Pollen characters and DNA sequence data converge on a monophyletic genus Iresine P. Browne (Amaranthaceae, Caryophyllales) and help to elucidate its species diversity

September 13, 2018


by Thomas Borsch, Hilda Flores-Olvera, Silvia Zumaya Mendoza, Kai Müller

Iresine is a neotropical genus of the Amaranthaceae with most of the species diversity in Mexico and Mesoamerica. It has suffered a complex classification history with considerably diverging views on the genus concept. We have carried out a phylogenetic analysis of Iresine and allied genera using sequence data of combined plastid introns (including the matK CDS) and spacers as well as ITS, and a dense sampling of species. Trees depict a clade of Iresine with Irenella and Woehleria deeply nested. This clade is sister to the remainder of Gomphrenoideae including Hebanthe and Trommsdorffia = Pedersenia). One of two maximally supported subclades of Iresine comprises mostly species restricted to the Mexican highlands and adjacent areas, whereas the other subclade is composed of more widespread Mexican-Mesoamerican taxa. Pollen grains of Iresine and relatives were examined using high resolution SEM, which yielded a matrix of 15 pollen characters. Ancestral character state reconstruction shows dodecahedral grains (in I. angustifolia and I. nigra) to have evolved within the Iresine clade, not involving the complete suite of character shifts associated with metareticulate pollen but just an increase of aperture diameter and a slight decrease of mesoporia width. To the contrary, four character state transformations occurred in the common ancestor of core Gomphrenoideae that led to metareticulate pollen (shifts to a distal orientation of punctae and microspines, to a sunken position of apertures relative to the distal part of mesoporia resulting in narrow mesoporia higher then wide, and a reduction in the diameter of mesoporia). The Iresine clade is characterized by pollen with well-separated ektexinous aperture membrane bodies, rounded or triangular, and gradually tapering into a single spine. For the monophyletic genus Iresine, 35 species are currently accepted. We provide a taxonomic backbone (including two new combinations and several lectotypifications) that also comments on the current understanding of species delimitation.

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