Alternations of Generation in Pteridophytes
Term alternation of generation was used by Hofneister (1851). He observed two types of morphologically distinct individual in the life cycle of mosses and fern in the vascular plants the sporophyte individual is a complicated, independent and a dominant generation, whereas the gametophyte is comparatively much reduced. The gametophyte in the homosporous forms, though inconspicuous and comparatively shortlived, is independent and may be surface living and green (autophytic) or subterranean and non-green (lycopodium). They are always exosporic and thus are not enclosed by the spore wall. They are also infected by an endophytic or a mycorrhizic fungus in some genera (Lycopodium, Psilotum and Tmesipteris). The heterosporous land plants, surprisingly enough, display a considerable reduction in their gametophytes. The gametophytes are reduced and endosporic. They have, as a rule, separate male and female prothalli. The male prothallus is extremely reduced and represented ony by a single prothallial cell. The female prothallus, on the contrary, is well developed because it has to nourish the developing embryo. Such a dioecism is unknown in homosporous land plants, but is a regular feature in the heterosporous ferns.
Origin of Alternation of Generations in Pteridophytes
Several theories have been proposed explain the origin of alternation of generation which are follows-
Interpolation or Antithetic Theory
It was put forth by Bower and later supported by Overton (1893), Scott (1896), Strasburger (1897), Williams (1904) and others. Bower discussed it in detail in 1935. The supporters of this theory believe that the gametophytes of the early land plants had an algal origin. They presumed that these early land plants migrated from amphibious situations; (where water was available in plenty for effecting sexual reproduction), to terrestrial habitats (where waster was available only during rainy seasons and extensive dew). Under such circumstances these early land plants could not depend entirely upon the sexual process for propagation and had to adopt another method. This was accomplished by the development of zygote into embryo and ultimately into a diploid individual called the sporophyte. Development of the sporophyte involved certain changes that can be listed below:
- Development of jacketed sex organs especially the archegonium.
- The retention of the fertilised egg within the archegonium that could provides it with sufficient food.
- The zygote or the fertilised egg divided mitotically and not meiotically to form a diploid embryo, which in primitive land plants distinguished into a sterile jacket enclosing a number of diploid cells or spore mother cells.
- The diploid spore mother cells underwent meiosis to form tetrads of haploid spores. Such a primitive sporophyte is exemplified by Riccia.
- Gradually the primitive sporophytes underwent progessive sterilisation and gave rise to complex sporophytes of liverworts like Marchantia, Anthoceros and Mosses, The sporophytes were dependent wholly or partially upon the gametophytes for food.
- Enhanced sterilisation of the sporophyte led to the development of an independent sporophyte that developed vascular tissue and possessed root, stem and leaves. These sporophytes also bore sporangia that produced spores and disseminated them efficiently.
The events of apogamy and apospory were dismissed by Bower as ‘ex post facto’ events. He stated, ‘They illustrate the potentialites of plants of present day rather than evolutionary features of the remote past, and will have no part in producing that normal alternation which is characteristic of the Archegoniate.
Homologous or Transformation Theory
This theory was put forth by Pringsheim (1876-77) and has a large number of supporters. Stebbins (1960) has also supported it. The proponents of this theory claim that the sporophyte is a modification of the gametophyte. They do not believe progressive sterilisation to be the cause of origin of photosynthetic tissues in the sporophyte. They do not regard the sporophyte and gametophyte individuals to be as different as postulated by the adherents of the antithetic theory. According to this theory the primitive land plants exhibited isomorphic alternations i.e., they had free living sporophytes and gametophytes that resembled each other in all respects except their genetic constitution and functions. They derived support from the following events:
- Existence of isomorphic alternation of generations in the algae e.g., Cladophora, Ulva, Ectocarpus and Dictyota. Both the generations are photosynthetic.
- Presence of Chloroplasts in the sporophytes of some bryophytes.
- Presence of tracheids in the gametophytes of some lower cryptogams e.g., Psilotum.
- Occurrence of phenomenon of apospory and apogamy. These deviations from the normal life cycle suggest that either generation can arise as an outgrowth from the other. These phenomena lend support to the view that sporophyte can be a modification of the gametophyte.
The supporters of this theory further believe that the free living sporophyte later became attached to the gametophyte and partially dependent upon it. Such a step leads to the reduction in complexity of the sporophyte as in the bryophytes. Further reduction leads to the origin of simple and completely dependent sporophytes of Riccia. This permanent retention of the sporophyte on the gametophyte is considered to be responsible for the development of morphological differences in the two generations and led to the development of heteromorphic alternation of generations in the bryophytes. In the vascular cryptogams also the embryo is retained within the gametophyte.
Modern evidence provides more support from the homologous origin of alternation of generations (Stebbins, 1960) but even if we accept this theory we have still to account for the morphological differences between the sporophyte and the gametophyte. There must be certain factors which initiate the development patterns responsible for the distinct morphological characteristic of the sporophyte and the gametophyte. Two hypotheses have been advanced to explain these difference.
