In this article we will discuss about the life cycle of bryophytes with the help of diagrams.
The life cycle of a bryophyte involves an alternation of generations—the gametophyte and sporophyte, which are different in morphology (heteromorphic) and differ in their level of ploidy (heteroploid). The gametophyte or gamete-producing generation is distinct in its morphology and is haploid, with a single set of genome.
The sporophyte or spore- producing generation is morphologically different from gametophyte and is diploid. The sporophyte undergoes meiosis to form haploid spores, which germinate to produce haploid gametophytes.
The gametophytes produce male and female gametes, antherozoid and egg that combine sexually to develop into a sporophyte. In this way, the life cycle is completed with an alternation of generations, and associated duplication and reduction.
It was ascertained quite early that in a life cycle there is not only an alternation between two morphologies but also an alternation in the levels of ploidy. However, what remains to be ascertained is: how and when the gametophyte and sporophyte generations and an alternation of these heteromorphic generations became operational in the life-cycle of land plants?
Origin of Gametophyte Generation:
A consideration of major events in the evolution of plant life is likely to provide some indication about the origin of gametophyte generation. Plant life is presumed to have originated in water and in the process of evolution from an independent free-living cell, the fundamental unit of structure and function, the first advance was the origin of sex.
The next advance was, probably, evolution of a multicellular structure, the gametophyte, with a division of labour amongst its cells.
Origin of Sporophyte Generation:
Two theories attempting to explain the origin of sporophyte generation, which have received considerable attention in the past are: homologous and antithetic.
The homologous theory also referred to as modification or transformation theory, maintains that sporophyte is not a new structure but is to be regarded merely a modification of the gametophyte under different developmental conditions.
Supporters of this theory hold that sporophyte is to be interpreted as a neutral generation with the primary function of spore production. Instances of isomorphic alternation between gamete-producing and spore-producing generations in algae are a strong evidence in support of this theory.
This theory essentially implies simultaneous and successful migration to land by two separate algal generations, something which is highly impossible. Also, the two generations in algae and land plants are analogous but not homologous.
Opposed to it is the antithetic theory also referred to as intercalation or interpolation theory. This theory maintains that sporophyte is a new and progressively a vegetative structure elaborated and interpolated in the life cycle, in connection with the migration of plants from water to land. This theory has been interpreted differently.
The proponent and some exponents have tried to explain the origin of sporophyte of land plants from green algae which already had alternation of generations and were of diplobiontic type; having two multicellular generations, gametophyte as well as sporophyte. However, this interpretation fails to explain as to how a free-living sporophyte generation became attached to or became parasitic on the gametophyte generation.
Another interpretation of this theory is that the sporophyte originated as a new component of life-cycle from algae which lacked the sporophyte. In other words, land plants originated from alga/e that was/were haploid and haplobiontic; having single multicellular generation.
Further, it has been proposed that a delay in zygotic meiosis could have led to the origin of a multicellular sporophyte. In this way, it is easy to explain sporophyte-gametophyte association, and dependence of sporophyte on the gametophyte.
The controversy concerning interpretation of alternation of generations attracted more attention when the phenomena of apogamy and apospory were discovered. Apogamy is the direct formation of a sporophyte from the gametophyte, without the intervention of gametic union. Apospory is the direct formation of a gametophyte from the sporophyte without the intervention of spore formation.
In view of these alternate pathways in the life cycle—apogamy and apospory—the idea of interpolation is untenable. Thus, it is possible to have an alternation of morphologies without an alternation in the levels of ploidy. In bryophytes, the phenomena of apogamy and apospory are of rare occurrence under natural conditions. Hence, their significance is limited in the normal life-cycle.
Nonetheles, these alternate pathways of life-cycle (Apogamy and Apospory) are of interest because:
(a) Haploid, diploid and polyploid generations are possible for a comparative study.
(b) Processes of differentiation either of a gametophyte or a sporophyte can be followed at cellular level.
(c) Direct derivation of one generation from the other is likely to provide a better understanding of normal life-cycle and of alternation of generations.
Thus, the account of alternation of generations in the life-cycle of land plants is to be reviewed in terms of the phenomena of apogamy, apospory, and multiple pathways of cellular expression. These are accounted here, one by one.
Apogamy:
Formation of a sporophyte from the vegetative tissues of a gametophyte, without the intervention of gametes and their union is apogamy (apo = without, gamy = marriage).
The first record of apogamy in bryophytes is in a moss, Phascum cuspidatum. In naturally-occurring diploid gametophytes of this moss swellings could be seen on leaf tips which spontaneously formed apogamous sporophytes. These sporophytes, however, remained sterile, i.e., failed to form spores.
