Abstract: This study was conducted to investigate the phylogenetic diversity of archaea in the rumen of adult and elderly yaks.  Six domesticated female yaks, 3 adult yaks ((5.3±0.6) years old), and 3 elderly yaks ((10.7±0.6) years old), were used for the rumen contents collection.  Illumina MiSeq high-throughput sequencing technology was applied to examine the archaeal composition of rumen contents.  A total of 92 901 high-quality archaeal sequences were analyzed, and these were assigned to 2 033 operational taxonomic units (OTUs).  Among these, 974 OTUs were unique to adult yaks while 846 OTUs were unique to elderly yaks; 213 OTUs were shared by both groups.  At the phylum level, more than 99% of the obtained OTUs belonged to the Euryarchaeota phylum.  At the genus level, the archaea could be divided into 7 archaeal genera.  The 7 genera (i.e., Methanobrevibacter, Methanobacterium, Methanosphaera, Thermogymnomonas, Methanomicrobiu, Methanimicrococcus and the unclassified genus) were shared by all yaks, and their total abundance accounted for 99% of the rumen archaea.  The most abundant archaea in elderly and adult yaks were Methanobrevibacter and Thermogymnomonas, respectively.  The abundance of Methanobacteria (class), Methanobacteriales (order), Methanobacteriaceae (family), and Methanobrevibacter (genus) in elderly yaks was significantly higher than in adult yaks.  In contrast, the abundance of Thermogymnomonas in elderly yaks was 34% lower than in adult yaks, though the difference was not statistically significant.  The difference in abundance of other archaea was not significant between the two groups.  These results suggested that the structure of archaea in the rumen of yaks changed with age.  This is the first study to compare the phylogenetic differences of rumen archaeal structure and composition using the yak model. 

Key words:  yak, archaea, rumen, diversity, high-throughput sequencing