Induced Hybridization in Graminicolous Smut Fungi. III. Further Studies of Ustilago Hordei × U. Bullata

SUMMARY

An account is given of hybridization between Ustilago hordei and U. bullata. Several contrasting spore characters were available for study in crosses between these species. Spores of U. hordei are small (5-8μ), smooth, and lighter colored on one side; those of U. bullata are larger (8-11 μ), verrucose, and, in some collections, lighter colored on two sides.

Seventy-four different monosporidial combinations, representing one collection of U. hordei and five collections of U. bullata, produced Ft spores on Elymus glaucus or E. canadensis.

The F₁ spores were echinulate; most were lighter colored on two sides and intermediate in size. The hybrid spores germinated, but lysis of the promycelia and sporidia defeated attempts to obtain complete sets of monosporidial lines. Therefore, hybrid teliospores were used as inoculum to obtain subsequent generations.

Sixty-two of the 74 F₁ hybrids produced a second generation. Segregation in the F₂ resulted in the recovery of parental types and in the appearance of a number of types unlike either parent. Most F₂ sori contained spores more or less echinulate and lighter colored on two sides like those of the F₁; some contained smooth spores lighter colored on one or two sides; and others contained a mixture of spore types. The F₂ spores ranged from smaller than U. hordei to larger than U. bullata.

Teliospores of representative F₂ segregates were used to obtain F₃ hybrids; representative F₄ types were then used to obtain F₄ hybrids. The F₄ generations represented nine of the original 74 monosporidial combinations of U. hordei and U. bullata.

In most cases, the F₃ and F₄ hybrids resembled their respective F₂ or F₃ parents; in others, segregation produced different types as in the F₂. In a few instances, segregates were recovered which possessed characteristics, not attributable phenotypically, to either parent. These included smooth spores notched on the lighter sides, echinulate spores with laminated exospore, and smooth spores broadly indented on one side.

Some of the segregates were sufficiently distinct to be considered as independent species. This production of new morphologic types by hybridization experimentally is indicative that new species may originate likewise in nature.

Populations were too small to permit ratio studies, but the predominance of intermediate types, the gradation of characters from one parental extreme to the other, and the occurrence of heterosis suggest that most of the characters are governed by multiple factors.