Pollen morphology of Nothofagus (Nothofagaceae,
Fagales) and its phylogenetic significance
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1 |
Sección Paleopalinología, MACN “B. Rivadavia”, Ángel Gallardo 470 (C1405DJR) C.A.B.A., Argentina |
2 |
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, Argentina |
3 |
Laboratorio de Sistemática y Biología Evolutiva (LASBE), Museo de La Plata, UNLP, Paseo del Bosque s/n°
(B1900FWA) La Plata, Argentina |
Online publication date: 2016-12-13
Publication date: 2016-12-13
Acta Palaeobotanica 2016; 56(2): 223–245
ABSTRACT
Nothofagaceae (southern beeches) are a relatively small flowering plant family of trees confined
to the Southern Hemisphere. The fossil record of the family is abundant and it has been widely used as a test
case for the classic hypothesis that Antarctica, Patagonia, Australia and New Zealand were once joined together.
Although the phylogenetic relationships in Nothofagus appear to be well supported, the evolution of some pollen
morphological traits remains elusive, largely because of the lack of ultrastructural analyses. Here we describe
the pollen morphology of all extant South American species of Nothofagus, using scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and light microscopy (LM), and reconstruct ancestral character
states using a well-supported phylogenetic tree of the family. Our results indicate that the main differences
between pollen of subgenera Fuscospora (pollen type fusca a) and Nothofagus (pollen type fusca b) are related to
the size of microspines (distinguishable or not in optical section), and the thickening of colpi margins (thickened
inwards, or thickened both inwards and outwards). In particular, Nothofagus alessandrii, the only extant South
American species of subgenus Fuscospora, presents distinctive pollen features that have not been observed in
any other species of the genus (i.e. a large granular infratectum and spongy apertural endexine). Species of
subgenus Lophozonia are characterized by having the largest pollen grains, with polygonal outline in polar
view, microspines distinguishable in optical section, long and non-thickened colpi, and a thin endexine. The
reconstruction of character states for the node corresponding to the common ancestor to genus Nothofagus leads
us to conclude that the ancestral form of Nothofagaceae should have had: equatorial diameter < 40 μm, circular
outline in polar view, microspines distinguishable in optical section, short colpi thickened inwards, and a thin
endexine. These features are fully consistent with those present in Nothofagidites senectus Dettmann & Playford,
the oldest fossil species of Nothofagaceae recorded in Campanian-Maastrichtian sediments of Gondwana.