On culture, these diploid gametophytes with swellings on leaf tips, showed a variable response. In moist conditions was seen regeneration of protonema, whereas under drier conditions or increased salt concentrations these swellings developed into sporophytes. Reduced hydration or drying of medium (by increased level of agar) promoted the apogamous response.
Induction of apogamy was possible in diplophase of Georgia pellucida when protonema developed on regeneration of a sporophyte formed structures which developed into sporophytes, bypassing the gametophytic stage. Also, on regeneration, the young sporophytes of Physcomitrium pyriforme formed sporophytes instead of protonema or gametophytes.
In studies on regeneration of sporophytes from different strains of Funarid hygrometrica it was found that sporophytes from haploid (n=14) and in vitro-induced diploid strain gave rise to protonema, whereas those from naturally-occurring strain (n=28) could be induced to develop both sporophytes and protonema.
This led to the conclusion that only tissues which are spontaneous diploid are capable of forming apogamous sporophytes. Following these reports, apogamous sporophytes have also been possible on diploid protonema as well as diploid gametophytes of Desmatodon spp. (Lazarenko, 1960), Pottia intermedia Amblystegium spp. and Brachythecium.
Apogamy in haplophase has been possible in D. randii the sporophytes developed directly on gametophyte axis (Fig. 5.1). Since then apogamy (Fig. 5.2A, B) in haplophase has been reported in Physcomitrium coorgense, Pottia intermedia Funaria hygrometrica, and Physcomitrium pyriforme.
Structure and Maturity of Apogamous Sporophytes:
The apogamous sporophytes are generally club-shaped or pear-shaped structures lacking a foot and without demarcation into seta and capsule. However, in pear-shaped sporophytes the globular region of isodiametric cells with dense contents, can be referred to as capsule and lower stalk region with elongated cells can be termed as seta. The apical region of apogamous sporophyte in F. hygrometrica resembles an operculum, but without any tissue differentiation.
The abnormal shape of apogamous sporophytes points .towards the controlling effect of calyptra in sporophyte development in mosses. This is strengthened by the finding that some changes in capsule size and shape were recorded in apogamous sporophyte of Amblystegium spp. when calyptras from some other species were placed on them.
In apogamous sporophytes originating from diploid tissues one often finds spore production. However, apogamous sporophytes developing from haploid tissue are sterile. It is very likely that the fertility is linked with duplication of genome. The apogamous sporophytes of Physcomitrium coorgense a highly polyploid form, formed spores in about 50% of the cultures.
Origin of Apogamous Sporophytes:
Apogamous sporophytes originate from an apical cell with two-cutting faces, this is contrary to the origin of a gametophyte from an apical cell with three-cutting faces on a linearly growing 1-D system or protonema. The apical cell in bryophytes is, however, a labile system; it can produce protonema, gametophyte as well as sporophyte.
It has been proposed that an apical cell with three-cutting faces passes through an apical cell with two-cutting faces. If conditions are conducive for apogamy, the apical cell with two- cutting faces gets stabilized. It results in the formation of apogamous sporophytes, whereas if conditions are conducive for gametophytic growth one finds the formation of gametophytes. This brings us to a discussion on conditions or factors favouring apogamy.
Factors Favouring Apogamy:
A number of factors have been found to affect the apogamous response. These can be broadly classified into endogenous and exogenous.
A. Endogenous Factors:
In an account of endogenous factors one finds references from genetic constitution to physiological state of the tissue, and finally to the elaboration of a factor which is conducive to apogamy.
1. Genetic Constitution:
There is no record of apogamy in liverworts, either spontaneous or induced. It remains to be seen whether it is due to low incidence of polyploidy in the group. Contrary to it, apogamy is not an unusual feature in mosses, it is of spontaneous occurrence as well as can be induced. Spontaneous occurrence of apogamy is on record in diploid Phascum cuspidatum.
As for induction, to begin with, it was proposed that apogamy in Funaria hygrometrica is preceded by diploidization of genome since spontaneous diploids alone formed apogamous sporophytes. Induction of apogamy has been possible with relative ease in polyploid systems, Physcomitrium coorgense and P. pyriforme with n = 51, and n = 72 chromosomes, respectively. From this, it can be concluded that an increase in gene dosage is conducive to apogamy. This is also supported by obligate occurrence of apogamy in ferns which are polyploid.
Apogamy in haplophase has been reported in Desmatodon randii. This species is, however, allopolyploid. The role of hybridization and consequent genetic imbalance is to be investigated in apogamy.
The role of mutation in inducing apogamy needs to be given a due consideration. Apogamy in haplophase of Desmatodon randii was possible due to the occurrence of a mutant shoot, which on vegetative propagation continued to produce apogamous sporophytes.
In this respect a parallel can be drawn with apogamy in haplophase of F. hygrometrica. In aseptic cultures of this moss, raised from spores, were seen gametophytes bearing apogamous sporophytes. The gametophytes on subculture continued to produce apogamous sporophyte (Fig. 5.2A, B).
