The Mechanism of Apomixis in Pennisetum ciliare

Abstract

1. Cytological studies of seven collections of Pennisetum ciliare from widely separated localities indicate that apomixis, involving apospory and pseudogamy, is the predominant mode of reproduction in this highly variable species. 2. Aneuploid chromosome numbers of 2n = 43 and 2n = 48 were found in two strains, and the tetraploid number 2n = 36 in the remaining five. 3. Meiotic behavior in microsporocytes is characterized by limited quadrivalent formation and considerable irregularity, particularly in aneuploid plants. 4. The products of meiosis in the ovules usually degenerate soon after formation, and one or more nucellar cells enlarge and develop into unreduced embryo sacs. Both four- and eight-nucleate mature embryo sacs are produced, although the latter are relatively uncommon. Evidence from Panicum maximum and from the present study strongly suggests that mature four-nucleate embryo sacs are typically unreduced and that eight-nucleate embryo sacs are typically reduced in these species. 5. Pollination is necessary for the initiation of endosperm development, which normally precedes division of the unreduced and unfertilized egg. 6. Closely similar reproductive behavior was found in material of a single collection of Cenchrus setigerus.