The Evolution of Pollen Tetrads in Onagraceae

Abstract

In Onagraceae, pollen is shed in mature tetrads in most Epilobieae, many species of Ludwigia (Jussiaeeae), and two closely related species of the large genus Camissonia (Onagreae). Mature tetrads of Camissonia cardiophylla and representative species of Epilobium and Ludwigia were examined with light, scanning, and transmission electron microscopes. Morphological diagnoses of monad units indicated that individual taxa could be readily distinguished. Statistical analyses of tetrads which remained after acetolysis treatment revealed significant differences in the strength of the binding mechanisms. Mechanisms of tetrad cohesion were found to consist of two principal types. Common to all taxa is cohesion of pollen wall surfaces at the aperture margins; this mechanism is well known in many angiosperm groups. With the exception of Camissonia, the remaining taxa also display binding by means of short exine fragments between adjacent pollen units. These fragments, termed bridges and reported here for the first time, are located in the area extending from the aperture margins to near the center of the proximal exine faces. Thin sections reveal that layers of the bridges are identical with those of the exine. Comparisons were made between bridges and viscin threads, both of which occur on the proximal faces of the grains. Viscin threads are present on all pollen grains in Onagraceae and exhibit distinctive morphologies, and bridges were viewed morphogenetically as related to viscin threads but including an endexine layer and occupying a position near the apertures where cohesion of wall surfaces also occurs. In an evolutionary sense, the formation of mature tetrads almost certainly occurred independently in Camissonia and may have done so in Ludwigia and the Epilobieae.

Investigations of Angiosperms From the Eocene of North America: A Mimosoid Inflorescence

“…14). Similar exine bridges have been noted in tetrads of the extant Onagraceae (Skvarla, Raven, and Praglowski, 1975). There can be little doubt that the pollen grains were shed at maturity in permanent tetrads; they are found in every instance (approximately 100 have been observed) to be in tetrads, there are binding exine connections between the grains, and similar tetrads are found in the dispersed pollen flora of the same formation (Potter, 1976, Fig.…”

Cytinaceae are sister to Muntingiaceae (Malvales)

“…obs.). The presence of mature pollen as tetrads in angiosperms is scattered among numerous families such as Monimiaceae (Sampson, 1977), Annonaceae (Tsou & Fu, 2002), Typhaceae (Skvarla & Larson, 1963), and Onagraceae (Skvarla & al., 1975), thus indicating that this feature has evolved multiple times independently. For this reason, and because the mode of binding together pollen grains in tetrads varies, it is not clear whether this feature is a synapomorphy between Cytinaceae and Muntingiaceae.…”

Pollen ontogenesis in Oenothera: a comparison of genotypically normal anthers with the male-sterile mutant sterilis

“…Heslop-Harrison (1971) states that one of the most significant early papers on the development of pollen walls is that of Beer (1905) on various species of Onagraceae. However, more recent morphological studies on the pollen structure of Onagraceae (Afzelius 1956;Cresti et al 1983;Cruden and Jensen 1979;Diers 1963;Hesse 1981;Jean 1971;Lepouse and Romain 1967;Rowley 1971;Rowley et al 1982;Skvarla et al 1975Skvarla et al , 1976 are not concerned with development and do not provide a clear delimitation of the stages of normal development.…”