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

Abstract

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 Xow cytometric seed screen (FCSS). Single seed FCSS of selected 14 tetraploid and Wve 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 diVered across ploidy levels and genotypes. Reconstruction of reproductive pathways yielded a total of eight diVerent pathways of seed development, arising by uncoupling/recombination between apomixis components. Amongst these, two pathways involving modiWcations 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 Wrst time. Some of the combinations, such as M I (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 identiWed. 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 Wrst time. Occurrence of several apomixis recombinants (phenotypic) in guinea grass lines suggested their hybrid origin, favors a multigene model for apomixis, with their penetrance aVected by modiWers and epigenetic mechanisms, in contrast to earlier reports of single locus control. Implications of partitioning components on apomixis research are discussed.

Development and Characterization of a Hexaploid <i>Pennisetum orientale</i> (2<i>n</i>=6<i>x</i>=54) Cytotype Recovered through B_III Hybridization

“…The recovery of 6x cytotype from a 4x maternal plant suggested the 2n+n (termed as B III ) hybridization pathway of its origin, possibly through fertilization of an unreduced egg cell (2n) with reduced male gamete (n) in the 4x cytotype (Kaushal et al 2008). As the 4x plant was aposporous in nature, it is likely to generate unreduced egg cells in its apomeiotically developed embryo-sac (Nogler 1984).…”

“…apomeiosis, parthenogenesis and functional endosperm development, are linked functionally and maintain clonal reproduction in apomictic seed development. However, uncoupling of these components has been demonstrated recently in several crops, thereby generating modified ploidies and diversity in pathways of seed development (Matzk et al 2005, Kaushal et al 2008, 2009). Uncoupling apomeiosis from parthenogenesis would require fertilization of an unreduced egg cell to develop an embryo and thus yielding 3n progeny through 2n+n hybridization and are termed as B III hybrids (Ozias-Akins and van Dijk 2007, Kaushal et al 2008).…”

“…However, uncoupling of these components has been demonstrated recently in several crops, thereby generating modified ploidies and diversity in pathways of seed development (Matzk et al 2005, Kaushal et al 2008, 2009). Uncoupling apomeiosis from parthenogenesis would require fertilization of an unreduced egg cell to develop an embryo and thus yielding 3n progeny through 2n+n hybridization and are termed as B III hybrids (Ozias-Akins and van Dijk 2007, Kaushal et al 2008). Such hybrids with 3n genomic constitution have already been reported in crops such as Poa pratensis (Albertini et al 2001), Panicum maximum (Kaushal et al 2009), Cenchrus ciliaris (Conner et al 2013) and Pennisetum spp.…”

Expressivity of apomixis in 2n + n hybrids from an apomictic and a sexual parent: insights into variation detected in Pilosella (Asteraceae: Lactuceae)

“…Apomixis is mostly ''dominant'' over sexuality and is conferred by a monogenic locus in some taxa (reviewed in OziasAkins and Van Dijk 2007). In other taxa, the independent genetic control of particular elements of apomixis is assumed to correspond to a two-or three-loci model (Ozias-Akins and Van Dijk 2007;Kaushal et al 2008). Assuming the control of particular apomictic elements by more than one locus each, a model involving up to five loci was proposed (Matzk et al 2005).…”

“…The modifier loci, in addition to those controlling basic apomictic components, are assumed to influence the expression of apomixis in various genetic backgrounds (e.g., Bicknell et al 2000;Koltunow et al 2000;Kaushal et al 2008;Batygina 2009;Tucker and Koltunow 2009). Some of the multiple locus models of inheritance of apomixis, however, may already involve ''modifiers'' that are not essential for the apomictic process and are shared with apomicts and sexuals (Tucker and Koltunow 2009).…”