Introductory Chapter: Reproductive Strategies and Biotechnologies in Terrestrial and Aquatic Animals

1. Introduction

Reproduction is a biological process by which new individual organisms are produced from their parents. Animal reproduction aims to renew generations for a given production purpose such as meat, milk, or wool according to species. In order to achieve this goal researchers look for the best control of reproduction in parents to provide a maximum number of new-born of the required quality [1].

Nowadays, many reproductive biotechnologies have been developed for the effective control of reproduction in terrestrial and aquatic animals showing great differences in reproduction types. The application of biotechnology offers many advantages to animal food production through enhancement and control of reproductive processes in terrestrial and aquatic animals.

2. Reproductive strategies

Reproduction success is a very important target for all animal species. Animal species take different strategies to achieve this target [2]. The objective of a reproductive strategy is to maximize reproductively active offspring depending on available energy and parental life expectancy [3].

There are two forms of reproduction known as asexual and sexual. There are several ways of reproducing asexually such as fission, budding, fragmentation and parthenogenesis. Asexual reproduction occurs in prokaryotic microorganisms (bacteria and archaea) and many eukaryotic, single-celled, and multi-celled organisms.

Additionally, some animal species (including sea stars and sea anemones, as well as some insects, reptiles, and fish) are capable of asexual reproduction. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation where part of a parent individual can separate and grow into a new organism.

On the other hand, sexual reproduction typically depends on the sexual interaction of two specialized organisms, known as gametes, which contain a half number (n) of chromosomes of normal cells and are created via meiosis. Typically, a male fertilizes a female of the same species to create a zygote (2n). In this way, it is possible to produce offsprings whose genetic characteristics are derived from those of the two parental organisms.

After fertilization, a series of developmental stages occur in which primary germ layers are established and organized to form an embryo. During this process, animal tissues begin to organize into organs to determine their future morphology and physiology.

It is known that all terrestrial animals perform internal fertilization, whereas aquatic animals perform various reproductive systems including internal fertilization with or without mating. On the other hand, many aquatic animals perform mainly external fertilization in different types such as viviparous, oviparous, and parthenogenesis [4].

2.1 Reproductive strategies in terrestrial animals

Terrestrial animals mostly perform internal fertilization and there are two ways of producing offspring via this way which are:

The first one is fertilizing of the eggs inside of the female’s body and the embryo receives nourishment from the egg yolk. When the offsprings are fully developed they are hatched.

The second one is fertilizing of the eggs inside of the female and the embryo receives nourishment from the mother’s blood through a placenta. The offspring develops inside of the female body and is born alive.

By the way of internal fertilization, the embryo is isolated within the female and is provided protection against the predators, which also increases the likelihood of fertilization of the egg by a specific male. Despite fewer offspring are produced via this fertilization type, but their survival rates are higher than that of external fertilization.

2.2 Reproductive strategies in aquatic animals

Aquatic animals mostly perform external fertilization. In general, this type of fertilization occurs in aquatic environments where both eggs and sperm are released into the water. The role of water is to protect the eggs from drying out during embryonic development. The gametes are released at the same time to the same location and provide increasing in the likelihood of fertilization of the eggs eventually. The embryo receives nourishment from the egg yolk.

Additionally, there are some strategies used by fish to ensure their offspring survive in aquatic animals. The reproductive strategies of aquatic animals are often reflected in the anatomical differences between the sexes. In this framework, aquatic animals, mainly teleost fishes, have developed a large variety of reproductive strategies, varying from mass spawning to parental care, from strict gonochorism to hermaphrodism, and from oviparity to viviparity.

2.2.1 Gonochorism or hermaphrodism

Most of the fish species are gonochoric, that is their sexes are separate during their lifetime. However, many fish species exhibit sex change that is called as hermaphrodism, which can be protandrous if the fish sex changes from male to female. Other hermaphrodite species can be protogynous hermaphrodite and their sex change from female into a male. Interestingly, simultaneous hermaphrodites also exist in aquatic animals, which behave almost simultaneously as males or females. In this way, individuals can change within minutes from displaying male sexual behavior, with sperm release, to female sexual behavior, with egg-laying [5].

2.2.2 Oviparity or viviparity

In oviparous fish, eggs are fertilized externally, after spawning. On the other hand, in viviparous fish, such as the guppy (Poecilia reticulata) or the mosquitofish (Gambusia affinis), the fertilized and developing eggs remain inside of the female body for a certain period and the offsprings are released as free-swimming larvae [5].

3. Reproductive biotechnologies

Biotechnology can be defined as a technique using living organisms to modify or improve products. Biotechnology has a great impact on species improvement, reproductive rate, and animal production. Reproductive biotechnology is modern technological technique using biological systems and organisms to develop and increase the quality of the products. The most common reproductive applications integrated with biotechnology are artificial insemination, short and long-term preservation of sperm, sperm sexing, synchronization, superovulation, embryo transfer, and in vitro embryo production [6].

3.1 Reproductive biotechnologies in terrestrial animals

The application of biotechnology offers numerous advantages to livestock production through control of the reproductive process in animals. These biotechnologies can be summarized as artificial insemination, estrus synchronization, embryo transfer, sperm cryopreservation, transgenesis, and in vitro fertilization. Theuse of these reproductive biotechnologies has contributed tremendously to meet the increasing demands of the modern dairy and beef industry [7].

