Selection for cytoplasmic differences

Colonies arising from single asexual spores o f the fungus Aspergillus glaucus show differences am ong them selves in germ ination, grow th rate, pigm entation and sexuality. These differences are transm issible, at least in part, to daughter colonies produced by asexual reproduction. T hey are independent o f the nucleus and m ust be referred to persistent differences in the cytoplasm , arising presum ably b y unequal distribution o f cytoplasm ic elem ents during spore form ation. Selection for these cytoplasm ic differences can produce m arked changes in phenotype com parable w ith those w hich arise during the norm al processes o f differentiation, cell delim itation, ageing and senescence, and m ay u ltim ately lead to biologically im portant differences, e.g. the com plete elim ination o f pigm entation and sexual reproduction. N ever­ theless, there are lim its, albeit w ide ones, to the changes realizable b y selection o f the cytoplasm , probably im posed by the inviab ility o f the m ore extrem e phenotypes. W ithin these lim its variation is continuous. The various characters are h ighly, but not com pletely, correlated in their response to selection. All the changes were reversible by back selection and could be wiped out by m ixture o f the selected cytoplasm s w ith th a t o f an unselected line. The invariable success o f back selection shows th at to ta l loss o f self-reproducing elem ents, or to ta l loss o f their function, is not involved in any o f the changes which m ust therefore be referred to change in the balance o f cytoplasm ic elem ents, in contrast to the petite variants in yeasts. The direct action o f natural selection on the cytoplasm m ust p lay its part in the adjustm ent o f the phenotypes in a fungus like A. glaucus, and w ould provide an elem ent o f flexibility , prospectively o f great im portance in adaptation o f a saprophytic fungus to its ever-changing environm ent.