After successful fertilization, the life cycle is complete with the production of new fern fronds. Ferns reproduce asexually by their modified stems, which are called rhizomes. Rhizomes spread just above or below the soil surface where they form roots on their undersides and new plants above.
Some ferns have clumping forms and others have spreading habits, but both kinds reproduce by their rhizomes. You can divide ferns by severing rhizome sections that contain roots and shoots and planting new ferns. Some ferns form new plantlets, called offsets, on their fronds. This is a form of asexual reproduction called vegetative propagation.
In nature, Monrovia notes that the plantlets fall to the soil where their roots secure them. You can clip the tiny plantlets from the mother plant after they form roots and start new plants. Equal parts of loam, peat moss and finely crushed terracotta spread to a depth of 2cm over a base of vermiculite also forms an excellent germination base. Once sown the containers should be covered with plastic or glass allowing some airspace and kept at around 20 degrees C in indirect light.
Spores take from 2 to 6 weeks to germinate. After a few weeks the germinating spores appear as a mossy growth. When the prothalli are formed and well developed they may be pricked off into a punnet containing a finely sifted soil mixture.
The container should be covered with glass or plastic until the fronds appear. The developing ferns should not be exposed to direct light. The hermaphrodite consists of a single sheet of cells with a distinct multicellular meristem that forms a meristem notch and multiple archegonia that develop adjacent to the meristem notch, which are highlighted in the SEM boxed area of the hermaphrodite.
The male lacks a meristem and almost all cells differentiate as antheridia. The SEM shows six antheridia, each having a ring cell and a cap cell that pops open to release sperm. When a male gametophyte is transferred to media lacking A CE , some cells divide and begin to form a hermaphroditic prothallus. Typical of other ferns, a C. The lateral meristem not only confers indeterminate growth to the gametophyte, but its formation coincides with a loss in ability to respond to A CE as well as the secretion of A CE.
Archegonia invariably initiate close to the meristem notch of the hermaphrodite, well after the lateral meristem is well developed. While the hermaphroditic program of expression cannot be reversed, the male program of expression is reversible.
Cells of the male gametophyte prothallus, when transferred to media lacking A CE , will divide to ultimately form one or more new hermaphroditic prothalli Figure 2E. Antheridiogen thus serves multiple functions in male gametophyte development: it represses divisions of the prothallus that establish the lateral meristem; it promotes the rapid differentiation of antheridia; it represses its own biosynthesis; and it serves to maintain in the gametophyte an ability to respond to itself.
All of the antheridiogens that have been structurally characterized from ferns are gibberellins GAs Yamane et al. Although the structure of ACE is unknown, GA biosynthetic inhibitors reduce the proportion of males in a population of C. Most recent studies aimed at understanding how antheridiogen determines the sex of the gametophyte have focused on two species of homosporous ferns: C.
Ceratopteris richardii is a semi-tropical, annual species and is useful as a genetic system for many reasons. Large numbers of single-celled, haploid spores typically 10 6 can be mutagenized and mutants identified within 2 weeks after mutagenesis. Gametophytes can be dissected and regrown, making it possible to simultaneously self-fertilize and out-cross a single mutant gametophyte.
Because C. Over 70 mutants affecting sex determination have been characterized, most falling into three major phenotypic groups: the hermaphroditic her mutants, which are hermaphroditic in the presence or absence of A CE , the transformer tra mutants, which are male in the presence or absence of A CE , and the feminization fem mutants, which are female in the presence or absence of A CE and produce no antheridia.
Through test of epistasis i. This pathway reveals that there are two major regulators of sex: TRA , which is necessary for lateral meristem and archegonia development female traits , and FEM , which is necessary for antheridia development the male trait.
TRA promotes the development of a gametophyte with female traits and represses the development of antheridia by repressing the FEM gene that promotes male development.
