Imprinting and Apomixis

Apomixis is defined as asexual reproduction through seed; consequently, the progeny of apomictic plants are genetic clones of the mother. Introducing apomixis into a desirable genetic background would allow its propagation while keeping its genetic composition intact, giving apomixis a great agronomical potential (Spillane et al. 2004). Apomictic plants have successfully overcome the fertilization-dependent barrier of embryo development. However, many apomicts require fertilization of the central cell, forming the triploid endosperm, implying that the 2m:1p ratio is crucial for successful endosperm development in these species (Koltunow and Grossniklaus 2003). Fertilization of an unreduced polar nucleus by a normally reduced sperm would result in an 'unbalanced' 4 m:1p genome ratio that is often lethal. In natural apomicts, two different strategies have been devised to prevent unbalanced genome ratios causing seed abortion: (1) alterations in either gameto-phyte development or the fertilization process, and (2) changes in the sensitivity towards unbalanced ratios of maternal to paternal genome ratios (Savidan 2000; Grossniklaus et al. 1998; Grossniklaus et al. 2001). Some species employ the first strategy: gametophyte development is altered such that the embryo sack contains four instead of eight cells,with a single,unreduced polar nucleus, and after fertilization the 2m:1p ratio is restored (Nogler 1984). Ranunculus auricomus uses an alternative mechanism,where the double fertilization process is modified, with both reduced sperm nuclei fusing with the unreduced polar nuclei, restoring a 2m:1p genome ratio (Rutishauser 1954). The second strategy is used in Tripsacum dactyloides and Paspalum spp.,where a single reduced sperm fertilizes the unreduced polar nucleus, giving rise to unbalanced genome ratios with no negative impacts on endosperm development (Grimanelli et al. 1997; Quarin 1999). Furthermore, in many apomictic species of the Asteraceae, autonomous endosperm development occurs in conjunction with autonomous embryo development, resulting in a 2 m:0p genome ratio in the endosperm. How natural apomicts manage to bypass the genome dosage sensitivity in the endosperm is a fascinating, but unresolved, question.

Another experimental approach to investigating apomixis utilizes the model plant Arabidopsis (Grossniklaus et al. 2001). In wild-type Arabidopsis plants, autonomous embryo and endosperm development does not occur. In mutants of the fis class,seed-like structures develop in the absence of fertilization (Chaudhury et al. 1997). However, embryo development does usually not occur, although a few embryo-like structures have been reported (Chaudhury et al. 1997; Kohler et al. 2003a), and endosperm development is abnormal, as it fails to cellularize. This aberration can be overcome when, e.g., the autonomous endosperm mutant fie is combined with a hypomethylated genome, allowing completion of endosperm development (Vinkenoog et al. 2000). This shows that the requirement of a paternal genome for successful endosperm development can be bypassed under certain conditions even in Arabidopsis.

0 0

Post a comment