Parasexual Phenomena in Microorganisms

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Abstract
In the decade since the discovery of parasexuality, this process has been found to be operative in several ascomycetes, basidiomycetes, and deuteromycetes. A complete parasexual cycle has not yet been described in the phycomycetes even though heterokaryosis is prevalent in this group. Heterokaryons of Rhizopus javanicus do occasionally give rise to dissociates possessing new phenotypes; conceivably, transitory diploids may be formed and the recombinants produced by haploidiza-tion. To date, studies on parasexuality have emphasized the importance of the persistent diploid, yet this is not an essential aspect of the cycle. It must be noted that most fungi with clearly established, complete parasexual cycles have as a part of their life cycle, hyphal fusion, dikaryotic cells, and conjugate nuclear division. This correlation may be by chance or indicative of an evolutionary link between parasexuality and sexuality. The question then arises: can parasexuality be shown for all groups of microbes ? In algae such as Vaucheria and mycetozoa such as Physarum, heterokaryons can be produced, thereby providing ample opportunity for diploidization and subsequent segregation. Obviously, parasexuality is a mechanism for establishing new combinations of genes. New species, therefore, may be created by selection of organisms possessing new associations of characters resulting from intraspecific or interspecific hybridization, as well as by mutation. Interspecific parasexual crosses have been successful in the streptomycetes whose deoxyribonucleic acids have the same base composition. Clearly, new pathogens and new industrial strains can be found among the products of parasexual recombination. Strains producing more antibiotic or a new antibiotic will be present as a minority population, however, and the ultimate utility of parasexuality will be only as good as the screening program coupled with it. This is also true in plant and animal breeding programs. Moreover, many of the combinations tested to date are of a single lineage, which certainly reduces the amount of variation that can be expected among the progeny. The recombinational process in the streptomycetes resembles the parasexual cycle in all of its essential features except that the streptomycetes, in contrast to the molds, lack an organized nucleus surrounded by a double membrane. I believe that the fundamental genetic processes of bacterial conjugation resemble the parasexual cycle of the streptomycetes and Aspergillus more than true meiotic (sexual) recombination. Because E. coli grows as short rods, the diverse germ plasms of a heterogenomic organism are regularly separated; as a result, persistence of a segregating heterozygous diploid is rare. The search for the parasexual cycle in filamentous microbes has usually been successful; (paraphrasing Pontecorvo) it seems legitimate to conclude that this process cannot be a rare oddity.