What is remarkable about this pathway is that it is inherently flexible, which is consistent with what is understood about sex determination in this species by A CE. Figure 3. The SD pathway in C. T bars represent repressive events whereas arrows indicate activating events. While this model explains how male and female gametophyte identities are determined, it does not explain the hermaphrodite. One possibility is that in certain cells of the hermaphrodite, the activities of FEM and TRA are reversed, allowing FEM to be expressed in cells that will eventually differentiate as antheridia.
Testing this and other possibilities will require the cloning of the sex-determining genes and assessing their temporal and spatial patterns of expression in the developing hermaphrodite. The sex-determining pathway in C. Based on the similarities between the GA signaling pathway in angiosperms and the sex determination pathway in C. Lygodium japonicum is another homosporous fern species with an antheridiogen response.
This species has the distinct advantage of having its antheridiogens structurally well characterized. Two different GAs have been identified as antheridiogens in this species, including GA 9 methyl ester Yamane et al. GA 73 methyl ester is the most active antheridiogen and is able to induce antheridia formation at the incredibly low concentration of 10 —15 M. To test the hypothesis that antheridiogen is synthesized through the GA biosynthetic pathway, L.
Their expression patterns revealed that all but GA30ox were more highly expressed in older gametophytes that secrete antheridiogen, consistent with the expectation that antheridiogen biosynthesis genes are up-regulated in gametophytes that secrete it. GA3ox expression showed the opposite pattern of expression; i. To explore this further, the same authors assayed the effects of prohexadione, a GA3ox inhibitor, on antheridia formation in the presence of GA 4 which has an OH group at the C3 position or GA 9 methyl ester which lacks the OH group at C3 ; both GA 9 and GA 4 induce antheridia formation by themselves.
Whereas prohexadione plus GA 9 methyl ester inhibited antheridia formation, prohexadione plus GA 4 did not, demonstrating that C3 hydroxylation of antheridiogen is essential for inducing antheridia formation. In another series of experiments, the authors found that GA 9 methyl ester was converted to GA 9 in young gametophytes. Based on these and other results, a model was proposed whereby antheridiogen GA 9 methyl ester is synthesized via a GA biosynthetic pathway and secreted by older gametophytes.
When it is taken up by younger gametophytes, the methyl ester is removed by a possible methyl esterase then hydroxylated at the C3 position by GA3ox to GA 4 , where it is perceived and transduced by the GA signaling pathway in young gametophyte.
Because GA 9 methyl ester is more hydrophobic and more efficiently taken up by gametophytes than GA 9 , splitting the GA biosynthetic pathway between young and older gametophytes was proposed to enhance the sensitivity of young gametophytes to the secreted antheridiogen by their neighbors and, at the same time, promote the activation of male traits once inside the young gametophyte Tanaka et al. In addition to characterizing antheridiogen biosynthesis in L.
They found that a L. All told, the results of these experiments were used to define a model of the antheridiogen response in L. The elucidation of the antheridiogen biosynthetic and signaling pathways in ferns has only just begun and many questions regarding sex determination and sexual reproduction remain, many of which can be resolved by cloning all of the sex determining genes.
Some of these questions are: To what extent are other hormones involved in sex determination? Is the split GA biosynthetic pathway in L.
What is the relationship between the antheridiogen response in the gametophyte to GA responses in the sporophyte? Knowing that some mutations in C. Is antheridiogen also involved in the developmental decision to produce mega- and micro-sporangia in heterosporous ferns?
From an evolutionary perspective, was the antheridiogen signaling and responses in the gametophyte co-opted during or important for the evolution of heterospory from homospory in ferns? Addressing these and other questions will lead to a more comprehensive understanding of sex determination in ferns, including an understanding of the molecular mechanisms at play.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Banks, J. Sex-determining genes in the homosporous fern Ceratopteris. Development , — Sex determination in the fern Ceratopteris. Trends Plant Sci. Genetics , — Chun, P. Inheritance of two mutations conferring glyphosate tolerance in the fern Ceratopteris richardii.
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