Apogamy and Apospory in Pteridophytes

Apogamy and Apospory

(Abnormalities in the life cycle of Pteridophytes)

The normal life-cycle of a vascular plant has two alternating generations, these are the diploid sporophyte and the haploid gametophyte. Both alternate regularly in the life cycle and this alternation is brought by two significant steps known as the fertilisation and the meiosis. The regular alternation of chromosome number is sometimes impaired by the occurrence of two common phenomena known as apogamy and apospory. They will be detail with separately.

Apogamy

It was first reported by Farlow (1874) in Pteris cretica. Apogamy can be defined as the development of a sporophyte directly from the gametophyte without the intervention of sex organs and gametes. The sporophytes thus formed usually have the same chromosome number as the gametophyte i.e. haploid number for the species. Apogamy occurs in nature and has also been induced under experimental conditions. It is a common and a widespread phenomenon in the ferns. Natural apogamy has been reported in more than 50 species of ferns belonging to 20 genera. In some species of ferns apogamy appears to be a necessity and is a regular process. It is perhaps due to the inherited constitution of the plant. Natural apogamy is commonly known in Dryopteris, Pteris, Pellaea, Adiantum, Osmunda, Todea, Athyrium, Cheilanthes, Polystichum, Asplenium, etc. To this list can be added a number of genera and species in which apogamy has been experimentally induced. Cases of parthenogenetic development of the egg into the sporophyte are not included under apogamy, because apogamy is the development of vegetative tissue of the prothallus into the sporophytc. Apart from the ferns ogamy has also been induced in some species of Lycopodium and Equisetum. It has been estimated that 12 percent of the cytologically investigated homosporous ferns are apogamous.

Causes of Apogamy

Regarding the cause of apogamy, several explanation have been put forth. Lang (1898) induced the formation of sporophytic buds root, sporangia and tracheids in various fern prothalli by avoiding watering of the prothalli from above. Brown (1923) summarised literature regarding the induction of apogamy by avoiding fertilisation of the egg. Many workers regard failure of normal fertilisation as a cause of apogamous production of sporophytes. Mottier (1931), however, demonstrated that in Matteuccia struthiopteris failure of fertilisation does not induce apogamy. Brown (1923) induced apogamy in Phegopteris polypodioides by avoiding normal fertilisation. Other conditions favouring apogamy have also been suggested. These are: culture in bright light and at higher temperatures (Nathansohn, 1900); by lowering the vitality of the prothallus by fungal and algal attack: and failure of formation of functional sex organs under various unfavourable nutritional condition. Willams (1938) suggested that in addition to the environmental factors there must also be some internal factors such as the nature of inherent susceptibility due to abnormal nuclear composition and behaviour, that bring about apogamy.

Ageing of the prothallus has also been regared as one of the factors influencing apogamous development on the prothalli of some ferns. Recent work (Whittier and Steeves, 1960) on Osmunda, Adiantum and Pteridium has shown that apogamy can be induced by growing the prothalli on an agar culture medium rich in glucose. Wetmore and his associates (1963) demonstrated that when prothalli of Onoclea, Osmunda and Todea are planted erect on a medium containing one per cent sucrose, cylindrical and radially symmetrical growths with vascular strands are produced. De Maggio (1964) induced the formation of sporophytic buds on the prothalli of Lycopodium obscurum grown in culture media containing coconut milk and sucrose. These experiments reveal the effect of nutritional factors in inducing apogamy. Loyal and Chopra (1973) induced apogamy in Regnellidium diphyllum.

Cytology of Apogamy

Work during the last 15 or 20 year has revealed that one out of every 15 species of ferns has what is called the “apogamous life-cycle” in which both generations have the same chromosome number. The question now arises as to how this apogamous type of life-cycle is maintained cytologially. Two methods are known. According to one method, which is rare, both the processes of spore formation and fertilisation are eliminated from the life-cycle. The sporophyte or the fern plant produces a prothallus or the gametophyte which arises as a bud from the Ieaf and develops into a full-fledged prothallus. It is produced from the diploid tissue by mitosis and is therefore, dipoid in constitution. This diploid prothallus does not bear sex organs but instead gives rise to sporophytic buds that develop into diploid sporophytes. Such a method, therefore, involves the occurrence of both apospory and apogamy in the same plant. It is not a common method and has since been reported in Anthyrium filix foemina var. Clarissiana, Dryopteris filix- mass var. cristata apospora (Farmer and Digby, 1907); and in Trichomanes Kraussiana (Georgevitch, 1910). Sarbadhikari (1939) discovered it in Osmunda javanica. It has been induced in Pteris vittata by Palta.

