Gene Transfer from the Zygomycete Parasitella parasitica to its Hosts: An Evolutionary Link Between Sex and Parasitism?

Parasitella parasitica (synonyms: P. parasitica and Mucor parasiticus) is a heterothallic facultative mycoparasite which belongs phylogenetically to the Mucor group within the zygomycetes. It is a facultative parasite of several but not all mucoraceous fungi. The interactions between host and parasite present several peculiarities. (1) Infection of the host organism Absidia glauca especially is strongly mating type-specific. (+) strains of P. parasitica will infect exclusively (-) strains of A. glauca whereas (-) strains of the parasite are required for the infection of (+) type hosts. We propose the hypothesis that host/parasite recognition is mediated by the same gamone system as is required for communication between sexual partners within mucorales (trisporic acid). (2) P. parasitica is a fusion biotroph. Infection is accompanied by a limited cytoplasmic continuum between the two partners, which allows invasion of the host by nuclei of the parasite. P. parasitica is not very aggressive. The host, A. glauca survives the infection well and forms normal sporangiospores even close to infection sites. We were able to demonstrate by genetic and molecular techniques that genetic markers of the parasite are transferred into the host’s nuclei at high rates. Absidia and Parasitella form a naturally occurring parasexual system with high efficiency. (3) Sexually compatible pairs of P. parasitica are not able to form zygospores. In order to reproduce sexually at least one of the partners has to grow parasitically. Measurements of trisporic acid levels in cocultures of P. parasitica with A. glauca led us to assume that interaction with a host organism is required for the cooperative synthesis of trisporic acid at a level high enough to induce sexual differentiation.