Although both males and females contribute to conception failure, infertility in a bull is formidable since a single male is used to artificially breed thousands of females [1]. It is well proved that the male offspring born out of species hybridization (for instance crossing of cattle with yak) are sterile. On the other hand, the male offspring produced by crossing Bos taurus males with Bos indicus females, tend to have higher incidence of sub-fertility/infertility problems as compared to both the parent breeds [2]. It is reported that, crossbred males are prone for many reproductive problems and possess higher culling rate of 40–70% due to sub-fertility and poor semen freezability [3, 4]. Moreover, ejaculate rejection rate (owing to poor semen quality) in crossbred bulls ranged from 10 to 100% with the average of 55% [5–9]. Ejaculates were rejected for one or more reasons that include low sperm concentration, poor mass activity, initial motility and poor progressive motility. Further, it was observed that, post-thaw sperm motility decreased while the exotic inheritance of the crossbred bulls increased [10]. In the world of Zebu bulls, albeit libido is poor, infertility is less when compared to crossbred bulls [7, 11].

Though axiomatic evidences of inferior reproductive performances in crossbred bulls than Zebu bulls has been demonstrated in terms of genetic [10], hormonal [12], semenological [7] and andrological aspects [13], the etiology has not been well understood. Our group has been working towards the goal of unravelling the reasons for infertility in crossbred bulls, and found the differences in terms of proportion of sertoli cells in relation to spermatogenic cells [13], proteomic profile of spermatozoa [14], seminal plasma [15], and spermatogenic and sertoli cells [16]. A majority of these studies indicate altered sperm quality and functions, as the major factor contributing to high incidence of infertility in crossbred bulls. Several studies indicate the relationship of sperm transcripts with sperm function and fertility [17–20]. The development of spermatogonia into mature spermatozoa requires a lengthy duration and a series of complex physiological changes at testicular level. Spermatozoa carry thousands of different types of RNA from the testis [21–24]. Spermatozoal transcripts are having their putative purpose in spermatogenesis [25], sperm function [26], fertilization [27] and embryo development [28]. Number of mRNA found in the spermatozoa are only 50% of the mRNA found in the testis of men [29] and stallion [30]. The testis specific transcripts such as CatSper (Cation channels of sperm) has been reported to be involved in calcium influx into the sperm tail resulting in hyperactivated motility and capacitation [31]. Further, few earlier studies assessed the dynamics of transcripts in sperm having different motilities [32], after meiosis [33], after capacitation [34] and after cryopreservation [35] and indicated their role in sperm functions. It has been reported that compatibility between two gametes is essential for the consolidation process (transfer of RNA based information to a chromatized state), which will be impaired when crossing is carried out between species or between breeds [36]. Moreover, it causes dissimilarities between pseudoautosomal regions (a short identical sequence exists between X and Y chromosomes) of sex chromosomes, which results in impaired sex chromosome pairing and segregation during meiosis [37–39]. In this line, breed variations in testicular transcriptome and associated sexual function development has been reported [40, 41]. While a plethora of reports are available about spermatozoal transcripts, information on transcriptomic profile of testis, an important reproductive and endocrine organ critical for spermatogenesis and sperm quality, is very limited. Therefore, we envisaged that studying the testicular transcripts between Zebu and crossbred cattle would help us to understand aetiology of infertility in the later breed.

In the present study, we performed transcriptomic profiling of testicular tissue collected from crossbred and indigenous bulls using Agilent Bos taurus GXP 8X60k AMADID: 29411 Chip. Our objective was to identify the molecular signatures and their pathways associated with the higher incidence of subfertility in crossbred bulls as compared to Zebu breed.