Introduction:
Asclepiadaceae includes about 250 genera and over 2000 species. Wide-spread in tropical and subtropical regions, especially in Africa and Southern South America, with a moderate representation in northern and South Eastern Asia. Members of the family Asclepiadaceae are unique due to the association of pollen grains that form a sac-like definite structure called pollinia (Singular: Pollinium). They are the product of only one anther, but are transferred during pollination as a single unit. This is also seen in orchids. The pollinarium of most Asclepiadaceae is composed of two or more pollinia, each of which contains all of the microspores of a single anther locule embedded in a hard matrix and a translator apparatus, which develops from a stigmatic secretion and mechanically attach the pollinia to a pollinator (Corry, 1883a; Schill and Jakel, 1978; Kunze, 1994; Swarupanandan et al., 1996). Pollinial wall is acetolysis resistant (Namboodiri and Sreedevi, 1980; Erdtman, 1952).
The plants of the family Asclepiadaceae is unique among the dicotyledons in having a pollination mechanism closely related to that which makes the Orchidaceae unique among the monocotyledons. In both the pollen is removed in masses by insect, but in Periplocoidae the pollen is in tetrads, falls naturally out of the anthers into or on to special structure (spoon like translator) which may be removed from the flowers on the heads or bodies of visiting insects. In the larger sub group the Cynanchoideae, the stamens and style are combined into a fused structure called gynostagium (some time spelt gynostegium) and pollen is aggregated into more definite pollinia.
These pollinia become jointed in pairs of structures called pollinarium which make it possible for them to be removed from the flowers on the legs or mouth parts of visiting insects.
It is general practice to use the term “Pollen” and “Spare” as synonyms.Literature abounds with discussions on the distinction between spores and pollen and there are divergent views on the homology of the two. In spore studies particularly, there is disagreement on usage of the terms “microspore and megaspore”. In the botanical sense, the microspore is a reproductive body which germinates to form the male gametophyte, where as megaspore germinates to form a female gametophyte. A pollen grain can be said to be germinated microspore of the seed plants. The term “Polospore” has been introduced into Palynology to include pollen and spores.
The study of morphology of living organism is one of oldest branch of science. Strictly morphology means no more than the study of form or structure, for the study of structure alone would be dull and useless indeed, but the aim should be to see each living organism in its relationship to other living beings. Arber (1950) was also in favour of this view and painted out that the aim should be of wider interest. The study of pollen grains and spores is of importance in medicines (allergy and immunology), genetics, biochemistry, agriculture, taxonomy, evolution, agriculture physiology, fossil botany, oil palynology and tissue culture etc.
Pollen is a male partner in the fertilization process of spermatophyte. Pollen grains are every where in the air we breathe, in water we drink, in the food we eat and in the soil we walk. No wonder that study of pollen has led to pollination ecology, ecotoxicology, adulteration control, palynology. Biochemistry and genetic engineering; pollen biology as direct relevance in agriculture, horticulture, forestry, plant breeding and biotechnology.
The study of pollen morphology has a four century old history, it is only during the last two decades with the publication of book “Pollen morphology and Plant Taxonomy” by Erdtman (1952) then it has gained importance in comparative morphology and taxonomy.
The word Palynology was coined in 1944 by two British scientists Hyde (Botanist) and Williams (Medical Doctor, Allergologist) in Cardiff. The word is derived from Greek Polynein = to scatter meal or powder i.e. morphology of pollen spores and their dispersal. The history of palynology is strongly associated with development of microscope, Hooke’s microscope and with help of similar microscopes. Grew and Malpighi managed to depict pollen grains. Grew’s Botanical work consists of one volume, “The Anatomy of Plants” (1682) wherein morphology of stamens shape, etc. of pollen grains are described. He compared the grains with cheesemites, mustard seeds etc. for reader’s imagination. Almost contemporarily with Grew (1682), Malpighi (1685) published his work “Opera-Omnia”, where he described the almost atomic globulets in the stamens.
The early history of pollen morphology, pollinations etc. from time of Assyrians (800 BC) to the twentieth century is described by Wodehouse (1935, 1965). Care Hugh Fishcher is widely accepted as the founder of pollen morphology from the modern view point, although only his late nineteenth century work dealt with pollen.
From 1940, palynology is strongly associated with three researchers Scandinavians. G. Erdtman (Sweden), R. Faegri (Norway) and I. Iversen (Denmark). Faegri and Iverson focused on pollen analysis and its implication in vegetation history, archeology, and pollination ecology. Erdtman became more and more oriented towards pollen morphology and plant taxonomy.
