Characterization of sus scrofa piRNA clusters.
(A) 1124 piRNA clusters were identified by scanning 10 kb genomic loci that produced at least 10 uniquely mapping piRNAs, and were thus ranked by the number of uniquely mapping piRNAs that they produced. Black diamonds and red curve show the genomic size of each cluster and the cumulative fraction of piRNAs contributed by clusters, respectively. (B) piRNA cluster expression were calculated as total reads of piRNAs within a cluster. (C) Cluster 1 on SSC7, and Cluster 3 on SSC14 were taken as the examples to describe the expression levels of individual piRNA in a cluster. (D) There were four types of piRNA clusters including divergent, plus-strand, minus-strand, and mixed type according to the definition of piRNA cluster type described by Lau et al. (2006).