All alleles were present within both herds, with minimum allele frequency and expected heterozygosity higher than 0.10 and 0.179, respectively. A total of fifteen (ten within population and five for the entire sample) Hardy-Weinberg equilibrium tests were performed. With the exception of GNRHR
haplotypes in Herd 1 and the combined population, none of these tests exhibited significant deviations from theoretical proportions (P > 0.05) (Additional file 1
: Table S1). These results evidenced that all the studied SNPs were suitable for association.
In bovine, several QTLs related to reproductive traits have been detected using microsatellites, some of which were associated with age at puberty http://www.animalgenome.org/cgi-bin/QTLdb/BT/search
. Most reported studies have used simpler indirect measurements (e.g
. scrotal circumference, calculated age at puberty, age at first insemination) [5
]. Recently, SNPs have been associated with timing of puberty in females [6
], but to date, they still remain unknown in males.
In the present work, we report the first evidence of markers associated with age at puberty in male cattle. Significant associations were detected between IGF1-SnaBI SNP and age at SC 28 cm (p < 0.05), but it were not associated with age at M 10% and C 50 million. Genotype CC exhibited an average age at SC 28 cm of 7 and 11 days higher than CT (p = 0.037) and TT (p = 0.012), respectively. This SNP explained 1.5% of the genetic variance of age of puberty at SC28. LHR-I499L, GNRHR-SNP5 and GNRHR-SNP6 were not associated with any of the measurements. However, GNRHR haplotypes showed a suggestive association with age at SC 28 cm (Table ). A resume of average phenotypic data was detailed in Additional file: Table S2.
Table 1 Average estimated puberty age and standard error (SE) in days, for the genotypes of the SNPs LHR-I499L and IGF1-SnaBI, and the haplotypes of GNRHR; and the P-val for the genotype effect in the model are presented for the two criterions for age of puberty (more ...)
Even though GNRH
and its receptors play a major role in coordinating sexual differentiation and reproduction in mammals, and there are strong evidences that genetic variation within these genes contributes to the regulation of pubertal timing in human and mouse populations [15
], the present study showed no effect of GNRHR
SNPs on analyzed puberty traits. Our results are in agreement with a previous work [13
] that also did not find correlations between early puberty in Nelore females and mutations in the LHR
genes. Nevertheless, other polymorphisms in linkage disequilibrium within these genes could be associated with the analyzed traits. Further studies are necessary to evaluate the influence of GNRHR
variations in the events that precede and initiate puberty in bull calves, as well as testis growth and gametogenesis.
Physiological and gene expression data showed that IGF1
is involved in the events that precede and initiate puberty in bull calves. Recent evidence showed that IGF1 can regulate the hypothalamic-pituitary-gonadal axis via direct actions at the pituitary and gonadal levels of this axis [11
]. In this sense, pre-pubertal IGF-1 serum concentrations have been genetically correlated with adult scrotal circumference and sperm motility, in bulls; and with the age at first calf and calving rate, in females [16
]. In mammals, IGF1 has been identified as a regulator of testicular growth, influencing the development of the seminiferous tubules and Leydig cells [12
]. Positive correlations were also found between serum concentrations of IGF1 and LH and testicular LHR concentrations in bull calves. Moreover, the IGF1-SnaBI
has been demonstrated to influence IGF1
gene expression and IGF1 blood level in cattle [17
]. In addition, IGF1 up-regulates LHR
and testosterone secretion in Leydig cells, and it has been shown to have an additive effect of LH and IGF1 on testicular cells number. Furthermore, IGF1 influences GNRH neurons during puberty through its receptor (IGF1R). For these reasons, IGF1
has been considered a strong candidate gene for the regulation of testicular growth in the bull calf during pre-pubertal period [11
Regarding this, IGF1-SnaBI SNP was reported to affect animal weight at different ages in cattle [18
]. However, in bovine and human, puberty has been associated more with stature or BMI than with weight [22
]. Nevertheless, the effect of weight (estimated W at 300 days) was included as covariate in the used association statistical model. Considering the correlation between these traits, a permissive effect of IGF1on reproductive development, rather than a direct effect as discussed above, could explain the observed association between this SNP and estimated age at SC28 [24
]. Regardless of the involved effect (direct or permissive), this findings support the hypothesis that IGF1-SnaBI SNP could be a useful predictor of early age of puberty.