Data from: A continuous morphological approach to study the evolution of pollen in a phylogenetic context: an example with the order Myrtales

The study of pollen morphology has historically allowed evolutionary
biologists to assess phylogenetic relationships among Angiosperms, as well
as to better understand the fossil record. During this process, pollen has
mainly been studied by discretizing some of its main characteristics such
as size, shape, and exine ornamentation. One large plant clade in which
pollen has been used this way for phylogenetic inference and character
mapping is the order Myrtales, composed by the small families Alzateaceae,
Crypteroniaceae, and Penaeaceae (collectively the “CAP clade”), as well as
the large families Combretaceae, Lythraceae, Melastomataceae, Myrtaceae,
Onagraceae and Vochysiaceae. In this study, we present a novel way to
study pollen evolution by using quantitative size and shape variables. We
use morphometric and morphospace methods to evaluate pollen change in the
order Myrtales using a time-calibrated, supermatrix phylogeny. We then
test for conservatism, divergence, and morphological convergence of pollen
and for correlation between the latitudinal gradient and pollen size and
shape. To obtain an estimate of shape, Myrtales pollen images were
extracted from the literature, and their outlines analyzed using elliptic
Fourier methods. Shape and size variables were then analyzed in a
phylogenetic framework under an Ornstein-Uhlenbeck process to test for
shifts in size and shape during the evolutionary history of Myrtales. Few
shifts in Myrtales pollen morphology were found which indicates
morphological conservatism. Heterocolpate, small pollen is ancestral with
largest pollen in Onagraceae. Convergent shifts in shape but not size
occurred in Myrtaceae and Onagraceae and are correlated to shifts in
latitude and biogeography. A quantitative approach was applied for the
first time to examine pollen evolution across a large time scale. Using
phylogenetic based morphometrics and an OU process, hypotheses of pollen
size and shape were tested across Myrtales. Convergent pollen shifts and
position in the latitudinal gradient support the selective role of
harmomegathy, the mechanism by which pollen grains accommodate their
volume in response to water loss.