OXYGEN CONSUMPTION OF MIXTURES OF HETEROTHALLIC AND HOMOTHALLIC SPECIES IN RELATION TO ‘IMPERFECT HYBRIDIZATION’ IN THE MUCORACEAE

Abstract

Attempts by several investigators to produce hybrids between different species of Mucoraceae have failed in most cases. Even where hybrid zygotes were not developed there was frequently a modification of the hyphae in regions where thalli were opposed. These modifications were usually more or less reminiscent of the structures developed in normal sexual reproduction, and in some cases gametangia-like structures or azygospores developed. Blakeslee (I904) described all such phenomena as 'imperfect hybridization'. They have been observed between different heterothallic species and between heterothallic and homothallic species. They have been described on two occasions between different homothallic species. Amongst heterothallic species the occurrence of 'imperfect hybridization' was explained by supposing that it developed between cultures of opposite mating type of each species. This hypothesis enabled Blakeslee to determine the mating type of all cultures of heterothallic species relative to arbitrarily designated mating types of a standard tester species, races 'c' and 'd' of Mucor V ( =M. hiemalis). In experiments with homothallic species, most were found to interact predominantly with one of the mating-type cultures of the heterothallic species with which hybridization was attempted. Blakeslee drew attention to the fact that the occurrence and vigour of these reactions appeared to be independent of the taxonomic relationships of the fungi involved. I am not aware of any attempt to integrate these observations with other aspects of the reproductive physiology of the Mucoraceae. Experimental results described in this note are an attempt at such integration. The oxygen consumption of mixtures of cultures of heterothallic species and of heterothallic and homothallic species was measured manometrically. The technique used has been described elsewhere (Burnett, I953). The species were selected on the basis of published observations of 'imperfect hybridizations'. This information and the results of the experiments are summarized in Table i. Small but significant increases in oxygen consumption were often observed in mixtures of species which were reported to have shown 'imperfect hybridization', but never in those mixtures where it did not occur. The results were obtained both with mixtures of heterothallic species and mixtures of heterothallic and homothallic species. Mixtures of compatible cultures of several Mucoraceae are known to show an increased oxygen consumption (Burnett, I953). I supposed that this increase was due to the action of diffusible, extra-mycelial agents produced at the outset of sexual reproduction.