2. Sporogon Factor:
Of the reports on apogamy, more interesting is the regeneration of protonema from sporophytes of P. pyriforme as well as hybrid sporophytes; arising out of reciprocal cross between F. hygrometrica x P. pyriforme . The regenerant protonema produced apogamous sporophytes as long as it was in continuity with the parent sporophyte. In absence of continuation only gametophytes appeared on the protonema.
This finding led the author to postulate that there is “sporogon factor” elaborated by parent sporophyte, which is responsible for new sporophytes. Another interesting finding was that it is possible to have a cluster of apogamous sporophytes, without an intervening protonema, provided the sporophyte taken for regeneration is young, and there is drier condition due to increased agar concentration.
In an extension of this work it was found that sporophytes of P. pyriforme and hybrid sporophytes of F. hygrometrica x P. pyriforme form a callus tissue which differentiates into apogamous sporophytes. A follow up revealed that the age of hybrid sporophyte determined the nature of regenerants (Fig. 5.3). From a mature sporophyte only protonema was possible whereas an embryonic sporophyte yielded variable results.
The extreme apex proliferated into apolar mass of cells (callus). A portion from the zone lower than this formed seta-like structures and protonema only up to some extent. The segments from the lowermost zone formed only protonema and the tissue in between the lowermost and upper two zones showed a transitional behaviour; it produced protonema which developed apogamous sporophytes, instead of gametophytes.
Hormonal nature of sporogon factor became evident when bryokinin (an adenine derivative) isolated from callus tissue, originating from hybrid sporophyte was found to promote apogamous response in Splachnum ovatum. A new evidence in support of hormonal nature of sporogon factor is the finding that red light promotes synthesis of bryokinin and also promotes apogamy.
There is also an evidence for a factor for apogamy in gametophytic cultures of F. hygrometrica which are responsive to apogamy. The response increases with increase in the level of sucrose. At low level of sucrose the gametophytes multiply but without an apogamous response.
Similarly, in gametophytic cultures of P. pyriforme the factor accumulates and results in apogamous sporophyte formation. A high intensity of light and absence of sucrose inhibit apogamous response.
An old sporophyte always regenerates to form a protonema, it is irrespective of its different parts or tissues. On the protonema in due course differentiate shoot-buds. By contrast, a young sporophyte reveals a differential regeneration; tissue from apical region regenerates to form a callus, tissue from meristematic region directly regenerates to form apogamous sporophytes; tissue from extension region regenerates to form protonema on which appear apogamous sporophytes; tissue from differentiated region always regenerates to form protonema on which appear shoot-buds (Fig. 5.3).
B. Exogenous Factors:
Different factors affecting apogamous response in different systems, are reduced hydration, increased carbohydrate level, low light intensity, presence of chloral hydrate and hormones in the nutrient medium.
Reduced hydration of medium, in terms of increased agar concentration, favoured apogamy in Phascum cuspidatum, Georgia pellucida, Physcomitrium pyriforme, Desmatodon ucrainicus, and Splachnum ovatum. On transfer of gametophytes of Splachnum sphaericum to very dry culture medium the male and female gametangia transform into sporogon-like structures. Also in Funaria hygrometrica drying of cultures was a crucial factor favouring apogamous response.
Enhanced carbohydrate level is a prerequisite for apogamous response. Apogamous sporophytes fail to appear in a sugar-free medium on the gametophytes of P. coorgense, F. hygrometrica and P. pyriforme. In F. hygrometrica an increase in level of sucrose from 1 to 4% promoted apogamous response.
It has been proposed that sucrose exercises some sort of hormonal control over the production of a factor for apogamy, possibly by interacting with endogenous substance/s.
Light also affects apogamous response. In gametophytic cultures of Physcomitrium coorgense there is formation of gametophytes as well as apogamous sporophytes in diffused light, whereas in the dark, only sporophytes are formed. It remains to be seen whether apogamous response is influenced by quality of light.
The reduced apogamous response of Phascum cuspidatum in daylight could be greatly increased in yellow light. However, in Physcomitrium pyriforme high intensity of light is inhibitory and low intensity daylight (complete spectrum) is promotory for the response.
Growth hormones at low concentrations are promotory for apogamous response. The effect, however, cannot be described as specific. Indole acetic acid at low level promotes apogamy in Georgia pellucida. This is also true of F. hygrometrica where IAA, kinetin and GA3 individually promote the response. A combination of IAA and kinetin appreciably promotes the response but a combination of IAA and GA3 is inhibitory.
