Academic journal article Genetics

The Genetic Control of Apomixis: Asexual Seed Formation

Academic journal article Genetics

The Genetic Control of Apomixis: Asexual Seed Formation

Article excerpt

ABSTRACT Apomixis (asexual seed formation) is the result of a plant gaining the ability to bypass the most fundamental aspects of sexual reproduction: meiosis and fertilization. Without the need for male fertilization, the resulting seed germinates a plant that develops as a maternal clone. This dramatic shift in reproductive process has been documented in many flowering plant species, although no major seed crops have been shown to be capable of apomixis. The ability to generate maternal clones and therefore rapidly fix desirable genotypes in crop species could accelerate agricultural breeding strategies. The potential of apomixis as a next-generation breeding technology has contributed to increasing interest in the mechanisms controlling apomixis. In this review, we discuss the progress made toward understanding the genetic and molecular control of apomixis. Research is currently focused on two fronts. One aims to identify and characterize genes causing apomixis in apomictic species that have been developed as model species. The other aims to engineer or switch the sexual seed formation pathway in non-apomictic species, to one that mimics apomixis. Here we describe the major apomictic mechanisms and update knowledge concerning the loci that control them, in addition to presenting candidate genes that may be used as tools for switching the sexual pathway to an apomictic mode of reproduction in crops.

WITHIN flowering plants (angiosperms), reproduction through seeds occurs by sexual and surprisingly, by asexual pathways. The former generates variation, while asexual seed reproduction (apomixis) produces genetically identical progeny. (See Table 1 for definitions.) The ability to produce genetically identical progeny via seed is of sig- nificant value to agriculture for its power to fix complex favorable genotypes, including the higher yields often found in F1 hybrids. Apomixis is documented in more than 120 angiosperm genera (Carman 1997); however, no major seed crop species are apomictic, and attempts to introduce the apomixis trait to crops from apomictic relatives by cross- pollination have been largely unsuccessful (Savidan 2000). Apomixis is also of interest from developmental and evolu- tionary perspectives. Most natural apomicts are classified as facultative because they have the ability to reproduce both sexually or asexually. To understand the different pathways of apomictic seed formation, it is instructive to first examine the sequence of events required for sexual seed formation.

Sexual Seed Formation in Flowering Plants

Seed formation in angiosperms begins with the develop- mental decision to switch from a vegetative to a reproductive mode of growth. This manifests with the formation of a flower containing male (anther) and female (ovule) reproductive organs, and within these organs, diploid gamete precursor cells form (Figure 1). Many diploid male gamete precursor cells differentiate in the anther of the flower; however within the ovule, which is the progenitor of the seed, a single female gamete precursor cell differentiates. This cell is called a mega- spore mother cell. Both male and female gamete precursor cells undergo meiosis followed by mitosis to form the mature multicellular male and female gamete-containing structures, known as the pollen grain and embryo sac, respectively. Figure 1 illustrates the events leading to the formation of the most common form of embryo sac, called the Polygonum- type, which is found in .70% of flowering plants. The diploid megaspore mother cell undergoes meiosis, forming four hap- loid megaspores. One of these megaspores is selected to be- come the functional megaspore and the others degenerate. The haploid functional megaspore then undergoes three mi- totic divisions without cytokinesis, to produce a coenocyte containing eight haploid nuclei surrounded by a cell wall (stage not shown in Figure 1) (reviewed by Drews and Koltunow 2011). The nuclei migrate and cellularize, producing a seven- celled mature embryo sac. …

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