Tripsacum dactyloides (Poaceae): a natural model system to study parthenogenesis

Diplosporous apomeiosis, formation of unre- duced embryo sacs primarily of the Antennaria type, fol- lowed by parthenogenetic embryo development and pseudogamy (fertilization of the central cell) describe gametophytic apomixis within the Tripsacum agamic complex. Tripsacum dactyloides (Eastern gamagrass) is a close relative of domesticated maize and was chosen as a natural model system to investigate gene expression patterns associated with parthenogenesis. The genome size of diploid sexual and polyploid apomictic T. dac- tyloides was estimated by flow cytometry to be 7.37 pg (2C), 14.74 pg (4C) and 22.39 pg (6C), respectively. The diploid genome size is thus approximately 1.35× larger than that of maize. The apomeiotic-pseudogamous path- way of seed formation was demonstrated at a rate of 92% by the flow cytometric seed screen (FCSS) with single mature seeds in tetraploid accessions. This num- ber includes twin embryos which were detected in 13% of the seeds analyzed. Fertilization of unreduced egg cells (B III hybrids) was measured in 10% of apomictic seeds. Autonomous (fertilization-independent) embryo development and fertilization-dependent endosperm for- mation were confirmed by pollination of tetraploid T. dactyloides with a diploid transgenic maize line carry- ing an actin::β-glucuronidase (GUS) reporter construct. GUS expression was detected after pollination in the de- veloping endosperm, but not in the embryo. In similar intraspecific crossing experiments with maize, GUS ex- pression was detected in both the embryo and endo- sperm. A protocol was established for microdissection of embryo sacs and early parthenogenetic embryos of T. dactyloides. Together, these techniques provide new tools for future studies aimed at comparing gene expres- sion patterns between sexual maize and sexual or apo- mictic T. dactyloides.