Floral Structure and Evolution in Lopezieae (Onagraceae)

Abstract

The Lopezieae present an interesting mixture of ancestral and derived characters: some members of the tribe retain the basic onagraceous chromosome number (n = 11), but the flowers are advanced in that they are mostly zygomorphic and always have a two‐merous androecium. Species differ in the position of the nectaries, also in the way in which floral parts are united above the inferior ovary. These differences, when analyzed with information from a new monograph of the Lopezieae, provide the basis for a phylogenetic tree. It is inferred that ancestral Lopezieae were bird‐pollinated woody perennials with regular flowers, two fertile stamens, and no floral tube distal to the ovary. Evolutionary events accompanying the emergence of modern taxa included abortion of the abaxial stamen (all surviving Lopezieae except Lopezia lopezioides), development of an epigynous floral tube (L. riesenbachia, L. semeiandra), decrease in floral symmetry without conversion to insect pollination (in two independent lines), and decrease in floral symmetry with conversion to insect pollination (in at least two independent lines). The prominent tubercles on upper petals of certain insect‐pollinated species apparently evolved from the less prominent swollen areas still present in some of the bird‐pollinated species. The tubercles and an associated snapping mechanism arose in response to increasing selection for fly pollination. Densely staining areas in some specimens may be osmophores; if so, scent plays a supplementary role in the orienting of insects to the upper petals. Interstaminal nectaries and the absence of a floral tube link the Lopezieae to Ludwigia; the relationship of these two taxa to Epilobium is presently unclear. Fossil records indicate that the Onagraceae had evolved by the beginning of the Tertiary Period and that the Ludwigia line is very old. The family's ancestral features are retained to a greater degree in Fuchsia, however, than in Ludwigia.

A survey of reproductive biology in Onagraceae

“…In both Fuchsia and Lopezia, hummingbird pollination is clearly primitive, with other modes of pollination derived (Eyde & Morgan 1973). This probably indicates an origin of these genera in the Eocene or subsequently (Sussman & Raven 1978).…”

“…It also seems to have evolved independently in the common ancestor of Lopezia. in North America, and then to have been lost in the common ancestor of the more advanced species of the genus in favour of fly-pollination (Eyde & Morgan 1973). In sect.…”

“…Although the family clearly belongs with the well-marked order MyrtaIes, with which it has been associated for over 150 years, its members share at least four features found nowhere else in the order: (I) a distinctive 4-nucleate embryo sac (Seshavataram 1970;Bhatnagar & Johri 1972, p. 91;Palser 1975, p. 641); (2) the abundant presence of raphides in the vegetative cells (Gulliver 1864;Metcalf & Chalk 1950, p. 1354Carlquist 1961Carlquist ,' 1975; (3) "paracrystalline beaded" pollen ektexine (Skvarla et al 1975(Skvarla et al , 1976; and (4) viscin threads (Skvarla et af. 1978).Onagraceous pollen appears in the fossil record in the Lower Paleocene (Krutzsch 1968, Eyde & Morgan 1973, and it is reasonable to assume that the family was in existence before the close of the Cretaceous (cf. Drugg 1967, p. 60, for a possible Maastrichtian record).…”

“…At any event, Ludwigia was present in both North America and Asia by the Eocene, and Circaea was in Europe by the Oligocene (review in Eyde & Morgan 1973), Lopezia, Circaea, Epilobieae, & Hauyeae, and Onagreae seem to have originated independently in North America from a common ancestor with Fuchsia, quite possibly after a number of independent dispersal events across a relatively wide water barrier between South and North America. Only 4 genera of Onagraceae occur as native plants in the Old World: (1) Circaea, with 5 of its 7 species in Asia, and the remaining 2 common to Asia, Europe, and North America (Boufford in press); (2) Epilobium, well represented in Eurasia (about 90 species) and Australasia (46 species; Raven & Raven 1976, West & Raven 1977, poorly so in Africa (10 species ;Raven 1967), but with sect.…”

Monographing in the 1980s

“…Furthermore, one will not have time to investigate all areas of specialization and still produce a monograph within a reasonable time period. In recent years there have been various notable contributions to taxonomy where a number of specialists in different fields have combined their efforts upon the same plant group, for example in the genus Erythrina (Raven 1974 and other papers in same volume) and other legumes, a concerted effort in the Onagraceae coordinated by Peter Raven, e.g., using evidence from floral anatomy (Eyde and Morgan 1973), wood anatomy (Carlquist 1975a); cytology (Kurabayashi, Lewis and Raven 1962), leaf architecture (Hickey 1980), pollen (Nowicke, Skvarla, Raven and Berry 1984), and flavonoid chemistry (Averett and Raven 1984), and for the Myrtales in a recent Annals of the Missouri Botanical Garden. Our study of the Lecythidaceae has been improved by collaboration with a palynologist in the Netherlands, a floral anatomist in Brazil, and a wood anatomist in Syracuse, New York.…”