Fine Structure of Mistletoe Pollen VI. Small-Flowered Neotropical Loranthaceae

Abstract

Pollen of small-flowered neotropical Loranthaceae (8 genera; ca. 135 species) was examined in the light, scanning, and transmission electron microscopes. Pollen is typically medium-sized and oblate. Pollen amb is more variable than equatorial shape, ranging from trilobate deeply concave to circular. Both isopolar and heteropolar pollen grains occur within the complex, the latter restricted to species of Phthirusa and Struthanthus in which the apertures differ at each of the polar faces. Simple apertures arranged in a diploaperturate configuration predominate. Such apertures range from diploporate to diplobrevidemicolpate to diplosyn-and diploparasyndemicolpate types. Compound apertures are rare, restricted to Oryctanthus and particular species of Cladocolea and Struthanthus. The (3-)4-5 colpate apertures of Ixocactus are unique in the family. Sculpturing is basically uniform with sculpturing elements ranging from perforations to shallow ridges and/or striato-rugulae. Ultrastructurally, the basic ektexine structure is composed of a thin, perforate tectum, granular/columellate interstitium, and a thick continuous foot layer usually twice as thick as the tectum plus interstitium. Pollen data suggest two basic groupings of small-flowered genera: Group I, composed of Dendropemon, Phthirusa pro parte, and Oryctanthus', and Group II, containing Phthirusa pro parte, Cladocolea, Struthanthus, Maracanthus, and Oryctina. Among Group I genera, Dendropemon is closely linked to Phthirusa through the species P. pyrifolia and P. platyclada. Oryctanthus is a highly derived genus with only remote ties to other Group I taxa. Among Group II genera, pollen data indicate a close relationship between Cladocolea and the Mexican species of Struthanthus. Pollen characters of Ixocactus indicate no relationship with any small-flowered neotropical genus. Rather its pollen features are closer to those of the Eremolepidaceae and the African species of Viscum.In a previous paper we detailed the pollen of also shows a marked tendency towards unisexual the large-flowered neotropical Loranthaceae flowers and dioecism. Only three of the eight (Feuer & Kuijt, 1980). The present paper focuses genera exhibit strictly bisexual flowers {Oryctanon pollen characters of the small-flowered gen-thus, Dendropemon, Ixocactus)', two genera era. Data from these two complexes will be used (Phthirusa, Cladocolea) include some species with to analyze pollen evolution among neotropical strictly bisexual flowers and others with exclu-Loranthaceae. sively unisexual flowers; three genera {Struthan-The small-flowered neotropical complex com-thus excluding S, panamensis, Maracanthus, prises eight genera: Struthanthus (ca. 50 spp.), Oryctina) demonstrate only unisexual flowers. Cladocolea (ca. 25 spp.), Phthirusa (ca. 30 spp.), Small flower size, the tendency towards unisex-Dendropemon (ca. 12 spp.), Cladocolea (ca. 25 ual flowers and dioecism, and an .v = 8 chrospp.), Phthirusa (ca. 30 spp.) Dendropemon (ca. mosome number (Barlow & Wiens, 1971) to-12 spp.), Oryctanthus ...

Evolution of pollen morphology in Loranthaceae

“…To complement and verify published data (Feuer & Kuijt 1978, 1980; Kuijt 1988; Liu & Qiu 1993; Han et al 2004; Roldán & Kuijt 2005; Caires 2012; Caires et al 2012, 2014; see File S4), we studied 35 genera of Loranthaceae, including 15 studied for the first time to our knowledge (Figure 2; File S5). The micrographs in Liu and Qiu (1993) and Han et al (2004) are of mediocre quality, typically out of focus, and do not allow identifying ornamental or structural details.…”

“…The pollen morphology of a number of extant Loranthaceae has been studied in detail using combined light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and revealed characteristic pollen features (Feuer & Kuijt 1978, 1979, 1980, 1985; Roldán & Kuijt 2005; Caires 2012). The micrographs of two studies focussing on Chinese Loranthaceae (Liu & Qiu 1993; Han et al 2004) are difficult to interpret.…”

“…]/Schoepfiaceae: Maguire et al 1974; Feuer 1978; Santalaceae: Feuer & Kuijt 1982; Feuer et al 1982) should have high potential to trace modern lineages back in the past. Feuer and Kuijt (1980) noted that pollen types are often genus-specific in the group of large-flowered Neotropical taxa and their putative Australian relatives, including all root-parasitic genera. The same holds for some of the Neotropical small-flowered taxa (Feuer & Kuijt 1985).…”

“…Feuer and Kuijt (1980) noted that pollen types are often genus-specific in the group of large-flowered Neotropical taxa and their putative Australian relatives, including all root-parasitic genera. The same holds for some of the Neotropical small-flowered taxa (Feuer & Kuijt 1985). Based on found similarities and dissimilarities and intra-generic variation in these groups, they established a first hypothetical framework for the evolution of pollen morphs.…”

“…However, there has been no attempt so far to link pollen morphology to molecular phylogenies and the current systematic framework for the family. Feuer and Kuijt (1980, 1985) based their evolutionary interpretations on the assumption that the small- and large-flowered Neotropical taxa form natural groups, a hypothesis rejected by later molecular phylogenies (Wilson & Calvin 2006; Vidal-Russell & Nickrent 2008b). …”