The Mechanism of Apomixis in Pennisetum ciliare

Abstract

Cytological studies of seven collections of Pennisetum ciliare from widely separated locations indicate that apomixis, including apospory and pseudogamy, is the predominant mode of reproduction in this highly variable species. Chromosome numbers of 2n = 43 and 2n = 48 were found, in addition to the numbers 32, 36, 40, and 54 previously reported for this species. Meiotic behavior in PMC's varies widely in the different strains, but, in general, is characterized by limited quadrivalent formation and considerable irregularity and chromosome nondisjunction. The products of meiosis in the ovules usually degenerate soon after they are formed, and one or more cells of the nucellus enlarge to become unreduced embryo sacs. Both 4- and 8-nucleate mature embryo sacs are produced, the latter being relatively uncommon, however. Evidence from Panicum maximum and the materials involved in the present study strongly suggests that mature 4-nucleate embryo sacs are typically unreduced and that 8-nucleate embryo sacs are typically reduced in these species. Pollination is necessary for the initiation of divisions in the endosperm, which precede the division of the unreduced and unfertilized egg. Closely comparable reproductive behavior was found in material of a single collection, of Cenchrus setigerus