The yak has unique physiological mechanisms to adapt to the cold and low-oxygen environment of the Tibetan plateau [29]. The impact of ruminal microbiota and tissue on production efficiency of Yak is undeniable. It is widely acknowledged that the rumen is incompletely developed both metabolically and physically at birth [4]. During the first weeks of life, when the animals are still suckling milk, the rumen is not functional: the suckled milk does not pass through it due to closure of the esophageal groove (the “nursing reflex”). The relative proportions of the rumen are smaller than that in the adult, and some of its functional components such as the rumen wall papillae, which serve to absorb volatile fatty acids, are not yet developed [21]. In neonates, the rumen does not have the high degree of keratinization characteristic of the mature organ. Following the initiation of solid feed intake by the neonate and the subsequent establishment of ruminal fermentation, the rumen goes through both physical and metabolic development. Physical development of the rumen can be further segmented into 2 aspects: growth of the papillae and increases in rumen mass [14]. Changes in the physiological and structural properties of the rumen with age are connected with the development of microorganisms, as their fermentation products are crucial for the development of the rumen wall papillae. In our study, the rumen index rose gradually from 1 day to 5 years of age, underscoring how the rumen plays a more important role as the yak grows older.
The extent of morphophysiological variation in the yak rumen reveals a degree of adaptability to a particular feed. The mature yak had longer and wider papillae in the rumen during the green compared with the dry season. Although the number of papillae and muscular thickness of the rumen did not significantly change between the green and dry seasons, the width and length of papillae increased in the green compared with the dry season [16]. Physical stimulation by feed should lead to measurable increases in both musculature development and rumen weight. When milk is infused directly into the rumen, resulting in short-chain fatty acids (SCFA) production, papillary growth is stimulated. In this study, the thickness of muscularis, length, and width of rumen nipple of the yak rumen increased gradually from 1d to 5y of age, which were mainly driven by physical development and diets, contributing to increasing the absorption of nutrients in the yak rumen.
Despite inter-species diversities in community structure and function, ruminal microbiota play a beneficial role in host metabolism and immunity across various species [30]. In ruminants, the common ruminal microbes are Fibrobacter succinogenes, Ruminococcus albus, Butyrivibrio fifibrisolvens, Ruminococcus flflavefaciens, and Prevotella [31]. Bacteroidetes and Firmicutes were the predominant bacterial phyla in the yak rumen [11]. While, Firmicutes, Fibrobacteres, Bacteroidetes, Euryarchaeota, and Proteobacteria were the predominant phyla in the cattle-yak rumen microbial community [10]. The rumen microbial groups varied through the growth of yaks from neonatal to adult and compared with the protozoan and fungal groups, the bacterial and archaeal groups were more sensitive to changes in growth stages [32]. A total of 7200 operational taxonomic units (OTUs) were gained from the yak rumen using 16S rRNA gene sequencing, and 23 phyla within 159 families were identified by taxonomic summarization [18]. Different forage growth stages changed the diversity, composition, and function of ruminal microbiota in the yaks which grazed naturally without feed supplementation in the alpine meadow of the Qinghai-Tibet Plateau [16]. Temperature changes likely had a direct effect on plant productivity, which in turn influenced the ruminal microbiome of both male and female yak.
Previous studies showed that the diversity and number of the organisms residing within the gut ecosystem are regulated by physiological and environmental factors including habitat and diet [33]. Ruminal microbes constantly interact among themselves and with the host ruminal epithelia. Different types of interactions are present, but most are commensal. The composition of the microbial community changes obviously between and within host species, suggesting that host genetics could influence functional genetic potentials of the rumen microbiome [33]. Seven individual yaks from four altitude populations clustered together, suggesting that the yak’s gut microbiota is extremely conserved for inter-species comparisons, despite inter-individual variation [15]. Diet also plays an important role in determining the composition of the resident gut microbes [34, 35]. Therefore, both host genotype and diet were likely to be major factors in determining the composition of the resident ruminal microbes.
There are now various approaches for studying ruminal microbiota, for instance, denaturing gradient gel electrophoresis, temperature gradient gel electrophoresis, real-time PCR, high-throughput sequencing of 16S rRNA gene, and metagenomics. Among these methods, metagenomic analysis can more truly reflect the microbial composition and interaction in the sample and enables the identification of novel genes and proteins of industrial interest [36, 37]. In the present study, we investigated the metagenomics of the yak rumen at 5 different ages. Relative abundance results showed that Bacteroidetes and Firmicutes were the two phyla with the highest abundance in 5 groups, which were in accordance with a report in mature yak [11]. There were obvious differences in microbes between the 5 groups, which also revealed rumen microorganisms were in a dynamic process and might mainly be regulated by host genotype and diet within different growth stages.
Functional metagenomics has the potential to discover new enzymes and metabolic pathways in the rumen in case of innovative strategies for screening are developed. The enzymatic machinery necessary to hydrolyze structural plant polysaccharides is an important target, though a new carbohydrate-binding domain has been reported in buffalo rumens [38]. One-hundred fifty glycoside hydrolase (GH) genes were annotated as fibrinolytic proteins in the yak rumen, and the majority (69%) were clustered or linked with genes encoding related functions. The predominant cellulase/ hemicellulase genes were GH5, GH9, and GH10, yet no GH48 exocellulase gene was detected. These findings suggested that the SucC/SucD-involving mechanism plays a vital role in lignocellulose degradation in yak rumen [19]. Recently, a total of 145,489 genes were annotated using the Carbohydrate-active Enzymedatabase, which identified GH as the most highly represented enzyme family in the cattle-yak rumen [10]. In our study, a total of 115,401 genes were annotated on the CAZy database. The glycoside hydrolase (GH) had the highest relative abundance, followed by glycosyltransferase (CT), and carbohydrate-binding modules (CBM). The discrepancies between the above reports may be due to the differences in animals and methods that have been applied. These newly discovered genes and enzymes need to be further studied to facilitate their possible application in breeding production.