying microtubules.

Two different methods, myogenesis and mitogenesis, are used in such studies. Myogenesis occurs by inhibition of the outflow of the second polar cell. The egg is fertilized by spermatozoa whose chromosome material has been eliminated. After a while, it is subjected to shock to prevent the outbreak of the second polar cell, and thus, meiotic gynogenetic embryos with two sets of chromosomes are formed [10]. Mitogenesis produces completely homozygous progeny because it is carried out by inhibition of the first mitotic division after genome duplication. By using this breeding method, after two generations, homozygous-related lines of genetically similar fish can be obtained [4].

2.2.4 Androgenesis

Androgenesis, unlike gynogenesis, is a technique in which the genetic material of the egg is eliminated and fertilization and embryo development continue from the chromosome set of the spermatozoa [6]. In other words, androgenesis is a technique that allows all individuals to carry XY chromosomes, that is, to obtain a male individual, as a result of fertilization of inactivated eggs with normal sperm cells [11].

2.3 Gene transfer

The process of transferring special gene sequences of a certain length to the deoxyribonucleic acid (DNA) structure, which is known as the basis of genetic material, using genetic engineering techniques is called gene transfer [12]. Transgenic creatures are obtained by transferring a foreign gene to the genome using gene transfer techniques. Transgenic organisms are defined as organisms that carry a recombinant gene from another organism in their genome [13]. Rapid developments in recombinant DNA technology increase the importance of gene transfer technologies day by day.

2.4 Gamete cryopreservation and cryoinjuries

Success in aquaculture depends on many technical factors applied in hatcheries. In this regard, induced breeding is considered viable to obtain the required quantity of larvae in hatcheries. On the other hand, apart from induced breeding, new biotechnological techniques such as cryopreservation should be considered in aquaculture programs to provide continuous gamete and larvae production. In this context, FAO has endorsed cryopreservation as a major strategy for the conservation of fish resources [14]. Cryopreservation boosts the viability of gametes for several years without any severe change in the fertilizing capacity of the gametes by lowering the temperature, usually to −196°C [15]. Almost all biological activities are suspended at this temperature, including biochemical reactions that lead to cell death and DNA degradation [16].

Preservation of gametes is of great importance in selection programs applied in aquaculture. Because rearing the same fish species in the same environment and conditions throughout generations causes rare genes to be lost in the current population and heterozygosity to decrease. This reduction in genetic variation limits the potential of existing fish stock to be used in future selection programs, which manifests itself in low survival rates, low growth rates, reduced feed conversion efficiency, increased risk of disease, and increased mortality in juvenile fish [17].

Currently, the cryopreservation technique is routinely applied to the conservation of sperm, embryos, tissues, and cells. Researchers involved in aquaculture are especially working on the cryopreservation of fish sperm [18]. The cryopreservation technique was first used in the field of aquaculture in 1953 to fertilize herring (Clupea harengeus) eggs with frozen semen and is now used successfully in many fish species [19].

The most important purpose of cryobiology for use in aquaculture is to create a sperm bank or gene pool belonging to the cultured species. Creating a gene pool by applying cryobiological methods not only allows the preservation of genes but is also a necessity in hybridization, genetic manipulation, and stock enrichment programs [20].

During the cryopreservation of sperm, cryoinjury is practically inevitable. In general, cryoinjuries occur regarding freezing and thawing processes during conventional cryopreservation protocols. Most of the cryoinjuries take place when the temperature is between 0 and −40°C due to heat removal, cryoprotectant toxicity, cold shock, pH fluctuation, osmometric effects, and ice crystal formation [21]. Major cryoinjury can disrupt and damage cellular structures like DNA, the acrosome, and the plasma membrane by oxidizing cellular compounds and causing oxidative stress [22]. It is important to note that permeability plays a critical role in the formation of cryoinjury in the cells. Cells that are permeable demonstrate more tolerance to the cooling and freezing stages of the cryopreservation process [23].

3. Conclusion

Thanks to the biotechnological methods applied in aquaculture, the growth and reproduction rates of aquatic organisms increase, the existence of conditions such as diseases and DNA damage decreases, healthier, more productive individuals with desirable characteristics can be obtained, and the genes of endangered species can be conserved for years. The fact that modern biotechnological methods are effectively used in the field of aquaculture, as well as in farm animals, will allow the sector to develop significantly and will make an important contribution to the national economy.