Unisexual Conidia from Bisexual Mycelia

The opinion prevails, in the absence of adequate proof to the contrary, that a heterothallic or a homothallic species of the Mucoraceae remains fixed so far as this particular characteristic is concerned. Only unisexual spores are developed in a sporangium of Phycomyces nitens. Only bisexual spores are formed in a sporangium of Sporodinia grandis. Species of the ascomycete genus Neurospora produce conidia of the Monilia sitophila type.1 Neurospora sitophila and N. crassa are heterothallic; therefore, in order to obtain perithecia it becomes necessary to mate in culture a pair of haplonts, and B, which may be conceived as being of opposite sex. It is customary to obtain such unisexual mycelia by germinating ascospores. A very interesting question arises when one considers the nature of the conidia produced in a mixed culture containing two mycelia of reciprocal sexuality. Clearly each mycelium produces very early its own unisexual conidia. One naturally assumes that in such a culture there would occur hyphal fusions particularly between branches which are of opposite sex. In that case, do such fusion cells bud out and finally produce bisexual conidia? Neurospora tetrasperma is ordinarily homothallic. A monosporous mycelium produces perithecia under proper cultural conditions. By selecting an abnormally small ascospore to be found in almost any spore print, one obtains a mycelium which develops perithecia only when mated with another haplont of the opposite sex. In other words, by properly selecting asco? spores one can secure heterothallic strains of a species which is commonly homothallic. The writer has recently shown why the