The Floral Morphology of Secamone and the Evolution of the Pollinating Apparatus in Asclepiadaceae

Abstract

The floral anatomy and morphology of several species of Secamone is discussed. Ontogenetic studies reveal that the anthers are 4-Ioculan In this respect Secamone differs from other members of the subfamily Cynanchoideae of Asclepiadaceae. The bilocular anthers of other Cynanchoideae are a result of phylogenetic suppression of the two outer locules of each anther, as shown by ontogenetic studies and by the origin and nature of the tapetal tissue. Ontogeny of the pollinum sacs is followed in Secamone. Pollen mother cells undergo considerable elongation before the formation of the tetrads. The two meiotic divisions are simultaneous, contrasting in this respect with the successive divisions of other Cynanchoideae and agreeing with the Periplocoideae and Apocynaceae. The resulting tetrads have different configurations depending on the direction of the second meiotic spindle; they are rhomboidal or T-shaped. This is in sharp contrast to the linear tetrads characteristic of all other members of Cynanchoideae, but similar to Periplocoideae and Apocynaceae. The ontogeny of the stigma head is followed, which reveals that at maturity the entire stigma head of Secamone is glandular, contrasting sharply with the five restricted glandular regions of this structure in other Asclepiadaceae and agreeing with similar structures of the Apocynaceae. The five glandular furrows of Secamone in which the simple translators are secreted are not well defined. Each translator consists of a more or less solid portion which is pushed upward and slightly outward by another more or less fluid substance. Thus the whole translator of Secamone is comparable to the stalk-adhesive disc of Periplocoideae and the corpusculum of higher Cynanchoideae. Secamone thus presents several primitive characters shared with Periplocoideae and Apocynaceae but not with other Cynanchoideae (where it has been placed by taxonomists). It shares with other Cynanchoideae, however, the presence of pollinium sacs and a primitive development of the translators. It is difficult to draw a sharp line between members of Apocynaceae and Aslepiadaceae since they apparently represent closely consecutive levels of floral evolution.

A revised classification of the Apocynaceae s.l.

“…Because it was unnamed, the status of this third group was ambiguous, and it was therefore subsumed in the Cynanchoideae (= Asclepiadoideae) in subsequent classifications by some authors (Bentham, 1876;Schumann, 1895). More commonly, however, it has been given equal rank (Endlicher, 1838;Decaisne, 1844;Safwat, 1962;Bruyns & Forster, 1991;Liede & Albers, 1994).…”

“…They are clearly more similar to each other than to the rest of the Gentianales, and in a number of characters there is a gradation from the Apocynaceae to the Asclepiadaceae. Three suggestions for recognition and expression of this relationship have appeared: as one family (Hallier, 1905;Demeter, 1922;Safwat, 1962;Stebbins, 1974;Stevens, 1976;Thome, 1976Thome, , 1992Judd et al, 1994;Struwe et al, 1994;Takhtajan, 1997); as an order separate from the Gentianales (Tsiang, 1934;Hutchinson, 1973); and as a suborder within the Gentianales (Rosatti, 1989;Nicholas & Baijnath, 1994;Omlor, 1998).…”

A phylogenetic study of floral morphology of some members of Asclepiadaceae R.Br

“…It should be noted, however, that the sparse sampling in the present study may obscure homoplasious convergence to apical attachment among clades coded as uniformly basal reversal to basal attachment in Ascleoiadaceae (Endress and Bruyns, 2000). A rigorous basis for assessing the homology of ambiguous cases would be a comperative ontogenetic study of polinaria (Safwat, 1962, Kunze, 1993.…”

“…Corolline coronas have been considered to be prevalent in Periplocoideae, but very uncommon in Asclepiadoideae (kunze, 1990; Nilsson et al, 1993). They have been reported from Secamonoideae, but their distribution in this subfamily has been little discussed (Safwat, 1962;kunze, 1990). Corolline coronas are also found widely among non-asclepiad Apocynaceae (kunze, 1990; Endress and Bruyns, 2000).…”

Pollinium structure in periplocoideae (apocynaceae)

“…The families Asclepiadaceae and Apocynaceae have been kept separate in most systems. Proposals to unite them were made by Demeter (1922), Safwat (1962), Thome (1992) and Struwe et al ( 1994). In a recent study, using rbcL sequence data, Sennblad & Bremer (1996) showed that the position of the Asclepiadaceae within the Apocynaceae is well supported, and therefore recommended the amalgamation of the two families.…”