SUMMARY: Nine mutants of which require arginine as a nutrient but cannot use citrulline were obtained from various sources. These fall into two classes, according to location in linkage group I or VII. (Enzymic tests, reported elsewhere, indicate that the two classes of mutants affect the enzymes which control the two reactions between citrulline and arginine.) Heterokaryon tests between mutants of the same group were negative, and crosses between mutants of the same group were semi-sterile, most of the aseospores being non-viable. Crosses between the five group I mutants produced no progeny, and separate mapping tests on four of them indicate that they are either allelic or closely linked. All crosses between the group VII mutants gave many arg progeny. For the one pair of mutants which was studied in detail, origin of the by means other than crossing-over (or gene conversion) has been virtually eliminated. However, mapping studies place the two mutants only 0-6 units apart. It is concluded that the high frequency is due to selection, and that the mutants might be pseudoalleles.


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