Lang introduced his hypothesis in the year 1909. He emphasized that the spore and the zygote, which are the first cells of the gametophyte and the sporophyte respectively develop under different environmental conditions. The spores after dissemination are free from the influence of the sporophyte and are freely exposed to external environments. The zygote, on the other hand, is retained within the venter of the archegonium and is not free from the influence of the surrounding gametophytic tissue. It is, therefore, subject to various restraints and other influences. Lang considered that the different conditions during early development of the two pioneer structures of the alternating generations may be sufficient to account for the differences between them. This hypothesis has recently been put to experimental tests and some favourable results have been achieved. To test the validity of Lang’s hypothesis, Bell (1959) argued that if this were true, then by interchanging the position of spore and zygote, it should be possible to change their sequence of development. It was not possible to produce a sporophyte from the spore, but surgical experiments on zygote under reduced or complete absence of archegonial restraint, yielded results contrary to the hypothesis. Ward and Wetmore (1954) and Jayasekera and Bell (1959) were successful in reducing the archegonial restraint on the zygote by cutting away archegonial tissue. De Maggio and Wetmore (1961) dissected embryos of various ages or even the zygotes from the archegonial venter and were successful in germinating them under experimental conditions free from the influence of the gametophyte. In most of the these experiments the freed tissue underwent abnor.nal development, but ultimately was able to produce a sporophyte. It, however, shows that gametophytic influences are significant in the development of the sporophyte, but not necessary. In Todea barbara the freed zygotes produced small thalloid structures of gametophytic nature, but it was not possible to achieve the formation of sex organs on these thallus-like structures. The older embryos, however, produced normal sporophytes.
Blackman (1909) attached no importance to the difference environmental conditions to which the zygote and spore are prone. He expressed the opinion that differences in the chromosome numbers and other intrinsic differences between spore and the zygote are responsible for morphological differences in the structures produced by their germination. Bell (1963) made cytochemical and ultrastructural studies on the fern egg cell and concluded that there are many cytochemical and cytological differences between the spore and egg cell.
It appears that the views expressed in both the hypotheses are partially correct. Environmental, cytoplasmic, nutritional and hormonal and genetic factors coupled together appear be responsible for the distinct developmental differences between the two generations. None of these hypotheses have taken into consideration the effect of organic and inorganic nutrition and of hormones on the developmental pattern of the spore and the zygote. Recent experimental data collected on a number of ferns indicate the importance of hormonal and nutritional factors in controlling the development of the zygote. Bistow (1962) experimented with the callus tissue derived from the sporophyte of Pteris cretica. He found that the callus remained undifferentiated on a medium containing sucrose and high concentration of auxin. In a medium containing sucrose and high concentration of auxin., In a medium containing sucrose along the callus developed into a sporophyte. If the callus is grown in a medium containing only mineral nutrients it differentiated into a gametophyte. Whittier (1962) and Whittier and Steeves (1960,1962) could induce the apogamous production of sporophytes from a number of othwerwise normal fern gametophytes by the incorporation of suitable sugar concentrations in the culture medium. Wetmore et al (1963) demonstrated that when prothalli of Onoclea, Osmunda and Todea are planted erect on a medium containing one per cent sucrose, cylindrical and sturdy growths, containing a vascular strand of xylem elements are produced. De Maggio (1964) was able to induce the formation of sporophytic buds and roots on the prothalli of Lycopodium obscurum grown in a culture medium containing coconut milk and sucrose. These experiments reveal the importance of nutritional factors in determining the morphological differences between the two alternating generations of the vascular cryptogams.
The studies on tissue culture and the effect of sugar or lack of sugar on the development of sporophytic or gametophytic structures from calli lead us to postulate two hypotheses:
(A) The Gene Block Hypothesis which postulates the presence of 4 gene blocks in fern systems and any one of them can be stimulated into action. These gene blocks control the formation of various organs in fern sporophytes and gametophytes. A gene block controlling the formation of root, leaf or gametophyte can be stimulated into action in any cell whether it is of a sporophyte or that of a gametophyte. Every gene block has a master gene for that block and it is necessary to activate this master gene in order to trigger into action any sub-division of a gene block.
(B) The second hypothesis concerns the phenomenon of alternation of generations. These studies lead us to disbelieve the Antithetic and Homologous theories of alternation of generations; but instead confirm our belief in a third theory called the Genetic theory of alternation of generations. This theory postulates that a change in the genetic system in the early progenitors of land plants brought into being two distinct and independent individuals in the life cycle of the land plants. These individuals may be identical or dissimilar in appearance. Bell (1970) opined that the totality of evidence leads us to perceive the life cycle as an integrated sequence of events, each key event coded by a specific set of the genome. No one questions the action of genes in the organistion of a living system but what is not clear is the location of gene switches. Smith (1979) is of the opinion that certain metabolic changes may be responsible for determining the developmental pathways during the germination of spore, rather than specific gene action. One of the major mysteries in cell biology is the missing link between the macromolecules and visible structural organisation.
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- Sexual Reproduction in Rhizopus stolonifer
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