RT Journal
A1 Murchison, Elizabeth P.
A1 Kheradpour, Pouya
A1 Sachidanandam, Ravi
A1 Smith, Carly
A1 Hodges, Emily
A1 Xuan, Zhenyu
A1 Kellis, Manolis
A1 Grützner, Frank
A1 Stark, Alexander
A1 Hannon, Gregory J.
T1 Conservation of small RNA pathways in platypus
JF Genome Research 
JO Genome Research 
YR 2008 
FD June 01 
VO 18 
IS 6 
SP 995 
OP 1004 
DO 10.1101/gr.073056.107 
UL http://genome.cshlp.org/content/18/6/995.abstract 
AB Small RNA pathways play evolutionarily conserved roles in gene regulation and defense from parasitic nucleic acids. The character and expression patterns of small RNAs show conservation throughout animal lineages, but specific animal clades also show variations on these recurring themes, including species-specific small RNAs. The monotremes, with only platypus and four species of echidna as extant members, represent the basal branch of the mammalian lineage. Here, we examine the small RNA pathways of monotremes by deep sequencing of six platypus and echidna tissues. We find that highly conserved microRNA species display their signature tissue-specific expression patterns. In addition, we find a large rapidly evolving cluster of microRNAs on platypus chromosome X1, which is unique to monotremes. Platypus and echidna testes contain a robust Piwi-interacting (piRNA) system, which appears to be participating in ongoing transposon defense.