Nonreciprocal Hybridization Failure in Crosses Between Annual Wild-Rice Species (Zizania palustris x Z. aquatica: Poaceae)

Abstract

Natural interspecific hybridization has been proposed to account for morphological variation in the annual members of the genus Zizania, commonly known as wild-rice, although isozyme data, as reported in a single study, have disclosed no evidence of introgression. Experimental hybridizations were conducted in all possible combinations among the five varieties of the two annual species of wild-rice. Meaningful intraspecific variation in crossability was not detected, which is in agreement with morphological and isozyme data. A unilateral interspecific crossability barrier was found between the two annual species. Pistillate florets of Z. palustris typically failed to produce viable fruits when pollinated by Z. aquatica. The reciprocal interspecific cross was successful at a low level of crossability. This evidence further strengthens the taxonomic separation of the two annual species previously defined by spikelet morphology, ultrastructure, and isozyme varia- tion. Examination of pollinated stigmas of the infertile types of crosses disclosed that pollen tubes had penetrated the entire length of the stigma. The crossing barrier is therefore not due to a pollen/ stigma interaction that severely interferes with pollen tube growth. Interspecific F, hybrids ex- pressed the spikelet morphology of the Z. aquatica parent exclusively, although other morphological characters were typically expressed as intermediate to those of the parent species. The potential for natural hybridization between Z. aquatica and Z. palustris is discussed.