**2. The overview of ovulation induction**

Ovulation is the release of a fertilization-competent oocyte (mature female germ cells) from the ovary into the fallopian tubes in the female abdominal cavity where male sperm cells fertilize; thus, it is an essential and sophisticated biological process for sexual reproduction. Ovulation is an ovarian response that is initiated due to the surge of luteinizing hormone (LH) through the anterior pituitary gland. Thus LH surge triggers ovulation and estrus [1, 2] and the development of the corpus luteum, which initiates a series of ovarian activities in females. It works with the secretion of follicle-stimulating hormone (FSH), which plays a fundamental role in regulating for development of ovarian follicles as well as selection. It also stimulates granulosa cell differentiation, regulating gonadal functions, including steroidogenesis [3, 4]. LH, along with FSH, are considered gonadotrophic hormones because of their role in controlling the function of the ovaries in the development of preovulatory follicles to stimulate certain molecular events. This complex spectrum of events includes various types of ovarian cells, the activation of various signaling pathways, and the controlled expression of specific genes affecting the overall mechanism. LH and FSH levels are regulated and highly dependent on the pattern of release of gonadotropin-releasing hormone (GnRH) from the hypothalamus [5, 6].

Up to the last decade, a significant focus on ovulation and related features has been the association analysis of known candidate genes. The entire process of ovulation varies in mammals, following where they can be spontaneous or induced manners [7]. Spontaneous ovulation is the ovulation process in which females exhibit a constant cycle of reproductive hormones and does not require to be aroused in any way through a male to generate a preovulatory LH surge associated with reproduction. Species that are naturally ovulating through estrous produce mature ova through a process necessary for fertilization to occur. The females are ovulating spontaneously include mice, rats, domestic dogs, sheep, goats, horses, pigs, monkeys, and humans [8, 9]. The entire cycling process of ovulation varies among species. For example, while humans and primates experience monthly menstruation processes, all other animal species ovulate through various other ovulatory mechanisms [10].

Whereas female who displays mating-induced ovulation will have spontaneous development of follicles to maturation due to some component of coitus that is an externally-derived and receipt of genital stimulation during, or just before mating. Therefore, despite exhibiting high estradiol levels resulting from follicular maturation, they do not ovulate because they entail priming of males resulting in a long mating cycle to ensure successful fertilization [11].

Often, steroid-induced LH surges are not seen in ovulation types induced during reproductive periods, indicating insufficient or reduced secretion level of GnRH due to lack of positive feedback from estrogen and progestin hormones upon gonadotropin secretion. However, paradoxically, some natural ovulating species may undergo an occasionally induced preovulatory LH surge due to mating. Species in which females are triggered in induced ovulation include rabbits, domestic cats, ferrets, and camels [8].

Reproduction is a highly dynamic process and has significant consequences on livestock profitability. Reproductive success is conditioned by fertility, productivity, and fecundity. In particular, there are several minor genes and some major genes, which are fecundity related genes (Fec) that significantly affect reproductive traits, like ovulation rate, prolificacy, and litter size genetically [12].

### *The Incidence of Ovulation and Detection of Genes Associated with Ovulation and Twinning… DOI: http://dx.doi.org/10.5772/intechopen.99936*

Livestock species are mainly classified either as monotocous species, like cattle, water buffaloes, and horses, or multiparous species, like goat, sheep, and pig based on ovulation rate depending on the characteristic of a species [13].

Biological factors for the consistent multiple ovulations and how to improve or control the ovulation rate in other single-ovulating species in livestock have been of interest to some researchers to understand and intervene in the process of follicular development by applying assisted reproductive technologies. Therefore, identifying various experimental animal models with multiple ovulation rates could efficiently enhance the selection response in farm animals.

Specifically, the reproduction process is primarily influenced by genetic and environmental factors for a transformation from primordial follicle to mature ovulatory stage and typically has low to medium inheritance; thus, traditional phenotype-based selection methods are often time-consuming processes due to a lack of efficiency.

It is more effective to select breeding animals based on their genotypic structure to increase ovulation rate, prolificacy, and litter size as reproductive abilities in livestock species. Eventually, selecting animals based upon highly polymorphic marker information for reproduction efficiency (MAS) will be of great importance for future breeding programs in the livestock production system.
