Apomixis in plants: structural and functional aspects of diplospory inPoa nemoralis andP. palustris

Abstract

Summary. Data on structural and functional aspects of mitotic diplospory and later stages of apomictic seed formation are reported for Poa palustris and P. nemoralis. In this study, the plant material of two Russian populations of P. nemoralis and P. palustris were used for transmission electron microscope observations. Seed formation was investigated by phase contrast microscopy in two populations of P. nemoralis collected in The Netherlands. The processes of transformation of the megasporocytes to the megaspore mother cells of diplosporous embryo sacs, and thereafter to one-and two-nucleate diplosporous embryo sacs (Antennaria type) were characterized by an increase of cell size, structural and functional reorganization of the nucleus, nucleolus, and cytoplasm, and cell isolation as a result of thickening of the cell wall. These were accompanied by an increase in the cell metabolic activity inferred from visual evidence of the activation of nucleus, nucleolus, endoplasmic reticulum, dictyosomes, mitochondria, and from the appearance of a dense population of ribosomes and polysomes. The diplosporous embryo sac of the Antennaria type was characteristic for both P. nemoralis and P. palustris. No signs of the presence of synaptonemal complexes were observed during the process of diplosporous-embryo-sac megaspore mother cell differentiation and division. About 90-95% of the diploid egg cells of diplosporous embryo sacs were able to produce apomictic embryos. These embryos developed before anthesis. However, many of them degenerated at the globular stage because of lack of endosperm. The ultrastructural events occurring during the process of diplospory of apomictic species, and meiosis and megagametogenesis of sexually reproduced plants are discussed. Abbreviations: DMC megaspore mother cell of diplosporous embryo sac; TEM transmission electron microscopy; ER endoplasmic reticulum.

Understanding Apomixis: Recent Advances and Remaining Conundrums

“…Of the native monocot apomictic systems under study, most work focuses on aposporous plus pseudogamous species in the genera Panicum (Savidan, 1980;Chen et al, 1999), Pennisetum and Cenchrus (Roche et al, 1999), Brachiaria (Pessino et al, 1997(Pessino et al, , 1998, Paspalum , and Poa (Naumova et al, 1999;Albertini et al, 2001aAlbertini et al, , 2001b) and the diplosporous plus pseudogamous genus Tripsacum (Grimanelli et al, 1998a(Grimanelli et al, , 1998bSokolov et al, 1998). The eudicot genera under study include the daisy genera Taraxacum and Erigeron (diplosporous plus autonomous) (Noyes and Rieseberg, 2000;van Dijk et al, 2003) and Hieracium (aposporous plus autonomous) (Koltunow et al, 1998.…”

“…Some have proposed that the optimum expression of apomixis may be achieved only in conjunction with a polyploid genome . Because rare diploid gametophytic apomicts have been reported (Asker and Jerling, 1992;Bicknell, 1997;Kojima and Nagato, 1997;Naumova et al, 1999;Koltunow et al, 2000), polyploidy does not appear to be absolutely required for the expression of the apomixis. In these examples, however, asexual seed formation often was poor, so polyploidy may enhance the expression of apomixis in many systems rather than ensuring its presence per se.…”

The Development of the Female Gametophyte in Fragaria x Ananassa Duch. cv. Selva

“…Ultrastructural changes during the process of diplospory were described in Poa nemoralis and Poa palustris (Naumova et al, 1999). Despite changes in metabolic activity, the transformation of the megasporocyte to produce a one-nucleate diplosporous female gametophyte and its division to produce a two-nucleate stage was accompanied by an increase in cell size and reorganization of the nuclei and cytoplasm.…”

Apomixis for crop improvement

“…It also occurs in Mangifera spp., Malus spp., Ribes spp., and Beta spp. (see Nogler 1984, Naumova 1993. In Citrus spp., sexual and apomictic processes occur concurrently within the same ovule, and the apomictic embryo development from the nucellus is morphologically similar, if not identical, to the stages of sexually derived embryo development.…”

“…Unfortunately, it is found mostly in distantly related polyploid wild species, and only a few agricultural crops such as Citrus spp. and tropical forage grasses possess this trait (Willemse and Naumova 1992, Naumova 1993, Koltunow 1993, Khush et al 1994. Despite many years of breeding research, apomixis has not yet been exploited in commercial breeding programmes, and its genetic regulation is poorly understood.…”