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Floral development

Assessing homology of morphological features in families that contain large genera is often difficult for several reasons. Such families are frequently comprised of one or two large genera and several smaller genera. In the larger genera, rapid species radiation may have occurred, often resulting in diverse morphologies that are difficult to interpret across the rest of the family. The situation may be complicated when a well-supported familial phylogeny is unavailable, possibly due to limited sampling in the smaller genera or from a lack of phylogenetic signal in rapidly radiating lineages. A primary example of this situation is the Passifloraceae. This family consists of approximately 17 pantropical genera, the largest of which are the primarily New World Passiflora L. and Old World Adenia Forsk. Both of these large genera have floral and vegetative morphologies that are difficult to relate to the smaller, often monotypic genera in the rest of the family. Passifloraceae has been traditionally divided into two tribes, Passifloreae and Paropsieae. Passifloreae is comprised of approximately 12 genera that are primarily climbing plants with tendrils (MacDougal, 1994). Paropsieae primarily consists of shrubs and small trees and has been viewed as transitional between Malesherbiaceae, Turneraceae, and Flacourtiaceae (De Wilde, 1971; MacDougal, 1994; Bernhard, 1999).

The taxonomic circumscription of Paropsieae has been difficult because many of its characters suggest conflicting familial affinities. While stamen number does not exceed 10 in the tribe Passifloreae, it may range from five to 20 or more in Paropsieae, especially in the genera Viridivia and Smeathmannia (De Wilde, 1971). Very little is known about the primitive floral condition in Passifloraceae. Bernhard (1999) examined floral development in members of both Passifloreae and Paropsieae to determine the primitive androecial condition in the family and to address questions of homology among the corona and disk structures observed across the genera. The rarity of many of the smaller genera in Passifloraceae restricted sampling in that analysis, thus limiting conclusions about ancestral floral character states. Recent molecular phylogenetic analyses by Alford (2005) on the Flacourtiaceae sensu lato suggest that the monophyletic tribe Paropsieae is sister to the remainder of the Passifloraceae. However, a well-supported phylogeny at the familial level is still unavailable, leaving many unanswered questions about the evolution of this group.

Passiflora, the largest genus in the family, contains approximately 490 species of vines, lianas, and small trees native to Central and South America, with approximately 22 additional species endemic to Southeast Asia and the Austral Pacific. The genus is well-known for the complex and diverse morphologies observed in both its vegetative and floral organs, but few of these features have been studied in an evolutionary context. All species have axillary tendrils, and many species are distinguished by the presence and type of laminar and petiolar nectaries. Passiflora flowers are characterized by an androgynophore, one to several series of coronal filaments, five stamens, and three carpels (Ulmer and MacDougal, 2004). There is a high degree of variation in floral morphology across the genus, even within small groups of closely related species (MacDougal, 1994; Kay, 2003; Porter-Utley, 2003). The morphological details of the hypanthium, coronal filaments, operculum, and limen are extremely variable in Passiflora. For this reason, it has been difficult to assess the homology of these organs within the genus and even more difficult to compare these features to other genera in Passifloraceae and allied families.

Relatively few studies have addressed the developmental patterns leading to the complex floral morphology of Passiflora. Payer (1857) was the first to examine the development of the flower in the hybrid Passiflora ×loudonii. Masters (1871) provided the most extensive treatment of development and homology in the genus. This work was the first of its kind to identify and define the morphological features unique to Passiflora. There has since been little investigation into the nature of these features. The ontogenetic sequence of floral development in the economically important Passiflora edulis Sims. and P. quadrangularis L. was characterized by Moncur (1988), but these are the only complete ontogenetic sequences available for the genus to date. Bernhard (1994) investigated the developmental pattern of the corona and operculum in several species of Passiflora and Adenia. Most recently, Bernhard (1999) examined the development of the androecium, corona, operculum, and floral disk in P. racemosa to assess homologies of these structures among the genera of Passifloraceae.

Within Passiflora, the number of reproductive structures is generally uniform, with most of the ca. 520 species consistently exhibiting five stamens and three carpels united on an androgynophore. However, three Asian taxa, Passiflora tonkinensis W.J. De Wilde, P. moluccana var. glaberrima (Gagnep.) W.J. De Wilde, and P. siamica Craib have significant variability in these features. These taxa have between five and eight stamens and three to five carpels. In P. siamica, the number of stamens ranges between five and seven, and the number of carpels may be either three or four. In P. moluccana var. glaberrima, the number of stamens is either seven or eight, and the number of carpels either four or five. Passiflora tonkinensis has been documented with seven stamens and between three and four carpels (De Wilde, 1972a). All other morphological features in these taxa are similar to the general conditions observed across the rest of the genus.

In previous hypotheses of relationships in Passiflora,the 17 species endemic to Asia were thought to be sister to the remainder of the genus (Cusset, 1968; Tillett, 1988). This was supported by two characters observed in some of the Asian taxa: the presence of branched inflorescences and the insertion of additional stamens and carpels in certain species. Branched inflorescences are common in Passifloraceae, but this feature has been greatly reduced in Passiflora, suggesting that this might be a plesiomorphic state in those species retaining this feature. The second hypothesized ancestral feature, the presence of additional stamens and carpels, is similar to the situation in the dioecious genus Adenia Forssk.and in the hermaphroditic Mitostemma Mast., Dilkea Mast.,and Ancistrothyrsus Harms. Based on these similarities, increased stamen number was thought to be a plesiomorphic character state in the Asian species.

Krosnick and Freudenstein (2005) showed that the 22 species of Old World Passiflora are a monophyletic group and supported their current placement in subgenus Decaloba (DC.) Rchb.supersection Disemma (Labill.) J.M. MacDougal & Feuillet. Disemma was shown to be nested within subgenus Decaloba, refuting previous hypotheses that placed Disemma as sister to the rest of the genus. Additionally, the morphologically similar supersection Multiflora (Small) J.M. MacDougal & Feuillet was shown to be sister to supersection Disemma. Within supersection Disemma, two monophyletic groups were resolved: an Asian clade spanning India, China, and Southeast Asia, and an Austral-Pacific clade. While it is now clear that the Old World Passiflora are not sister to the rest of the genus, their anomalous morphology raises several new issues. De Wilde (1974) hypothesized that the ancestor of Passifloraceae was diplostemonous or possibly even triplostemonous. According to his hypothesis, the five stamens would represent the innermost whorl, while the outer whorls have been reduced and transformed into the limen that protects the nectary at the base of the flower. De Wilde proposed that the Asian species retained some members of these outer whorls, resulting in six to eight functional stamens. Bernhard (1999) refuted this hypothesis based on both morphological and histological differences between the secretory cells characteristic of the limen and the true staminodial tissue in the androgynophore. Moreover, the delayed initiation of the limen until long after the differentiation of the stamens suggests that these structures are not likely to be homologous. Thus, at present there is no convincing explanation for the presence of additional stamens and carpels in these anomalous species. It is possible that the condition observed in the Asian species is a synapomorphy for the anomalous taxa, resulting from a unique ontogenetic pathway unrelated to the polyandry characteristic of other genera in Passifloraceae. Alternatively, the Asian species may represent a reversal to the plesiomorphic polyandrous condition for the family.

Material from:

  • Krosnick SE (2006) Phylogenetic relationships and patterns of morphological evolution in the Old World species of Passiflora (subgenus Decaloba: supersection Disemma and subgenus Tetrapathea). Ph.D. Dissertation, Columbus: The Ohio State University.
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