Of the other substances promoting apogamous response a mention may be made of chloral hydrate. This compound significantly promoted apogamous sporophyte formation in spontaneously responding apogamous strain of Splachnum luteum. Also, the protonema regenerating from sporophytes of F. hygrometrica x P. pyriforme fails to produce apogamous sporophytes if it is isolated from the parent sporophyte. However, it can be made to do so on culture to a medium containing chloral hydrate.
Apospory:
Formation of a gametophyte on regeneration of sporophyte, without the formation of spores, is apospory. A regenerant originating from a sporophyte is diploid and hence, it is of interest in raising diploid and polyploid lines. However, there are not many successful reports of regeneration of sporophytes in bryophytes. It appears that sporophytic cells are relatively refractory to regeneration.
Wounding is an essential requirement for regeneration of seta. There is also an evidence of polarity and apical dominance in regeneration of setae of some mosses. A low frequency regeneration was possible from seta segment of Physcomitrium with attached capsule, whereas high-frequency regeneration was possible from seta segments without capsule. On culture of seta most of the cells die out, but some cells of epidermis and parenchyma remain alive.
These give rise to either a protonema/germ tube or rarely to a callus mass. Tissue degeneration is a prerequisite for regeneration. When a large number of cells die it disrupts cell communication. Consequently, the surviving cells act as individuals and regenerate. On culture of seta of Blasia pusilla the regeneration was possible on sucrose-free medium. In this respect, apospory is reverse of apogamy.
Multiple Pathways of Cellular Expression:
Multiple modes of cellular regeneration from different regions of an embryonic sporophyte of the hybrid F. hygrometrica x P. pyriforme have already been mentioned (Fig. 5.3) in this account, and need not be repeated. Of interest in this context is comparative account of regeneration of gametophytes of Funaria hygrometrica and apogamous sporophytes developing on these gametophytes.
The gametophyte and apogamous sporophytes although of a similar genome, differ in their regeneration response. The gametophytes readily regenerate on basal medium without any age specificity but fail to form a callus.
A young sporophyte on the other hand, readily formed a callus tissue on a medium with an increased carbohydrate level. Cytokinin in the medium promoted callusing as well as differentiation of new sporophytes. The growth of callus tissue on isolation from parent sporophyte could be supported on medium containing coconut milk and it differentiated into numerous sporophytes (Fig. 5.2C, D).
Under cultural conditions it is possible to evoke multiple pathways of cellular expression. The tissue is not constrained by its genetic constitution for growth and organization. So far, this account has been concerned with various modes of regeneration from an organized tissue, either a gametophyte or sporophyte.
However, more revealing is regeneration of an unorganized tissue (callus) and control of its differentiation, at will, into either a gametophyte or a sporophyte. Multiple pathways of cellular expression have a bearing on alternation of generations.
Callus formation occurs on medium containing 2% sucrose from protonema of P. coorgense. This is also true of P. pyriforme. The origin of callus on protonemal cells could be traced to the differentiation of a small intercalary cell, amongst the elongated cells. This intercalary cell divided irregularly to form a callus. These cells of callus behaved like zygote by differentiating into sporophytes. The differentiation of P. pyriforme callus cells could be controlled (Fig. 5.4).
In high intensity of light the callus differentiated predominantly into gametophytes whereas differentiation in low intensity light was predominantly sporophytic. Differentiation of sporophytes was always direct, whereas differentiation into gametophytes was always indirect, through the formation of protonema.
Furthermore, when the callus of P. pyriforme was sub-cultured on medium containing cytokinin it differentiated into sporophytes with the virtual exclusion of gametophytes.
A New Theory on Alternation of Generations:
A new theory that can account for deviations like apogamy and apospory, maintains that two generations are genetically similar but have different genetic drives to different responses. Basically, the question that needs to be answered is: how can one and the same genome produce plants of two diverse morphologies?
Differential Behaviour of Germ-Cells and Alternation of Generations:
In the life cycle of lower land plants, (bryophytes and pteridophytes) two germ cells—spore and zygote—initiate two dissimilar phases. The differential behaviour of these germs cells results in alternation of generations. While the products are two dissimilar phases, gametophyte and sporophyte, there is little direct evidence to account for the causal differences of these two generations.
Considerations for differential behaviour of cells, spore and zygote, have been engaging attention. Quite early, it was pointed out that one must look for answers in causal morphology rather than insist upon heredity or chromosome number. To be precise, it was pointed out that one should consider the range of possibilities, within which a cell or a plant can express itself, rather than insist upon the fixed routine of alternation of generations.
Further, it was suggested that either spore or zygote are inherently different structures which result in distinct developmental patterns, or the initials are similar but the environment (physical and chemical) in which they develop determines their response.
The spore germinates as a free-cell, whereas zygote develops within the confines of an archegonium. It was reasoned that in contrast to a freely germinating spore on a substrate away from the main plant, the zygote is within the confines of venter and neck of an archegonium.