The second method which is very common among the ferns involves the formation of spore mother cells that actually double their chromosome number by the fusion of daughter nuclei. During this method the number of spore mother cells is reduced. Normally the sporangia in the leptosporangiate ferns possess 16 diploid spore mother cells, but in apogamous ferns there are only 8 spore mother cells with their nuclei having double the chromosome number (tetraploid). They will undergo meiosis and produce 32 diploid spores. These spores will produce diploid gametophytes. So the number of chromosome in both the generations is same. These gametophytes will bear sporophytic buds that will develop into full-fledged diploid sporophytes. Sex organs are usually not produced by such gametophytes and there is thus no fertilisation. Such a method is known to occur in a large number of ferns and is by far the commonest.

In India it has been reported by Mehra (1944) in Adiantum lunulatum; and by Mehra and his students (see Mehra, 1961) in thirty species of Himalayan ferns (Pteris cretica, P. aspercaulis, P. blumeana, P. biaurita; Dryopteris paleacea, D. fibrillosa, D. odontoloma, D. attrata; Adiantum caudatum and others). Mehra (1961) has given a vivid description of the apogamous and aposporous Himalayan ferns and has discussed their bearing on speciation and phyletic relationship among the ferns.

Evans (1965) has reported an interesting case of apogamy in a species of Polypodium. In this case he reported the formation of 32 mitospores in the sporangia. The sporangia have 16 spore mother cells which do not undergo meiosis, but instead divide mitotically into 32 spore. These spores are reniform in shape and occur in diads rather than tetrads. He reported this sequence to occur regularly in all sporangia. The spores germinate readily to form prothalli which bear numerous apogamous sporophytic buds. The prothalli bear stomata but produce no ex organs. Virginia, M. Marzenti (1967) reported apogamy in Asplenium curtissii and A. plenum. In both the species the spore mother cells do not undergo reduction division and act directly as spores. These spores germinate to produce gametophytes that bear antheridia containing viable sperms, and also bear apogamous sporophytes.

A very interesting fern gametophyte (Gametophyta appalachina) was discovered by a bryologist A. J. Sharp from the Appalachin region of North America. It is a ribbon like and branching prothallus that reproduces mainly by means of marginal clusters of gemmae. It has not been seen to produce sporophytes under natural conditions. Wagner regards it as a species of Vittaria (shoe string fern) that has somehow lost its sporophyte during evolution. Stokey (1951) cultured these prothalli for three years and was able to induce the formation of small young sporophyte directly from the gametophyte. These apogamously produced sporophytes had certain diagnostic features of the Vittariaceae. Investigations on the cytological basis of apogamous life cycles in ferns enable us to learn a lot about speciation among ferns, and the evidence collected so far seems to indicate that majority of apogamous ferns arose by hybridisation or reticulate evolution.

Cytological studies

Cytological studies on pteridophytes were initiated after the First World War. The first pteridophyte whose chromosome number was studied was Psilotum triquetrum. Okabe (1929) found that the haploid chromosome number n was 52 while the polyploid of the same species had 104 chromosomes for the haploid state. Dopp (1938) published a review of the work done before the War. Manton (1950) pteridophytes in Britain and published his observation in a magnum opus “Problems of Cytology and Evolution of in Pteridophyte”. Manton’s researches stimulated world wide as a result of which cytological information is available for more than 2000 species and more is added everyday.

Chromosomes number of a few genera of pteridophytes is given below:

(i) Low chromosome number pteridophytes

Selaginella: n = 9,10

Osmunda: n = 22

Hymenophyllum: n = 13, 19

Isoetes: n = 10, 11

(ii) High chromosome number pteridophytes.

