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Good timing: the key to good breeding
Apomixis (setting seed without a conventional sexual union) is receiving more attention as a tool for plant breeders. The generation of an elite genotype followed by induction of apomixis is a clear route to the maintenance of that genotype in succeeding generations. However, apomixis is also a barrier to crop improvement by conventional breeding, as in Paspalum notatum (Bahiagrass) for example, an important forage grass in Central and South America. Many races of P. notatum are tetraploid and obligately apomictic, making outcrossing impossible. The particular form of apomixis involves the development of a 2n embryo sac (apospory) followed by fertilization of the central nuclei to form the endosperm (pseudogamy). However, some tetraploid lines are facultatively apomictic and an individual plant may set seed both apomictically and sexually. The `choice' between the sexual and the nonsexual routes has been investigated by Argentinian collaborators (Espinoza et al., pp. 165-170). They used conventional breeding methods with a facultatively apomictic line, applying the heterologous pollen at different times before and after anthesis in the potentially female parent (taking care to avoid self-fertilization after anthesis). The genetic makeup of the resulting embryos was determined by RAPDs (a form of DNA finger-printing). The results show that early fertilization leads to the formation of apomictic embryos while fertilization at anthesis favours the formation of sexually derived embryos. This happens because the aposporous embryo sacs develop faster than the meiotic sacs and thus are available earlier. However, after anthesis, for reasons that are not understood, the balance tips back strongly towards apomictic reproduction. The adaptive significance of all this is not clear but the implications for plant breeders are: `...it is possible to manipulate the...pollination of facultative apomictic plants to obtain progenies with variable representation of maternal genotypes'. Sex or apomixis: its all in the timing.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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