Comparison of the large subunit ribosomal DNA ofNeosporaandToxoplasmaand development of a new genetic marker for their differentiation based on the D2 domain

The latest release of the large subunit ribosomal database contains 429 sequences, yet only 10 (six nuclear and four mitochondrial) are derived from parasites of the phylum Apicomplexa. Three of these (allToxoplasma gondii) were previously contained in the 1994 release of the database. As an initiative towards an understanding of ribosomal gene organization in the Apicomplexa, the primary sequence of the large subunit (LSU) rDNA ofNeospora caninumis presented, and compared with a consensus sequence derived for the LSU rDNA ofT. gondii. Nucleotide differences observed between these two taxa in the D2 expansion segment (or domain) (also called the C1/C1′ region) of the LSU rDNA were incorporated into a primer that forms the basis of a species-specific polymerase chain reaction (PCR) forN. caninum. The D2 domain of the LSU rDNA, therefore, represents a new genetic marker that can be used for the differentiation and identification ofNeosporafrom other cyst-forming coccidia.