3.1.1 Artificial insemination

Artificial insemination is one of the earliest reproductive biotechnologies and permits using of superior males for breeding purposes. It involves sperm collection from superior males, dilution of sperm, freezing of sperm, and transferring of frozen–thawed sperm to the female reproductive tract [7].

3.1.2 Estrus synchronization

Estrus synchronization is a grouping of females for parturition at the same time. It is used at commercial dairy farms for uniform milk production throughout the year. It is closely linked with artificial insemination and is also the first step of embryo transfer. Using estrus synchronization is a good strategy to overcome breeding problems, especially during the summer months [7].

3.1.3 Embryo transfer

Embryo transfer is a process by which embryos are collected from a donor female and then transferred into a recipient female where the embryos complete their development. It is the most commonly used biotechnology after artificial insemination and estrus synchronization. Embryo transfer is profitable for producers of pure-bred animals and genetically superior females that produce more offspring than natural reproduction. This technique is used in several species of domestic animals including cows, horses, goats, and sheep [7].

3.1.4 Sperm cryopreservation

Research on sperm cryopreservation can be traced back to the discovery of the protective peculiarity of glycerol for freezing of avian sperm by Polge et al. [8]. Since then, cryopreserved sperm of livestock has grown into a near-billion dollar global industry. Additionally, since the first successful cryopreservation of bull semen, cryopreserved sperm has been used to propagate the rare and endangered species [9].

It is clear that sperm cryopreservation contributes to the expansion of reproductive techniques, such as artificial insemination and in vitro fertilization [10]. Additionally, sperm banks are currently more developed for rare domestic breeds (cattle, sheep, and goats) than for non-domestic species. Nowadays, using of sperm banks to facilitate management and conservation of endangered species is being promoted extensively [11].

3.1.5 In vitro fertilization

In vitro fertilization (IVF) is the collection of oocytes from a donor female and fertilization of that matured oocyte in a laboratory dish. The eggs after collection are placed in CO₂ incubators in the IVF laboratory. Most viable spermatozoa are recovered after processing for inseminating the eggs. The addition of a large number of viable spermatozoa to each ova will disperse the follicular cells and also ensure fertilization of an egg by one spermatozoon. In vitro fertilization has been used to treat many infertility issues such as when both fallopian tubes are blocked and compulsory fertilization of the egg cell has to take place outside of the body [7].

3.2 Reproductive biotechnologies in aquatic animals

Successful reproduction of cultured fish broodstock is essential in terms of the sustainable aquaculture of aquatic organisms. In this concept, reproductive biotechnologies mainly include cryopreservation of male gametes, genetic control of sex, and production of transgenic fish.

3.2.1 Cryopreservation of male gametes

Cryopreservation is the process of freezing the biological materials at the temperature of liquid nitrogen (LN₂) (−196°C). In this way, it is possible to storing the biological materials as unchanged for centuries with the capability of recovering the cell functionality following the thawing process [12]. Today, sperm management techniques have been established for freshwater and marine fish species [13, 14, 15].

Cryopreservation of male gametes is an important biotechnological tool for aquatic species and has great concern for aquaculture. Following successful cryopreservation of avian spermatozoa using glycerol as cryoprotectant by Polge et al. [8], cryopreservation of male gametes became possible in aquatic animals [9]. First time, Blaxter [16] applied a similar approach for fish sperm and reported achieving approximately 80% cellular motility following thawing of Atlantic herring sperm cells. Since then, cryopreservation of fish sperm has been studied and succeeded in more than 200 species [17]. In addition, it is possible to reconstruct the original strain, population, or variety following required environmental restoration via this biotechnology [9].

3.2.2 Genetic control of sex

The genetic control of fish sex could be useful where one sex displays advantageous characteristics such as larger adult size, production of high-value caviar, faster growth rate, or higher age at first sexual maturation. In this concept, monosex populations of the most advantageous sex may be produced through genetic control (crossing, gynogenesis, androgenesis, hybridization) or steroid treatment of broodstock (hormonal treatment) [18].

Genetic control of sex is possible via crossing, gynogenesis, androgenesis, and hybridization. In this framework, it is possible to control sex by the crossing of sex-reversed adult broodstock (administering androgens to produce “neo” males and estrogens to produce “neo” females) with normal males or females to produce single-sex progeny. Another sex control method is gynogenesis which is the fertilization of oocytes with inactivated sperm (irradiated) with normal sperm, to eliminate the female chromosomes and produce all-female progeny. Also, it is possible via androgenesis that is fertilization of inactivated oocytes (irradiated) with normal sperm, to eliminate the female chromosomes and produce all-male progeny. Additionally, another way of genetic sex control is the hybridization of two different species from the same genera producing single-sex progeny [18].

3.2.3 Production of transgenic fish

The domestic fish production through transgenic techniques offers many potential economic advantages for commercial aquaculture production. The traditional method of producing transgenic fish is still microinjection. However, some success has also been shown using particle bombardment [18].

4. Conclusion

Reproduction is the backbone of animal production and productivity is the key element for development. Reproductive inefficiency is one of the most important reasons of economic losses in animal industries. Despite the remarkable advancement in the field of reproductive physiology, low conception rate, and the high embryonic mortality rate remains a major problem in terrestrial and aquatic animals as well.