Partitioning Apomixis Components to Understand and Utilize Gametophytic Apomixis

Abstract

Apomixis is a method of reproduction to generate clonal seeds and offers tremendous potential to fix heterozygosity and hybrid vigor. The process of apomictic seed development is complex and comprises three distinct components, viz., apomeiosis (leading to formation of unreduced egg cell), parthenogenesis (development of embryo without fertilization) and functional endosperm development. Recently, in many crops, these three components are reported to be uncoupled leading to their partitioning. This review provides insight into the recent status of our understanding surrounding partitioning apomixis components in gametophytic apomictic plants and research avenues that it offers to help understand the biology of apomixis. Possible consequences leading to diversity in seed developmental pathways, resources to understand apomixis, inheritance and identification of candidate gene(s) for partitioned components, as well as contribution towards creation of variability are all discussed. The potential of Panicum maximum, an aposporous crop, is also discussed as a model crop to study partitioning principle and effects. Modifications in cytogenetic status, as well as endosperm imprinting effects arising due to partitioning effects, opens up new opportunities to understand and utilize apomixis components, especially towards synthesizing apomixis in crops.

Keywords: Panicum maximum; apomeiosis; apomixis; endosperm; parthenogenesis; partitioning.

FIGURE 1

Consequences of partitioning apomixis components leading to formation of triploids (3n) and haploids (1n) in the progeny of diploids (2n), through B_III and M₁ pathways. Apomixis is achieved when an unreduced egg cell (in apomeiotic embryo-sac) develops through parthenogenesis, however, in sexually reproducing plants meiotically derived haploid egg cell fertilizes with a haploid sperm cell. In both cases, diploid status (2n) of embryo is conserved. In presence of genetic determinants for both these functions, as in heterozygotes, recombination between these elements may occur. Consequently, apomeiotic egg cell might require fertilization for embryo development leading to triploid (2n + n = 3n; B_III) progeny. Alternatively, a meiotically derived haploid egg cell acquires the parthenogenetic capacity and develops without fertilization, leading to formation of haploid (1n + 0 = 1n; M₁) progeny.

FIGURE 2

Representative ES of guinea grass (cleared ovules). (A) Aposporous ES, (B) Sexual (or meiotic) ES, (C) Multiple ES (three ES seen), (D) Ovule showing proliferating polar nuclei in absence of pollination, as an indicator of AED, a cluster of four nuclei is visible in one plane, (E): Ovule showing an aborted ES. e- egg cell, p-polar nucleus, a- antipodals, ES-Embryo sac. Reprinted by permission from the Springer Nature: Kaushal et al. (2018).

FIGURE 3

Illustrative Single seed-FCSS histograms showing various progeny classes obtained in P. maximum, exhibiting (A) 2n embryo (B_III or M₁ origin; with 2Emb:3End genome ratios); (B) 3n embryo (B_III seed; 1Emb:1End ratio; (C) 1n embryo (M₁ seed; 1Emb:3End ratio); and (D) a seed with twin embryos with n (M₁ origin) and 2n genomes and sharing common endosperm. In histogram, x axis- relative fluorescence, y axis- number of nuclei; Peaks 1 and 2 represent embryo and endosperm peaks, respectively, other unmarked peaks arise from endo-polyploidization events of embryo/endosperm cells. Reprinted by permission from the Springer Nature: Kaushal et al. (2018).

FIGURE 4

Scheme for production of ploidy series (Kaushal et al., 2009, 2015b). Plants representing different ploidies viz., 3x, 4x, 5x, 6x, 7x, 8x, 9x, and 11x, were generated from a single 4x progenitor through HAPA. The recovery of plants with specified ploidy and their pathways of formation (M₁, B_II or B_III) is depicted. Information in parenthesis shows maternal (m) and paternal (p) genomic contribution. In all cases depicted here, male gamete was always reduced, while female gamete might be reduced or unreduced, and the embryo development may be through parthenogenesis or fertilization dependent. Reprinted by permission from the Springer Nature: Kaushal et al. (2018).

FIGURE 5

Morphological, flow cytometric and meiotic chromosomes characterization of a ploidy series in guinea grass, represented by 3x, 4x, 5x, 6x, 7x, 8x, 9x, and 11x cytotypes, generated through HAPA. Upper lane: morphological features of plants representing different ploidies. Vertical scale represents 100 cm height; Middle lane: Flow cytometric histograms from plant sample alongwith internal control, obtained from leaf tissues of plants representing various ploidies represented in upper lane, respectively. A triploid (3x) plant was used as the internal control, represented by peak 1 in all FCM histograms; peak 2 was of the sample. Smaller peaks arise from G2 phase cells. Leaves of 4x plant were used as the internal control for 3x, and hence the histograms of 3x and 4x plants were same; Lower lane: meiotic chromosome configurations of plants representing ploidies 3x till 9x and 11x. Adapted and modified with permission from Kaushal et al. (2009). Reprinted by permission from John Wiley and Sons: Kaushal et al. (2009).