J Mol Evol. 2008 Dec;67(6):594-607. doi:10.1007/s00239-008-9149-6

Marz M, Kirsten T, Stadler PF.

Evolution of spliceosomal snRNA genes in metazoan animals.

”The snRNAs of the minor spliceosome were in most cases single-copy genes.” The minor spliceosome was not found in Oikopleura dioica, but it was found in Ciona. “SnRNA locations are not syntenically conserved, i.e., snRNA behave like mobile elements in their genomic context.”