SUMMARY: Colonies of arising from single asexual spores or hyphal tips of the same or different homokaryon clones often vary in their adaptability to new environments. This shows itself as differences in the percentage survival and mean lag period of inocula taken from colonies growing on a normal medium when transferred to one containing a poison such as mercuric chloride, or unfamiliar sugar sources, e.g. arabinose, galactose, lactose and xylose.

The differences in adaptability to these new media between homokaryon clones are characteristically nuclear in origin, while those between colonies of the same clone are characteristically cytoplasmic. The nuclear system might be formally regarded as epistatic to the cytoplasmic system involved in these adaptive changes.

Selection for the cytoplasmic differences can produce marked changes in the adaptability of the asexual spores of single colonies, even though the selection technique is such that colonies in the direct line of selected descent are at no time exposed to the new media.

The changes in adaptability to mercuric chloride are independent of those for adaptability to the new sugars and only the latter shows any correlation with changes in rate of growth. There is therefore evidence of two or possibly three cytoplasmic systems with much the same properties of variation but independent in action and transmission.


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