Erdtman (1943) described an introduction to pollen followed by a series of text book. His subsequent contributions have been numerous and today he stands as a major figure in world palynology. A fair degree of knowledge on the architecture of pollen wall has been presented and its value to taxonomy and palynology has been demonstrated by Wodehouse (1935). The founder of modern pollen analysis was the Swede Lennart Von Port (1884-1951).
The beginning of pollen morphological study started with the publication of “Pollen grains” by Wodehouse (1935) to which may be associated the publication ‘‘Pollen Morphology and Plant Taxonomy – Angiosperms’ by Erdtman (1952). In 1830, the first successful use of pollen characters in plant classification appears to be that of John Lindley in his genera and species of Orchidaceous plants. The study of pollen morphology in India started by various workers (Nair, 1960; Srivastava, 1976). Importance of this aspect was initiated with monographic work on pollen morphology of Acanthaceae by Bhaduri (1948). Later, Sayeeduddin et al. (1949) published their findings on the flora of Hyderabad. In addition, there are series of papers on pollen grains of Indian plants by Nair et al. (1960, 1974a) . Erdtman has also touched upon several Indian plants in the exhausted account of the pollen morphology of Angiosperms. Programmed in the study of pollen has been received and presented in the book ‘Pollen morphology of Angiosperms: A historical and phylogenetic study by Nair (1970a) in India. The application of pollen morphology in plant taxomomy and phylogeny has been demonstrated at various taxonomic levels e.g. Nair (1970b) in a series of communication led the principles and methods for applying pollen morphology in phylogenetic treaties and also enunciated the triphylatic theory of phylogeny of plant kingdom. The evolutionary process in plants lies in the sex unit, called the spore and the pollen and what is reflected in them represents a fair degree, the general trends of plant evolutions (Nair, 1974a).
Pollination in the plants of family Asclepiadaceae compares some of the most complex floral mechanisms in angiosperms. The floral structure and mode of pollination in Asclepiadaceae have been studied by Koelreuter and Sprengel in the eighteenth century and Brown, Delphino, Hildebrand, Corry and Mullar in the nineteenth century (Galil and Zeroni, 1965). Re-examination of floral morphology of Secamone sp. and Asclepias sp. by Safwat (1862) and Galil and Zeroni (1965, 1969), respectively, have dealt with the functional and evolutionary significance of the extremely complex floral modifications in the Asclepiadaceae.
Pollen grains of the members of the family Asclepiadaceae are produced in paired packages known as Pollinia. These pollinia are removed from the flower by the pollinating insects. Quantitative studies of the reproductive biology of Asclepias were attempted in 1940’s, when potential commercial uses of milkweeds were investigated (Stevens, 1945; Moore, 1946a,b, 1947; Sparrow and Parson, 1948).
Frost (1965), Macior (1965) have shown that species of Asclepias may differentially attach pollinia on various parts of pollinators body and have suggested that this type of mechanical isolating mechanism may play a role in speciation in the genus.
Analysis of nectar production and pollination behaviour was done for several species of Asclepias (Morre, 1947, 1982; Galil and Zeroni, 1965;Wilson et al., 1979; Bertin and Willson, 1980; Kephart et al., 1980; Fritz and Morse, 1981; Southwick, 1983; Southwick and Southwick, 1983).
Nectarines of Asclepias and Vincetoxicum was studied by Galil and Zeroni (1969), Chirst and Schnephf (1985),while extrafloral nectarines of Calotropis and Wattakaka by Arekal and Ramakrishna (1980).Pollinial morphology is taxonomically significant like the pollen morphology of other angiosperm. The size and shape of pollinial sac, nature of corpusculum, position of pollinia, structure of caudicle or translator, etc. are important features for analysis of phylogenetic study. These characters are also required for efficiency of pollinia attachment to the pollinator during pollination. The species diversification through the pollinial structure is established in this study. The pollination efficiency of pollinia is well studied by different authors (Wyatt, 1976, 1978; Broyles and Wyatt, 1993). Pollen morphology of Asclepiadaceae has been surveyed and summarized by Brown (1810b), Corry (1883a), Schill and Jakel (1978), Swarupanandan et al. (1996), Civeyrel et al. (1998) Verhoeven and Venter (1998b, 2001), Sajith and Sreedevi (2005).
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