Interspecific transfer of pycnial nectar induces pycniospore caps in rust fungi in a manner related to mating type within species

Pycnial nectar of one mating type is known to induce cap formation on pycni ospores of opposite mating type within several species of Puccinia and Urom yces. To learn if caps are induced by nectar transfers between species, we used interspecific pairings involving six species of Puccinia and three of Uromyces. Overall, caps were induced in 14 pairings between different speci es involving all tested species, except Puccinia helianthi which has no int raspecific cap induction. Nectar (with pycniospores) exchanged in reciproca l transfers between individual pycnial clusters of two different species ga ve pycniospore caps in nine of 16 cluster pairings, comparable to rates wit hin species. Spore-free nectar combined from five or more pycnial clusters of one species (to ensure that nectar of two mating types was present) usua lly induced caps in pycniospores from single pycnial clusters of a second s pecies. This occurred in all tested pairings of species except pairings inv olving P. helianthi. In experiments with pycniospore-free nectar of one cap ping type specificity from P. recondite, caps were induced in about 50% of pycnial clusters of unknown capping type from P. triticina or P. hordei and only in pycnial clusters of one capping type from P. triticina or P. reich ertii in experiments in which type within species was determined. Coupled w ith the fact that capping type specificity and mating types are coincident within species, the results indicate that mating type-specific induction of pycniospore caps by nectar extends across species boundaries. Although aec ia were never produced in interspecific pairings, cap induction occurred as it does in intraspecific pairings where it precedes aecium formation in sp ecies exhibiting the capping phenomenon.