Characterization of specific cDNA clones of the zygomycete Parasitella parasitica, derived from mRNAs which are regulated by the pheromone trisporic acid

Sexuality and parasitism share common regulatory pathways in the fungus Parasitella parasitica

Parasitella parasitica, a facultative mycoparasite of zygomycetous fungi, forms cytoplasmic fusions with its hosts during infection. Thus, the organism is an efficient donor of genetic material in parasexual host-parasite interactions. Recognition between parasite and host is mediated by trisporoids, which are also responsible for sexual communication. The TDH gene for one of the key enzymes of trisporic acid biosynthesis, 4-dihydromethyl-trisporate dehydrogenase, was cloned and its transcription analysed. TDH was cloned on a 6175-bp insert and was found to map in a complex cluster of genes that suggest post-transcriptional antisense regulation. Histochemical TDH analysis in developing parasitic or sexual structures shows high enzymatic activity in Parasitella. TDH is linked to a gene for a putative acyl-CoA thioesterase (ACT). Two ORFs were identified in the 5'-region of the TDH gene, a third one, coding for 176 amino acids overlaps the ACT gene in antisense direction completely. Expression levels of ACT and ORF1 depend on parasitic and sexual interactions.

Variability in genome organization of the zygomycete Parasitella parasitica

In addition to conventional methods for the identification of fungi, molecular techniques at the DNA level are increasingly being employed. In order to check the validity of such experimental approaches, we have analyzed the well-defined species Parasitella parasitica, which belongs to the family Mucoraceae (Mucorales, Zygometes). The seven strains of this species, which are available from international strain collections, were analyzed by several molecular methods: restriction fragment length polymorphism analysis (RFLP), the random primer-dependent polymerase chain reaction (RAPD-PCR), and electrophoretic karyotyping. Unexpectedly, these strains are highly diverse at the molecular level. By these techniques they can be divided consistently into two different groups. Nevertheless, all seven strains belong to a single species. They show no morphological differences and sexual spores (zygospores) were found in all possible combinations either within or between the two groups. Southern-blot analysis of genomic DNA of all P. parasitica strains with RAPD-PCR-derived labelled probes shows the existence of repetitive elements characteristic for only one group of P. parasitica. In addition, chromosome sizes, which were separated by rotating-field electrophoresis, were highly divergent, and ranged from 3 to 6.5 Mb in one group and between 2 and 4.5 Mb in the other. The RAPD-PCR patterns also discriminate both groups of P. parasitica. However, they are very similar if strains of a single group are compared. Therefore, we propose that the determination of fungal species by molecular techniques should be vetted at least by morphological and physiological parameters and, whenever possible, by mating experiments.