Genomic characterisation of the ichthyotoxic prymnesiophyte Chrysochromulina polylepis, and the expression of polyketide synthase genes in synchronized cultures

Abstract

The widely distributed prymnesiophyte species Chrysochromulina polylepis is prominent and well known for occasional formation of ichthyotoxic blooms. The chemical structure of the C. polylepis toxin(s) has not yet been elucidated, but the associated haemolytic activity, potent membrane disruption interactions and toxicity to finfish and protists have led to the suggestion that they may be similar to the prymnesins of Prymnesium parvum. Such polyether toxins are presumably formed partially or completely via polyketide biosynthetic pathways. In this genetic study of C. polylepis, we generated and analysed a genomic DNA and a normalized cDNA library. We estimated a genome size of approximately 230 mbp based upon analysis of >1000 genomic library clones. Of the cDNA library, 3839 clones were partially sequenced and annotated, representing approximately 2900 unique contigs. We detected several genes putatively related to toxin synthesis. Thirteen putative polyketide synthase (PKS)-related gene sequences were identified and phylogenetic analysis identified two of these as containing ketoacyl domains of the modular type I PKS. Semi-quantitative reverse-transcription polymerase chain reaction (RT-PCR) was used to follow the expression of PKS genes over the light/dark cycle of synchronized C. polylepis cultures. This is the first study showing the expression of PKS genes in marine microalgae, in this case in the toxigenic C. polylepis.