Psilotum: n = 52, 104

Lycopodium: n = 23 to 264

Tmesipteris : n =52

Equisetum : n=108

Ophioglossum eliminatum : n= 30

Ophioglossum reticulatum : n = 720.

Apospory

The phenomenon of apospory was recorded by Druery (1884) in a fern called Athyrium filix-femina var. clarissima. He observed the development of prothalli from the stalk of the sporangium as well as from the spore case of this fern. Later apospory was noticed in many ferns and some workers induced it under cultural conditions. During this phenomenon gametophytes or prothalli develop from the vegetative tissue of the sporophyte and not from the spores. Such prothalli are diploid i.e., they have the same chromosome number as the sporophyte. Normally the haploid spores germinate and give rise to the haploid gametophyte, but during apospory spores are not required. Apospory thus is the development gametophytes from the vegetative parts of the sporophyte without the intervention of spores. Bower reported it in two species of Trichomanes in 1885. In this case aposporous gametophytes were produced from soral regions of the leaf and from leaf tips. Later apospory was reported by several authors in many genera and species of ferns. Druery and Neurnberg (1938) and Steil (1939, 1951) reviewed the literature on apospory and apogamy in ferns. Bristow (1962)developed gametophytes from a callus tissue derived from the sporophyte of the fern Pteris cretica. This callus developed into gametophytes when grown in media containing only mineral nutrients. De Maggio and Wetmore (1961) isolated zygotes and undeveloped embryos from the archegonia of Todea barbara and cultured them. They were able to induce these isolated zygotes to develop into thalloid structures that resembled the gametophytes of this fern. Bell (1959) reviewed many cases of apospory in ferns and was tempted to state that the phenomenon of apospory must be general among ferns. Wetmore and De Maggio (1963) have also reviewed cases of apospory and apogamy among ferns.

Cytology and Artificial Induction of Apospory

In Osmunda javanica Sarbadhikari (1936) reported the formation of gametophytes from any vegetative part of the young or old sporophyte. He observed filamentous as well as heartshaped prothalli arising from the sporophyte. These prothalli borne antheridia containing spermatozoids but did not produce archegonia. Later apogamous sporophytes also developed from these aposporously produced gametophytes. Lawton (1936) recorded the presence of both antheridia and archegonia on the aposporously produced gametophytes of Osmunda regalis and Cystopteris fragilis. The gametophytes were diploid an so were the sex organs and the gametes. As a result of normal fertilisation tetraploid embryos and sporophytes were produced. Such tetrapoloid sporophytes have been induced to produce tetraploid aposporous gametophytes. This may lead to the formation of octaploid sporophytes. Lawton also observed cases where the diploid gametes of diploid aposporus gametophytes cross-fertilise with haploid gametes from haploid gametophyte and produce triploid sporophytes. Lawton (1932) also induced apospory in Aspidium marginale and Woodwardia virginica and produced tetraploid sporophytes. Manton (1932) induced apospory in Osmunda regalis and produced diploid, triploid and tetraploid gametophytes and sporophytes.

W.N. Steil (1944) induced the formation of aposporous gametophytes in the young leaves of apogamously produced sporophytes of Tectaria trifoliata. He was able to induce the formation of gametophytes from the margins of the young leaves and from the hair borne on the petiole of the first formed leaf of the sporophyte. In both the cases the gametophytes were produced in large numbers.

Goebel (1907) and Palta (1973) induced the formation of prothalli from the petiole and laminar surface of the pinnae in Pteris vittata. Goebel also observed the formation of thalloid structures bearing antheridia, rhizoids, stomata, and even vascular strands on the detached juvenile Ieaves of Alsophila van-geertii and Ceratopteris thalictroides. These thalloid structures resembled gametophytes in possessing antheridia and rhizoids and sporophytes in possessing stomata and vascular strands. He designated them as “Mittelbildungen”.

Development of tracheids and stomata are no longer regarded as indicative of aposporous or apogamous development, but are considered to have development under the influence of chemical environments under which the organs are cultured. So formation of stomata and tracheids do not  give these aposporously developed gametophytes the characteristics of sporophytes. They can be regarded as purely gametophytic structures because they bear antheridia and rhizoids.

