The Effects of Sugars and Ethylene on Apospory and Regeneration in <i>Ceratopteris richardii</i>

In land plants, two distinct generations, gametophyte and sporophyte, alternate to complete the life cycle. Sporophytes undergo meiosis to produce spores, from which gametophytes develop. Gametophytes produce gametes, which participate in fertilization to produce the zygote, the first cell of the sporophyte generation. In addition to this sexual reproduction pathway, some fern species can undergo apospory or apogamy, processes that bypass meiosis or fertilization, respectively, to alternate between the two generations without changing the chromosome number. Apospory is inducible in the laboratory in various fern species simply by altering the sugar level in the media. In sporophytes induced to undergo apospory, sporophyte regeneration is also observed. The ratio of aposporous gametophytes to regenerated sporophytes varies, in a manner consistent with being dependent on sugar level. Whereas the sugar signaling pathway is yet to be elucidated in lower plants, in angiosperms it has been shown to play a regulatory role in controlling essential processes including flowering and embryo development, which give rise to the gametophyte and the next sporophyte generation, respectively. Here, we present evidence for the role of different sugar levels on the balance of apospory and regeneration in the fern Ceratopteris richardii. The demonstration of crosstalk between sugar signaling and the hormone ethylene signaling in angiosperms prompted us to test the effects of this hormone in combination with sugar on apospory vs. regeneration. These results provide insight into how a group of redifferentiating cells determines which generation to become and lay the groundwork for further analysis of this asexual pathway.

Embryological Studies on Facultative Apomixis in Apluda mutica (Gramineae)

A cytological study on Apluda mutica L. revealed that this species was a facultative apomict with 60.74% of apomictic embryo sac formation. Its sexual reproduction exhibited the characteristics of polygonum_type embryo sac formation. After the 3 megaspores at the micropylar end had degenerated, leaving only the chalazal megaspore in the nucellus, a nucellar cell adjacent to the megaspore differentiated into an aposporous initial, which later developed into a panicum_type embryo sac. It was an aposporous 4_nucleate embryo sac without chromosome reduction and composed of an egg, a single synergid and a central cell with 2 polar nuclei. The chalazal megaspore degenerated before the onset of the division in the aposporous initial cell.

Facultative Apomixis in an Endangered Dioecious Species, Woonyoungia septentrionalis (Magnoliaceae)

The breeding system of an endangered dioecious species, Woonyoungia septentrionalis (Dandy) Law, was studied in two natural populations, the Mulun population and the Dahuangni population, in Guangxi Province, China from June 2001 to early September 2002. The female flowers from previously bagged buds were treated respectively with paper bags, net bags and hand pollination by comparing with natural pollination. The results showed that female flowers treated with paper bags, net bags, hand pollination, and natural pollination all set fruits and seeds, but there was great difference in their fruit set, seed set, pre-emergent reproductive success (PERS) and seed germination rate for two populations. In both populations, PERS of hand pollination and natural pollination were higher than those of paper bagged and net bagged treatments. Among them, PERS of hand pollination was the highest, and PERS of net bagged treatment was the lowest. Both sexual reproduction and apomixis occurred simultaneously in W septentrionalis and seeds from both sexual and apomixis could germinate into seedlings. Therefore, the breeding system of W septentrionalis belonged to facultative apomixis. The apomixis in Magnoliaceae was reported for the first time.