Nonreciprocal hybridization failure in crosses between annual wild-rice species (Zizania palustris X Z. aquatica: Poaceae).

To test the theory that morphological variation in annual Zizania results from natural interspecific hybridization (despite contrary evidence from isoenzyme data), all possible crosses (25 in all) were made between 3 botanical varieties of Z. aquatica and 2 of Z. palustris. No appreciable intraspecific variation in crossability was detected, but a unilateral interspecific cross incompatibility was found (pistillate florets of Z. palustris failed to set viable fruit when pollinated by Z. aquatica), the reciprocal showing a low degree of crossability. In infertile crosses, the pollen tubes penetrated the entire length of the stigma, indicating that the incompatibility mechanism must lie elsewhere. F1 interspecific hybrids had the spikelet morphology of Z. aquatica alone, though they were intermediate between their parents in other morphological characters.