Beyerle (1932) observed the formation of undifferentiated outgrowths on the isolated leaves of some ferns. He observed them to grow into prothalli in Anaemia densa, Pteris trenula, Anogramma leptophylla, Polypodium heracleum, Cibotium schiedei (leaf margin), Dicksonia fibrosa (leaf margin), and Drynaria heraclea (small and dying leaves).

Woronin (1908) reported the development of aposporous gametophytes on the attached primary leaves of Pellaea nivea. Koehler (1920) induced the formation of prothalli on the leaves of Platycerium bifuractum grown in dim light. Under strong light the leaves produced shoot buds.

Brown (1918) reported an interesting case of regeneration from the petioles of the leaves of Phegopteris polydioides. Under experimental conditions a cellular mass of green cells developed on the cultured petioles. It grew into a prothallus that borne rhizoids, true leaves, and structure intermediate between leaves and prothalli. It is a queer case of development of sporophytic and gametophytic structures together. Such instances clearly indicate that there is no inherent difference between the two alternating generations (sporophyte and gametophyte), and consequently no clear relation to the chromosome number.

Charles Morlang (1967) induced apospory in three species of  Asplenium (A. platyneuyron, A. rhizophyllum and A. montanum). He cultured the leaves cut from the sexually produced sporophytes of these ferns, under controlled conditions. The leaves were observed to produce two types of neoplastic growths. These were two dimensional growths and three dimensional growths. The former developed into a normal heart shaped prothallus that produced both types of sex organs that borne gametes. They had a diploid chromosome complement. The three dimensional growths developed into sporophytes.

Takahashi (1962) induced aposopory in Pteridium aquilinum latiusculum. He was able to induce the growth of gametophytes from petiolar epidermis from margins of pinnae from epidermal cells from mesophyll cell and from root epidermis.

Causes of Apospory

Several factors seem to influence the aposporous development gametophytes from vegetative tissues of the sporophyte:-

  1. Briston (1962) demonstrated that mineral nutrition is responsible for the formation of prothalli from callus tissue obtained from the sporophytes of Pteris cretica. When he supplied sucrose to such a callus tissue, it developed into a sporophyte.
  2. Goebel (1902) and Beyerle (1932) demonstrated that there is a pronounced relation betwee the stage of development of sporophytic cells (under culture) and the kind of organs regenerated. They observed that in Ceratopteris thalictroides aposporous gametophyte developed on decapitated young sporophytes with one or two leaves whereas in older sporophytes only shoot buds developed. Beyerle (1932) observed that in Davallia canariensis and in Nephrolepis biserata the prothalli develop on leaf tips and shoot buds at the basal and older parts of the leaf.
  3. In some ferns e.g., Drynaria rigidula, polypodium aureum and P. heracleum the leaves develop prothalli under dim light and sporophytic buds under strong light. Koehler (1920) also demonstrated that in Platycerium bifurcatium prothalli develop on leaves grown under dim light whereas the same leaves produce sporophytic buds on exposure to strong light.
  4. The work done in Punjab University Botany Laboratories (Mehra and Palta, 1969 to 1973) on tissue culture has revealed interesting results.

A few examples are being cited here. They obtained root callus tissue from the roots of a tetraploid Cyclosorus dentatus on Knudson’s medium +2% sucrose +2,4-D. The callus was fragile and its cell suspensions were obtained in sterile distilled water. The isolated cells of the callus were then placed on three different media:

  1. Knudson’s medium basal.
  2. Knudson’s medium + 1% sucrose,
  3. Knudson’s medium + 2% sucrose.

In A the root cells behaved as spores and germinated to give rise to a prothallus. This shows that under mineral nutrition the callus tissue is induced to form gametophytes because basal Knudson’s medium contains only minerals. In B the callus cells gave rise to structures that were partly gametophytes and partly sporophytes. They called such structures as Gameto-sporophytes. In C the callus cell gave rise to the complete sporophytes (regeneration).

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