Abstract
Apomixis, the asexual reproduction via seed, has many potential applications for plant breeding by maintaining desirable genotypes over generations. Since most major crops do not express natural apomixis, it is useful to understand the origin and maintenance of apomixis in natural plant systems. Here, we review the state of knowledge on origin, establishment and maintenance of natural apomixis. Many studies suggest that hybridization, either on diploid or polyploid cytotypes, is a major trigger for the formation of unreduced female gametophytes, which represents the first step toward apomixis, and must be combined to parthenogenesis, the development of an unfertilized egg cell. Nevertheless, fertilization of endosperm is still needed for most apomictic plants. Coupling of these three steps appears to be a major constraint for shifts to natural apomixis. Adventitious embryony is another developmental pathway toward apomixis. Establishment of a newly arisen apomictic lineage is often fostered by side-effects of polyploidy. Polyploidy creates an immediate reproductive barrier against the diploid parental and progenitor populations; it can cause a breakdown of genetic self-incompatibility (SI) systems which is needed to establish self-fertility of pseudogamous apomictic lineages; and finally, polyploidy could indirectly help to establish an apomictic cytotype in a novel ecological niche by increasing adaptive potentials of the plants. This step may be followed by a phase of diversification and range expansion, mostly described as geographical parthenogenesis. The utilization of apomixis in crops must consider the potential risks of pollen transfer and introgression into sexual crop fields, which might be overcome by using pollen-sterile or cleistogamous variants. Another risk is the escape into natural vegetation and potential invasiveness of apomictic plants which needs careful management and consideration of ecological conditions.
Highlights
Sexuality is well entrenched in all seed producing plants
When pollen is not essential for endosperm formation, as it is the case in most Asteraceae, a triploid, pollen-sterile, highly obligate apomictic lineage would rapidly become established by selection against the sexual pathway (Figure 2c). This scenario is confirmed by the occurrence of different natural populations of triploid apomicts showing autonomous endosperm development in Erigeron (Noyes and Rieseberg, 2000), Hieracium (Bicknell et al, 2000), Taraxacum (Tas and van Dijk, 1999), and by a mathematical model for origins of 3x Taraxacum clones (Muralidhar and Haig, 2017)
Developmental trait expected to have an enormous impact in plant breeding if introduced into main crops, both shortening the time required to develop a new variety and to increase revenues
Summary
Sexuality is well entrenched in all seed producing plants. Seeds are an integral part of diaspores that enhance plant dispersals and store all nutrients needed to start the new generation. Uncoupled activation of apomeiosis and parthenogenesis in a diploid cytotype would drive an increase in ploidy and a shift in dosage that can help to stabilize the coordinated expression of apomixis elements and the formation of a number of polyploid individuals producing clonal seeds In natural conditions, this mostly happens through a triploid intermediary that facilitates the formation of even polyploids, like in sexual systems. When pollen is not essential for endosperm formation, as it is the case in most Asteraceae, a triploid, pollen-sterile, highly obligate apomictic lineage would rapidly become established by selection against the sexual pathway (Figure 2c) This scenario is confirmed by the occurrence of different natural populations of triploid apomicts showing autonomous endosperm development in Erigeron (Noyes and Rieseberg, 2000), Hieracium (Bicknell et al, 2000), Taraxacum (Tas and van Dijk, 1999), and by a mathematical model for origins of 3x Taraxacum clones (Muralidhar and Haig, 2017). Apomictic pathways, as well as the population dynamics at local and regional scales can help us to decipher the strategies plants use to respond and adapt to the environment, but it provides valuable information to use on apomictic crop management and production practices
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