SUMMARY: The basis of variation in morphological appearance of first cultures of , isolated from diseased roots and corms of prematurely yellowed Gladiolus plants, was investigated. The occurrence of mutations is considered to be far too infrequent to account for the wide range of variability of the fungus in culture, and the existence of heterocaryons of mixed morphologically distinct genotypes is considered.

Cytological studies show that each microconidium contains one nucleus which is derived by mitosis from the single nucleus of the conidiophore. Hyphal tip cells contain an average of seven nuclei whilst the cells behind, in the older parts of the hyphae, contain only one nucleus each or, more rarely, two. Anastomoses between germ tubes from conidia of different morphological variants occur very frequently and may provide a means whereby heterocaryons are formed.

Synthesis of heterocaryotic cultures is achieved by simultaneous inoculation of plates containing plain water agar medium. Hyphal tips were separately removed from the mixed colonies thus produced and analysed by single-sporing the resulting colonies. Media varying in carbon content and in carbon/nitrogen ratio proved to have a selective action upon one or other of the homotypes constituting a heterokaryon of two known morphologically distinct variants. By use of such media, it was shown that the ratios of nuclei homotypic for ‘flat sporodochial’ and ‘aerial microconidial’ variants was altered according to the relative concentrations of carbon or nitrogen in the culture medium.

A discussion follows on the bearing of these results on the reliability of hitherto important taxonomic criteria in a classification of these fungi. The isolates of include a wide range of morphological variants which may be included in Snyder & Hansen's . The existence of neterocaryons between taxonomic variants and their plasticity under various cultural conditions, poses the question of their status in any satisfactory classificatory system.


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