Selection for cytoplasmic differences

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Abstract
Colonies arising from single asexual spores of the fungus Aspergillus glaucus show differences among themselves in germination, growth rate, pigmentation and sexuality. These differences are transmissible, at least in part, to daughter colonies produced by asexual reproduction. They are independent of the nucleus and must be referred to persistent differences in the cytoplasm, arising presumably by unequal distribution of cytoplasmic elements during spore formation. Selection for these cytoplasmic differences can produce marked changes in phenotype comparable with those which arise during the normal processes of differentiation, cell delimitation, ageing and senescence, and may ultimately lead to biologically important differences, e.g. the complete elimination of pigmentation and sexual reproduction. Nevertheless, there are limits, albeit wide ones, to the changes realizable by selection of the cytoplasm, probably imposed by the inviability of the more extreme phenotypes. Within these limits variation is continuous. The various characters are highly, but not completely, correlated in their response to selection. All the changes were reversible by back selection and could be wiped out by mixture of the selected cytoplasms with that of an unselected line. The invariable success of back selection shows that total loss of self-reproducing elements, or total loss of their function, is not involved in any of the changes which must therefore be referred to change in the balance of cytoplasmic elements, in contrast to the petite variants in yeasts. The direct action of natural selection on the cytoplasm must play its part in the adjustment of the phenotypes in a fungus like A. glaucus, and would provide an element of flexibility, prospectively of great importance in adaptation of a saprophytic fungus to its ever-changing environment.

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