Sexual reproduction in diploid angiosperms is characterized by double fertilization. After meiosis and the production of haploid sperm and egg cells within the anthers and ovules, respectively, two male sperm (released from a single pollen grain) enter the female gametophyte (embryo sac); one fuse with the egg cell to produce the diploid embryo, and the other fuses with two central cell nuclei to form the triploid endosperm tissue. In the female ovule, meiosis within the megaspore mother cell produces four haploid megaspores, three of which disintegrate soon thereafter and only one of which (the functional megaspore or “selected spore”) enlarges to form the embryo sac. During apomixis, meiosis is circumvented and an embryo develops in the absence of fertilization.
Apomixis in Hieracium initiates with aposporous embryo sac formation, meaning that an embryo sac structure develops from a somatic cell (aposporous initial cell) within the ovule, completely bypassing meiosis, and then both endosperm and embryo develop autonomously without fertilization. In other apomicts, the unreduced embryo sac is derived from the megaspore mother cell, but meiosis is disrupted in a variety of ways that result in an unreduced embryo sac (diplosporous apomixis). And in contrast to Hieracium, most apomicts (aposporous and diplosporous) are pseudogamous rather than autonomous, meaning that endosperm development (and seed viability) still requires fertilization of the central cell, even though embryo development proceeds without fertilization of the egg cell. Even within Hieracium, there is variation (between and within species) in the timing and cytological aspects of aposporous embryo sac formation.
Apomixis does not occur in Arabidopsis. Nonetheless, it is apparent that there is a great deal to be learned about the molecular mechanisms involved in apomixis from our best-characterized plant genetics model system. Postulated that genes isolated from Arabidopsis would be important for the study of apomixis, and this was the rationale for a genetic screen for Arabidopsis fertilization-independent seed (fis) mutants. The first is the use of reproductive molecular markers derived from Arabidopsis in an apomictic plant system with established transformation procedures (such as Hieracium), and the second is the use of comparative functional genetics to search for and investigate the function of homologs of Arabidopsis reproductive genes in other plant systems.
To examine the molecular relationships between sexual and apomictic pathways, Tucker et al. used fusion constructs of β-glucuronidase (GUS) with a variety of Arabidopsis genes that are associated with different aspects of sexual reproduction in this species. These markers were introduced into sexual and apomictic Hieracium plants, and their expression patterns were monitored during ovule and seed development. The same constructs also were introduced into Arabidopsis for a comparison with sexual Hieracium. The Arabidopsis marker sequences used were from sporocyteless nozzle, which is required for male and female sporogenesis in Arabidopsis. Overall, the results showed a remarkable conservation of expression patterns of reproductive marker genes in apomictic compared with sexual Hieracium plants during embryo and endosperm development. The major differences that were observed occurred early in ovule development, close to the point of divergence between sexual and apomictic processes.
(Department of Plant Breeding and Genetics)