The morphological diversification of pollinia of some members of Asclepiadaceae

Most of the natural systems of plant classification have been based on exomorphic characters. The flower is a primary reproductive character considered to be more important than any vegetative characters. The mass of pollen grains is basically called pollinia. Like pollen, the pollinium is also the key character for the identification of specific plant species belonging to the family Asclepiadaceae (Dicotyledons) and Orchidaceae (Monocotyledons). Pollinial characters are now being used as important taxonomical tool for reassessing the different types of plant groups. The morphological nature of pollinia is specific in each genus. The morphological diversifications of pollinium of different genera of Asclepiadaceae were studied with the help of light microscope and phase contrast microscope (Leica-DM1000). The shape, size, position, orientation of pollinia, translator attachment, furrow position, etc are important criterion for the studies of pollinial morphology. This study analyzed the pollinial morphology of some selected plant taxa like Calotropis gigantea (L.) Ait., Daemia extensa R.Br., Dregea volubilis Benth, Gymnema sylvestre R.Br, Hoya globulosa Hook.f. and Tylophora indica (Burm.f.) Merr. collected from different parts of West Bengal.


INTRODUCTION
Asclepiadaceae includes some 250 genera and over 2000 species, widespread in tropical and subtropical regions, especially in Africa and southern South America, with a moderate representation in northern and southeastern 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, 1883;Schill and Jakel, 1978;Kunze, 1993; *Corresponding author.E-mail: amalcaebotvu@gmail.com.Swarupanandan et al., 1996).Pollinial wall is acetolysis resistant (Namboodiri and Sreedevi, 1980;Erdtman, 1952).Pollinial morphology is taxonomically significant like the pollen morphology of other angiosperm.The size and shape of pollinial sacs, color of pollinia, nature of corpusculu, 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(Wyatt, , 1978;;Kephart, 1981;Broyles and Wyatt, 1993).Pollen morphology of Asclepiadaceae has been surveyed and summarized by Brown (1811), Corry (1883), Schill and Jakel (1978), Swarupanandan et al. (1996), Civeyrel (1999) Civeyrel et al. (1998), Verhoeven and Venter (1998b, 2001) and Sajith and Sreedevi (2005).

Pollinial morphology
The Asclepiadaceae has five pollinarium, each of which consists of two pollinia.Pollinia have a pair of pollinial sac connected to a central corpusculum through a pair of caudicles.The pollinial wall is made of amorphous sporopollenin enclosing the pollen mass each with a lamellate exine.Pollinial sac measurements relate to the length (from point of attachment to caudicle to the distal free end) and breadth (at the broadest point of the sac) of the sac.

RESULT AND DISCUSSION
The pollinia showed a great variation in form, varying from oval to globular.The size, shape, colour, pollinia attachment, orientation of pollinia, position of pollinia within anther, etc are valuable characters for the analysis of diversification of pollinia of the Asclepiadaceae.
The pollinial sacs are thin and flat.Their size, shape, colour, orientation of pollinia, position of caudicle or translator attachment to the corpusculum, differ according to genera.From Table 1, it was found that within the six members, the largest pollinial sac was found in Calotropis (Figure 4b) and smallest in Gymnema (Figure 1b).The shape of pollinial sacs are more or less oval but globular shape was found in Tylophora (Figure 2b).The genus Gymnema possesses intermediate shape which is neither oval nor globular.The colour also differs in different genera.
Pollinium orientation has been used as a diagnostic character for systematic studies in the family Asclepiadaceae.The classic division of Asclepiadoideae   into three tribes is based on the orientation of the pollinium relative to the translator.Three tribes have been recognized by erect (Marsdenieae), horizontal (Gonolobeae) and pendulous (Asclepiadeae) pollinia (Swarupanandan et al., 1996).In Calotropis and Daemia (Figure 5b), the pollinia are pendulously orientated within the anther locule and horizontal orientation of pollinia is found in Dregea (Figure 3b), Hoya (Figure 6b) and Gymnema.But the orientation of pollinia of Tylophora is questioned due to the fact that pollinia are slightly erect,  that is why this type of pollinia is recognized as erect pollinia.The structure of a typical pollinia is shown in Figure 7 .Position of attachment of translator or caudicle to the pollinia is also an important diagnostic feature to evaluate the morphological diversification of pollinia of different genera of Asclepiadaceae.The translator attachment is either apical or basal.In Periplocoideae, the translator attachment is absent because the mechanism in this subfamily is via adhesion, following flower maturation.This condition differs substantially from Secamonodeae and Asclepiadoideae in which the attachment of pollinia to the translator occurs much earlier in development by different mechanism (Brown, 1811;Swarupnandan et al, 1996).From Table 2, it was found that the basal attachment is found in Calotropis, Dregea and Hoya and apical attachment of pollinia was found in Daemia, Tylophora and Gymnema.

Conclusion
On the basis of our observation on pollinia, we conclude that the pollinia of different genera vary in morphology.These morphological features are accelerating for future studies of phylogenetic interrelationship among different

Table 1 .
Pollinial size of six genera of Asclepiadaceae.

Table 2 .
Colour, shape, orientation and translator attachment of pollinial sacs of six members of Asclepiadaceae