Is there a plastid in Perkinsus atlanticus (Phylum Perkinsozoa)?
Introduction
Protistan parasites of the genus Perkinsus are pathogenic microorganisms responsible for great mortality in populations of different species of bivalve molluscs of economic importance. Perkinsus atlanticus was first identified in Europe in carpet shell clams (Ruditapes decussatus) on the Atlantic south coast of Portugal and described as a new species (Azevedo 1989).
There is a recent proposal for the inclusion of the genus Perkinsus in a new phylum, Perkinsozoa, related to both Apicomplexa and Dinoflagellata in the infra-kingdom Alveolata (Nóren et al. 1999). However, there is no consensus about the existence of this new phylum (Cavalier-Smith and Chao 2004), consequently its taxonomic characterization remains unresolved. The principal evidence for the relationship between these three phyla comes from molecular analyses of nuclear encoded genes, pointing to a closer relationship of Perkinsozoa to Dinoflagellata (Reece et al. 1997; Saldarriaga et al. 2003). Studies on extranuclear genomes, however, could also be very helpful for a more precise definition of those phyla. In Apicomplexa, it is already well established that they possess one circular 35 Kb plastid genome, in a membrane-enclosed organelle named the apicoplast (Köhler et al. 1997). In Dinoflagellata, the plastids are heterogeneous and have been found to possess a genome composed of 15 mini-circles, each one carrying only one gene (Zhang et al. 1999). Although they possess different characteristics, the plastids in Apicomplexa and Dinoflagellata seem to share a common origin (Fast et al. 2001). In Perkinsozoa, there have been until now no conclusive results about the presence of any plastids.
In this work we identify a plastid-like organelle in Perkinsus atlanticus, using electron microscopy and assays of growth inhibition by thiostrepton, a thiazole-containing peptide antibiotic that inhibits protein synthesis and ribosomal GTPase activity by binding to a conserved region in the LSU rRNA of eubacteria and plastids, but not to the corresponding region in nuclear and mitochondrial LSU rRNAs (Rogers et al. 1997).
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Cell cultures
The cultures of Perkinsus atlanticus Azevedo, 1989, used in this study were kindly supplied by Prof. Leonor Cancela, University of Algarve, Portugal. They were sub-cultured in our laboratory in DME/Ham's medium (Gauthier and Vasta 1993) with antibiotics at 28 °C.
Electron microscopy
The parasite and the enriched plastid fraction were fixed in 3% glutaraldehyde buffered in 0.2 M sodium cacodylate (pH 7.2) at 4 °C for 10 h, washed overnight in the same buffer at 4 °C, and post-fixed in 2% osmium tetroxide with the same
Results and discussion
The results obtained in this work, by electron microscopy and inhibition growth assays with thiostrepton, indicate the presence of plastids in Perkinsus atlanticus.
Ultrastructural observations revealed a structure with four membranes, located in a region at the apical end, anterior to the nucleus and near the mitochondria, in zoospores of Perkinsus atlanticus (Figs. 1 and 2). It is interesting to verify that the location of the plastid is the same as in the apicomplexans (Hopkins et al. 1999).
Acknowledgments
The authors thank Mrs. Carla Oliveira, Mr. António Augusto Rocha and Mr. João Carvalheiro for the excellent technical assistance. This work was supported by the Foundation for Science and Technology (FCT/MCT) (Proj. no. POCTI/CTA/47583/2002).
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