ADAPTATION IN PANTOTHENATE-REQUIRING NEUROSPORA II. NUCLEAR COMPETITION DURING ADAPTATION

DAVIS, R. H. (U. Michigan, Ann Arbor.) Adaptation in pantothenate-requiring Neurospora. II. Nuclear competition during adaptation. Amer. Jour. Bot. 47(8): 648-654. Illus. 1960.-The process of adaptation in pantothenate-requiring Neurospora was studied by the use of heterocaryons constituted of the pan-i strain and a modified strain, pan-I m, derived from it. Although pan-l-m homocaryons grow well on a pantothenate concentration on which pan-i grows little or not at all, pan-i nuclei often have a selective advantage in pan-i + pan-l-m. heterocaryons grown on the same medium. This results in non-adaptive changes in nuclear ratios and labile growth rates of the heterocaryons. Nuclear competition does not occur when pantothenate is not limiting to the growth of either homocaryon. The results are discussed, and related to past work on adaptation and nuclear ratio changes in Neurospora hetercaryons. ADAPTATION in Neurospora, in the sense of genetic changes in the nuclear population of a mycelium which lead to more rapid growth, has been by no means thoroughly investigated. The coenocytic organization of Neurospora mycelia allows numerous nuclei to cooperate in the activities of a single cytoplasmic unit. Through mutation or through the fusion of genetically dissimilar mycelia, a heterocaryon may arise in which there is a heterogeneity in the nuclear population. Mutant nuclear types which will not sustain normal growth in pure cultures may, when combined in a "balanced heterocaryon," contribute to a normal phenotype through the mutual complementation of the wild type alleles of the mutant genes involved. Since the nuclei in heterocaryons are not autonomous, but interact with one another in an integrated cellular system, the change of nuclear frequencies is a more complex phenomenon than adaptation in populations of uninucleate, unicellular organisms such as bacteria. The change in nuclear frequencies of an adapting Neurospora crassa heterocaryon is the subject of this paper. Previous investigators have brought forth 2 different proposals regarding the interactions between the 2 components of a balanced heterocaryon. Beadle and Coonradt (1944) suggested, though without definitive evidence, that nuclear ratios are altered by selection among the hyphae at the growing frontier of a mycelium. They assumed that growth rates of individual hyphae are determined by their own nuclear ratios, and that selection for those hyphae with ratios optimal for growth leads to the establishment of an optimal growth rate l Received for publication December 19, 1959. This work was done while the author was a U. S. Public Health Service Predoctoral Research Fellow of the National Cancer Institute. Submitted as a portion of a dissertation in partial fulfillment of the requirements for the degree of Ph.D., Department of Biology, Harvard University, 1958. This investigation was conducted in the Biological Laboratories of Harvard University. It is a pleasure to thank the following people for their critical discussion of this work: Drs. K. C. Atwood, W. T. Ebersold, R. P. Levine, J. R. Raper, F. J. Ryan, and l'. S. Snider. (and nuclear ratio) for the heterocaryon as a whole. Pittenger and Atwood (1956), after a more extensive investigation of this matter, maintained that hyphal selection "rarely, if ever, operates" to alter the nuclear ratio of a fully established heterocaryon, even in cases in which change of nuclear ratio would result in higher growth rate. This idea is supported by experiments in which balanced heterocaryons of biochemically mutant nuclear types, carefully synthesized with highly disparate nuclear ratios, grew at stable submaximal rates for long periods of time. The submaximal growth rate was referable to the scarcity of 1 of the 2 nuclear types. The stability of nuclear ratios was presumed to depend upon equal division rates of the nuclear tvpes, dependence of growth rates of hyphae at the mycelial frontier upon the activities of the larger nuclear population proximal to them, and continual mixing of the nuclear population through cytoplasmic streaming and hyphal anastomosis. A phenomenon which was not, apparently, encountered in either of the above-mentioned investigations was described by Ryan and Lederberg (1946; cf. Ryan, 1946). Adaptations of the leucineless (leu-i) strain of Neurospora to limiting concentrations of leucine were observed to take place through the occurrence and proliferation of backmutant (leu-i +) nuclei. It was found, however, that the leu-i mycelium often inhibited ths growth of leu-i + sectors in limiting concentrations of leucine, leading to non-adaptive changes in nuclear ratios (i.e., changes less favorable to high growth rate) of a leu-i + leu-i + heterocaryon. Data reminiscent of this behavior were reported by Emerson (1947, 1948) and by Emerson and Cushing (1946), working on adaptations to sulfonamides. Such behavior cannot be accounted for in terms of simple hyphal selection or stability of nuclear ratios. Another case of non-adaptive changes in the nuclear ratio of a heterocaryon is described in this paper. The heterocaryon is constituted of a normal pantothenic-less strain, pan-i, and a modified strain, pan-i m, described previously (Davis, 1960). The 2 strains, isolated asexually from a singrle panto-