In ferns, apomixis, also called apogamy, describes a common kind of asexual reproduction involving the production of unreduced spores (diplospory) and the development of a sporophyte from somatic cells of gametophyte (prothallium) without the involvement of any reproductive organs (apogamy). In short, two critical steps in the life cycle of ferns are transformed (Manton 1950). Firstly, spores are formed without meiotic reduction from diploid to haploid. Secondly, the next sporophytic generation is formed without fertilization. As a result, both generations of the fern, gametophyte and sporophyte, have the same ploidy level (Manton 1950). Sexual recombination is completely absent in the life cycle of the majority of apomictic ferns because no functional reproductive organs are formed (Manton 1950).

The term apogamy has been frequently used to describe apomixis in ferns. Lovis (1977) provide compelling arguments that apogamy is a special form of apomixis.

Two pathways of apomixis have been described for ferns. Both pathway concerning the mechanism to obtain diplospores. The Doepp-Manton pathway (Doepp 1939; Manton 1950) is the more common one and apomictic ferns of this pathway appear to lack any kinds of sexual organs. The Braithwaite pathway (Braithwaite 1964) was described for apomicitc ferns of the Asplenium aethiopicum complex. Apomictic ferns showing the Braithwaire pathway appear to be able to generate functional antheridia.

Here, we focus on natural occurrences of apomixis in ferns, meaning apomixis in wild plants. However, apomicitc reproduction can be induced to many sexual ferns under laboratory conditions. Despite the importance of these results, we focus here on natural occurrence only.

The loss of sexual reproduction in land plants, such as angiosperms and ferns, is paradoxical. For example, apomixis is highly unevenly distributed among plants with apomictic species contribution about 0.1% of the current angiosperm diversity but up to 10% of fern species. Still lower percentages were estimated for gymnosperms, mosses and liverworts (Walker 1966; Lovis 1977; Asker and Jerling 1992; Mogie 1992; Pichot et al. 2001; Park and Kato 2003). In addition, apomixis is also unevenly distributed among fern lineages. Among ferns, two derived fern families, Dryopteridaceae and Pteridacaeae, contribute together about 70% of reported apomictic fern species but other species rich families, Polypodiaceae and Thelypteridaceae, comprise only a few apomicitic species.

Several characters are considered as indicators for apomixis in ferns. Of course, the final prove relies on observations confirming the development of sporophytes without fertilization (Huang et al. 2011). In general, reduced number of spores per sporangium is considered as an indicator because diploid spores do not undergo a regular meiosis (Doepp 1939; Manton 1950; Braithwaite 1964; Lovis 1977). In derived ferns, the waste majority of sexual reproducing ferns form 64 spores per sporangium, whereas apomictic ferns show reduced numbers with 32, 16 or 8 spores per sporangium. However, the reduced number of spores per sporangium can be also the result of other processes such as the reduction of spore mother cells. Another indicator is the occurrence of odd chromosome numbers. Ferns with triploid or pentaploid ploidy levels are either sterile or reproduce via apomixis. However, diploid apomictic ferns are known.

As pointed out above, apomicitc ferns possess a life cycle deviate from the life cycle of fern in two critical characters. For comparison, we recapitulate the life cycle of a sexual fern.

all other land plants, ferns possess a life cycle comprising two alternate phases. In sexually reproducing ferns, the haploid gametophyte phase alternates with the diploid sporophyte phase of development. The meiotic cell devisions (Meiosis = reduction of the chromosome number from diploid to haploid) happen in the sporangium. This structure is formed by the sporophyte and comprises diploid spore wall cells, tapetum cells, as well as the spore-mother cells. These spore-mother cells, usually eigth in a derived ferns, undergo one mitotic and the two meiotic cell divisions. The resulting 64 spores are haploid. These spores germinate (usually) after dispersal and form the haploid gametophyte, also called prothallium. The prothallium houses the sexual organs, the antheridia forming the spermatozoids, and the archegonia holding the egg cell. The fertilization process happens via the merging of the spermatozoids and the egg cell within the archegonium. The newly formed diploid zygote develops to a multicellular diploid embryo, which in turn will develop into the sporophyte.

Acknowledgement of images used in this website

The image of the leaf (Cyrtogonellum caducum) was created by Dr. Hong-Mei Liu during fieldwork in northern Vietnam. The images of the gametophytes were kindly shared by Mr. Robert Dyer (Natural History Museum, London), they show different stages of apomictic outgrowth in gametophytes of Asplenium monanthes.

Acknowledgement for this website

Mr. Wei Cao (Copunter Network Information Center, CAS) supported the design of the website by providing technical support. We thank Ms. Zhen Meng (Computer Network Information Center, CAS) and Mr. Ri-Hong Jiang (Guangxi Institute of Botany, CAS) for discussions on the contents and/or display of this website.