Small Subunit Ribosomal RNA Sequence Comparisons Demonstrate Link Between Proliferative Gill Disease (PGD) and Hennequya N. Sp. in Channel Catfish       L.M. Pote, L. Hanson, and R. Shivaji   College of Veterinary Medicine, Mississippi State University, Mississippi State, MS  39762       Proliferative gill disease (PGD) is associated with major fish losses in commercial catfish.  Past research has indicated that this disease is linked to a myxozoan parasite, Aurantiactinomyxon ictaluri.  Artificial infections with A. ictaluri, isolated from the aquatic oligochaete, Dero digitata, have demonstrated a myxozoan stage in the gills, identical to those seen in field infections.  In several challenge studies, parasite-free catfish were artificially infected with A. ictaluri.  Seven days post-infection, fish from each challenge group were necropsied and the typical PGD organisms were observed in the gills.  Three months post-infection, the remaining fish were necropsied and a newly described cyst stage of PGD was found, Henneguya ictaluri n. sp.  The H. ictaluri n. sp. spores were isolated and purified.  DNA was isolated from these spores and the rRNA gene was amplified using PCR primers specific for A. ictaluri.  The gene sequence in the highly variable region of these spores was found to be identical to the gene sequence of A. ictaluri, the PGD stages found in the gills of the artificially infected fish, and in naturally infected catfish. This is the first molecular evidence that A. ictaluri, isolated from D. digitata, can infect catfish and produce the typical PGD organisms in the gills and provides further proof that the actinosporean, A. ictaluri, is actually a life stage of the myxosporean Henneguya ictaluri n. sp.  Research supported by:  USDA/NRI:  96-02330.