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Reproductive pathways of seed development in apomictic guinea grass (Panicum maximum Jacq.) reveal uncoupling of apomixis components

Kaushal, P., Malaviya, D. R., Roy, A. K., Pathak, Shalini, Agrawal, A., Khare, Ambica, Siddiqui, S. A.
Euphytica 2008 v.164 no.1 pp. 81-92
Panicum maximum, apomixis, seed development, seeds, flow cytometry, meiosis, parthenogenesis, endosperm, embryo sac, germplasm, plant genetic resources, plant genetics, hexaploidy
One hundred and sixty accessions representing global germplasm of guinea grass (Panicum maximum Jacq.), an important apomictic (aposporous) fodder crop, were subjected to study on reproductive diversity in apomictic seed development utilizing ovule clearing and flow cytometric seed screen (FCSS). Single seed FCSS of selected 14 tetraploid and five hexaploid lines demonstrated uncoupling between the three apomixis components, viz. apomeiosis, parthenogenesis and functional endosperm development, in natural as well as experimental populations, though it differed across ploidy levels and genotypes. Reconstruction of reproductive pathways yielded a total of eight different pathways of seed development, arising by uncoupling/recombination between apomixis components. Amongst these, two pathways involving modifications in embryo-sac (ES) (presence of two polar nuclei in aposporous ES that fuse prior to fertilization) and fertilization process (fusion of only one polar nucleus in a sexual ES) have been reported for the first time. Some of the combinations, such as MI (haploids arising from parthenogenetic development of reduced egg cell), were found viable only in hexaploid background. Germplasm lines with higher expression of individual components were also identified. These components (including autonomous endosperm development) were also experimentally partitioned in hexaploid progenies (derived from a tetraploid parent viz. accession IG 04-164) that showed segregation in their reproductive capacities, and are reported for the first time. Occurrence of several apomixis recombinants (phenotypic) in guinea grass lines suggested their hybrid origin, favors a multigene model for apomixis, with their penetrance affected by modifiers and epigenetic mechanisms, in contrast to earlier reports of single locus control. Implications of partitioning components on apomixis research are discussed.