Eragrostis curvula, a Model Species for Diplosporous Apomixis

Abstract

Eragrostis curvula (Schrad.) Ness is a grass with a particular apomictic embryo sac development called Eragrostis type. Apomixis is a type of asexual reproduction that produces seeds without fertilization in which the resulting progeny is genetically identical to the mother plant and with the potential to fix the hybrid vigour from more than one generation, among other advantages. The absence of meiosis and the occurrence of only two rounds of mitosis instead of three during embryo sac development make this model unique and suitable to be transferred to economically important crops. Throughout this review, we highlight the advances in the knowledge of apomixis in E. curvula using different techniques such as cytoembryology, DNA methylation analyses, small-RNA-seq, RNA-seq, genome assembly, and genotyping by sequencing. The main bulk of evidence points out that apomixis is inherited as a single Mendelian factor, and it is regulated by genetic and epigenetic mechanisms controlled by a complex network. With all this information, we propose a model of the mechanisms involved in diplosporous apomixis in this grass. All the genetic and epigenetic resources generated in E. curvula to study the reproductive mode changed its status from an orphan to a well-characterised species.

Keywords: cytoembryology; ovule development; reproduction.

Figure 1

Scheme and safranin-fast green-stained sections of E. curvula embryo sac development in (A) sexual and (B) diplosporous apomictic (Eragrostis type) plants. Note in (B) the lack of meiosis during megasporogenesis, the absence of one round of mitosis in megagametogenesis, the fertilization of the polar nucleus (pseudogamy), and the formation of the embryo without fertilization (parthenogenesis). The scale bar equals 10 µm. (A) Sexual pathway (i) megaspore mother cell (MMC), (ii) meiosis I showing the dyad (D), (iii) meiosis II with the four cells in a linear tetrad, (iv) functional megaspore (FM) and degenerated megaspores (DM), (v) binucleated embryo sac (BEC) derived from mitosis I, (vi) tetranucleated embryo sac (TEC) from mitosis II, and (vii) embryo sac with antipodal cells (AN), polar nuclei (PN), egg cell (EGG), and one out of two synergid cells (SY). (B) Eragrostis type diplosporous apomixis, (viii) elongated megaspore mother cell (EMMC), (ix) binucleated embryo sac (BES) derived from mitosis I with nuclei at the micropylar pole, (x) tetranucleated embryo sac (TES) from mitosis II with nuclei at the micropylar pole, and (xi) embryo sac with polar nucleus (PN), egg cell (EGG), and synergid cells (SY).

Figure 2

Percentage of apomictic embryo sacs in apomictic F1 hybrids individuals of the mapping population coming from the cross OTA-S (sex) × Don Walter (apo).

Figure 3

Region linked to apomixis in E. curvula (Victoria cv.) with four linked GBS-SNPs markers (blue label) and two mapped QTLs (green label).

Figure 4

Hypothetical model of the quantitative regulation of diplosporous apomixis mediated by ROS1. Green circles attached to the genes represent methylated positions over putative genes involved in the sexual pathway. This situation is observed when plants grow under normal conditions and ROS1 is repressed or de-methylated. Blue ovals represent ROS1 de-methylating genes involved in the sexual pathway when plants are exposed to stressful conditions. As a consequence, the sexual ratio increases.