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Alternanthera (Amaranthaceae) is a diverse genus largely restricted to the American Tropics that belongs to the alternantheroid clade containing C4 and C3–C4 intermediate species. This research focuses on the study of pollen characters by studying 13 species, representatives of the two major clades and subclades of Alternanthera. General palynological comparisons were conducted with light microscopy (LM), scanning electron microscopy (SEM) and with confocal laser scanning microscopy (CLSM) for exine ultrastructure. Twenty-five characters were measured and described for Alternanthera and among these, 14 pollen characters were used to discriminate pollen groups using cluster analysis and canonical analysis of principal coordinates (CAP). Pollen form and ornamentation, pores number, spines length, number of ektexinous bodies and nanospines on the ektexinous bodies on pore membranes, arrangement of nanopores and spines on structural elements, and metareticula form were taxonomically important and ther...

Iresine rzedowskii Zumaya, Flores Olv. & Borsch and I. valdesii Zumaya, Flores Olv. & Borsch are described, illustrated, and compared to morphologically similar species. In addition, pollen SEM micrographs and distribution maps are provided for the new species. Iresine rzedowskii is a shrub with long clambering stems, the younger ones reddish with conspicuous lenticels and the older ones with suberose cortex. It is the only species of Iresine that has pollen grains with a foveolate tectum. Iresine valdesii is also a shrub but with stiff erect branching and small, perennial coriaceous leaves. The synflorescences of staminate plants are solitary, very small, usually once-branched thyreoid structures while those of pistillate plants are very different, with paracladia appearing terminal on most branches and erect, up to 13 cm long, and two to three times branched. Both species are restricted to Mexico. Iresine rzedowskii ranges from northwestern Chihuahua to northern Oaxaca, while I. valdesii is end...

The Amaranthaceae-Chenopodiaceae alliance has been the focus of several phylogenetic studies, but major questions concerning the internal relationships of Amaranthaceae still remain unanswered. This study aims to test the monophyly of the subfamily Gomphrenoideae and to examine the generic relationships within this group. Parsimony and Bayesian analyses of trnL-F and rpl16 show that the subfamily Gomphrenoideae is monophyletic. The clade of Gomphrenoideae represents a large and diverse radiation of the Amaranthaceae in the New World. Unilocular anthers are a morphological synapomorphy for Gomphrenoideae that was derived from bilocular anthers. Three major clades are resolved: the Gomphrenoids, the Alternantheroids, and the Iresinoids, which are largely supported by pollen morphology. The Iresinoid clade is sister to Alternantheroids plus Gomphrenoids, rendering metareticulate pollen as the synapomorphy for the latter two clades. Tribes and subtribes delimited by androecium and inflorescence characters are poly- or paraphyletic. Several genera are monophyletic including the large genus Alternanthera, whereas Gomphrena is polyphyletic. Irenella and Woehleria are resolved within Iresine whereas Blutaparon and Lithophila fall within the polyphyletic Gomphrena. The trnL-F and rpl16 sequence data are the most variable chloroplast regions examined to date for the family and are highly effective in resolving relationships in Amaranthaceae.

Twenty-three genera of Amaranthaceae occur in the New World. Two endemic genera occur in the Hawaiian Islands. Among the genera of the subfamily Amaranthoideae, Celosia, Cyathula and Achyranthes have their main distributions in the Old World; the two last-named genera are represented in the Americas only by widespread weeds. All the New World genera of the subfamily Gomphrenoideae are mainly or entirely restricted to this region. Characters of androecium and gynoecium are fundamental in the recognition of genera within the family. Androecia of different genera may be structurally and phylogenetically more similar than would appear from a cursory examination. It is suggested that the type of staminal tubes found in Pseudogomphrena and Froelichia can be derived from that in Alternanthera and Froelichiella by reduction of filament length and a fusion of pseudostaminodia with the filaments. The staminal tube in Gomphrena could result from a further decrease in distance between pseudostaminodia of the Pseudogomphrena type, and a deeper forking of the pseudostaminodia; each so-called apical filament lobe in Gomphrena would then be homologous with half a pseudostaminodium in Pseudogomphrena. Much of the variation in the androecia of these and other genera, as well as within genera such as Pfaffia, can be explained as the combined results of coalescence and splitting-up tendencies. Splitting up of staminal tubes may not necessarily take place along the borders of phylogenetically original filaments and pseudostaminodia. The Amaranthus-type of pollen is found in the majority of genera of the subfamily Amaranthoideae, but also in the Chenopodiaceae. A group of genera within the subfamily Gomphrenoideae also has pollen very similar to, or identical with, this type. Most genera of the subfamily Gomphrenoideae have pollen of the Gomphrena-type. Pseudoplantago has unilocular (at anthesis) anthers, a characteristic of the subfamily Gomphrenoideae, but floral structure as well as pollen morphology connect the genus to a group of genera within the Amaranthoideae, subtribe Achyranthinae. The combination of subcuboidal shape and opercula with radially arranged hooked protuberances, makes the pollen of Pseudoplantago unique among the angiosperms studied so far. Floral morphology and palynological characteristics indicate a close relationship between Pfaffia and Alternanthera. Both genera, as currently accepted, are relatively homogeneous from pollen morphological points of view. There are no correlations between pollen morphology and the variation in the androecium in Pfaffia, nor would pollen structure support recognition of Hebanthe as a distinct genus. Woehleria and Irenella may be derived from, or be of the same origin as, Dicraurus and Iresine. All four genera are placed in the subfamily Gomphrenoideae because of the bisporangiate anthers, but their pollen structure is very close to, or identical with, that of the Amaranthus-type. Pseudogomphrena combines characteristics of Gomphrena and Pfaffia.