Estimation of the genetic parameters for some of semen quality characteristics using animal models with combined phenotype and genotype data of Holstein bulls

Introduction: For the design of any successful breeding program, knowledge about semen quality traits is greatly important and per-required. Due to the critical role of bulls through artificial insemination in the genetic improvement of the dairy cattle herds, knowledge about semen characteristics and genetic factors which affect semen parameters is essential. In addition, semen with poor fertility requires more units of semen to establish a successful pregnancy and produce a live offspring. Nonetheless, both impacts are economically important factors, which are considered by the producers leading to rejection of bulls with poor semen quality, and investing mainly on highly fertile sires. Summary of previous literatures is highlighted semen traits and their key roles in upcoming fertility and pregnancy of cows within herds (Gacitua et al. 2005). Furthermore, some reports pointed DNA markers can identify the genetic background mechanism of resistance and susceptibility of sperm after freezing and thawing (Purdy et al., 2005). Additionally, previous reports supported  low hereditability scenario of semen characteristics in livestock species (England et al., 2010). Leptin is a globular protein with a tertiary structure similar to a haemopoietic cytokine synthesized by adipose tissue. It is involved in the regulation of feed intake, fetal growth, energy balance, fertility, and immune functions. The leptin molecule (16 kDa) is made up of 167 amino acids with an N-terminal secrotary signal sequence of 21 amino acids. In cattle, the leptin gene is located on the fourth chromosome. It consists of three exons and two introns. Only two exons are translated into the protein. The coding region of the leptin gene (501 nucleotides in length) is in exons 2 and 3, which are separated by an intron of approximately 2 kb. The leptin gene promoter region spans approximately 3 kb. Leptin is necessary for normal reproductive function, but when present in excess, it have detrimental effects on the male reproductive system. Human and animal studies pointed to leptin as a link between infertility and obesity, a suggestion that was corroborated by findings of low sperm count, increased sperm abnormalities, oxidative stress, and increased leptin levels in obese men. In addition, daily leptin administration to normal-weight rats has been shown to result in similar abnormalities in sperm parameters. The bovine growth hormone (bGH) is a 22 KDa single-chain polypeptide hormone, which is produced in the anterior pituitary gland. The encoding gene is approximately 1800 base pair (bp) and consists of five exons separated by four intervening sequences (Harvey et al. 2000). Recently, several studies have investigated the association between bGH locus and reproduction traits. A substitution of cytosine (C) for guanine (G) at the  position of 2141 causes an amino acid change from leucine (leu) to valine (val) at residue 127. This transversion enables the genotyping of this particular locus using the endonuclease AluI, since in the mutant bull, this enzyme does not recognize its target sequence. Several point mutations in the bovine growth hormone (GH) gene have been described, and as such, the Leu127Val polymorphism described by Lucy et al. (1993) has been extensively investigated based on  production and reproduction traits. In addition, Lechniak et al. (1999) have reported the relationship between individual semen quality traits and fertility. The purpose of the present study was to estimate the genetic parameters of some semen quality traits using animal models containing genotype of candidate gene data on Holstein bulls.
Material and methods: For this reason, 67 bulls were selected from two breeding stations in the Northwest of Iran (41 bulls) and the National Livestock Improvement Center (26 bulls), which were in the sperm production stage between 2003- 2013. In this regard, four related traits of semen quality were considered, including: volume of ejaculation, sperm concentration, live sperm percentage before freezing, and live sperm percentage after thawing. The DNA extraction was done from semen according to commercial DNA kit and quality and quantity tests were done using spectrophotometry and gel monitoring. The nucleotide polymorphisms were evaluated in two loci of the leptin gene (Exon 2 and intron 2) by the PCR-RFLP method and in the growth promoter receptor gene (promoter region) using PCR-SSR technique. Finally, using combined phenotypic and molecular information, genetic parameters were estimated through Bayesian statistics and Gibbs sampling. POPGENE software was used for molecular data and descriptive statistics (genotype frequencies, allele frequencies, and hetrozygosity index calculation). Univariate and multi variate analysis for semen characteristics was done using gibbs3f90 and renumf90 softwares.
Results and discussion: According to the molecular outputs, in all investigate genes nucleotide variation and polymorphism was observed. The results of univariate analysis of animal model for estimating heritability for traits of semen volume, sperm count, live sperm percentage before freezing, and thawing point were 0.22, 0.14, 0.20, and 0.10, respectively. Reproductive performance is controlled by the genetic make-up of dam, sire, and offspring, but it is largely affectedby environment. Thus, the reproductive efficiency of the breeding herd depends on the fertility of the bulls(Gredler et al., 2005). Bull’s fertility is also essential, since bull's DNA is the primary mechanism through which genetic improvements can efficiently be accomplished. Implementation of artificial insemination (AI) in dairy cattle production allowed improving selection of bulls for production traits. Also, frequent freezing and thawing process of bull semens significantly affected quality index and consequent evaluation of fertilization potential of a semen sample for AI in Holstein breeding stations (Mathevon  et al., 1998).However, the preselection of the samples, the high number of sperm per doses, and the high quality of the semen used in the AI programs  can reduce the variability, thereby giving a low probability of detecting fertility differences associated with seminal parameters. Spermatogenesis is a complex process that involves stem-cell renewal, genome reorganization, and genome repackaging;  so, it culminates in the production of motile gametes (Kealey et al., 2006). The process of spermatogenesis is regulated by reproductive hormones in gonadotropin axis and is controlled by a large number of genes(Sun et al., 2012). Therefore, hormone and their receptors are presumed to be good candidate genes for reproductive traits.
Conclusion: The results of this study indicated that the range of heritability values for the different semen quality characteristics are low to moderate, which may indicate that improvement of environmental factors is more effective than genetics basis.