Facultative versus obligate apomixis in angiosperms: insights from Paspalum (Poaceae) and Ranunculus (Ranunculaceae)

HOJSGAARD DIEGO H.; MARTÍNEZ ERIC J.; QUARIN CAMILO L.; HÖRANDL ELVIRA

Viena

Jornada; Vienna Plant Network Meeting; 2010

Angiosperms, or flowering plants, are the most diverse and widespread group of the Plant Kingdom. The normal sexual reproductive process involves meiosis, resulting in reduced spores, which develop into female and male gametophytes (embryo sac and pollen, respectively). Seeds are produced after fusion of reduced female and male gametes, plus fertilization of the endosperm. However, besides the sexual pathway, in some plants from different families an asexual pathway for seed formation (or apomixis) is also possible. In the apomictic development, at least three components are needed to be functional to generate a viable seed. First, the normal meiotic process must be bypassed (apomeiosis) to form an unreduced embryo sac. Second, the egg cell of the unreduced female gametophyte must develop parthenogenetically in a maternal embryo and, third, the development of the endosperm (via pseudogamy or autonomously) for these apomicts to produce viable seeds. From a reproductive view, plant individuals producing both sexual and apomictic seeds are considered as apomicts, independently of the percentages of apomixis. When a fraction of the progeny is sexual, the plant is considered as facultative apomict, but if the all progeny is of clonal origin that plant is considered as an obligate apomict. Here we present the results of our research on two aposporous apomictic case studies: the grass genus Paspalum (Panicoideae: Poaceae), and Ranunculus (Ranunculaceae). Both Paspalum and Ranunculus species integrate agamic complexes, with diploid sexual and polyploid apomictic cytotypes. Aposporous development in the autotetraploid Paspalum species differs from allohexaploid Ranunculus species by the final constitution and number of the mature embryo sacs in his ovules. The degree of apomixis in these plants is variable according to the time in the life cycle in which the character is measured. Embryology on flowers, flow cytometry on seeds and molecular data on progeny shows strong differences in the incidence of apomixis suggesting that unreduced embryo sacs are more efficient than reduced ones, and this tendency is accentuate on progeny. The nature and consequences of this selective pressure again the sexual reproductive way and its evolutionary consequences are discussed.