Dr. Jeffrey M. Osborn
Dean of the School of Science and Professor of Biology
The College of New Jersey
Cooper, R. L., J. M. Osborn, and C. T. Philbrick. 2000.
Comparative pollen morphology and ultrastructure of the
American Journal of Botany
The Callitrichaceae are an aquatic family of dicots that include the single,
geographically cosmopolitan genus Callitriche. Callitriche contains 40-50
terrestrial, amphibious, and obligately submersed species, and it is the only
known genus in the plant kingdom with co-occurring aerial and hydrophilous
pollination syndromes. Pollen morphology and ultrastructure were described
for 13 Callitriche species using scanning electron and transmission electron
microscopy. Representative taxa of each growth form were examined; these
included three terrestrial species (C. deflexa, C. peploides, and C. nuttallii)
nine amphibious species (C. brutia, C. cophocarpa, C. cophocarpa-stagnalis
hybrid, C. cribrosa, C. hamulata, C. heterophylla var. heterophylla,
C. lusitanica, C. marginata, and C. trochlearis), and one obligately submersed
species (C. truncata). Of the amphibious taxa, C. heterophylla var.
and C. trochlearis had internal geitonogamy, a type of internal self-fertilization.
Pollen from all taxa was spheroidal, small, intectate, and lacked well-defined
apertures. Taxa primarily differed with respect to exine thickness, surface
ornamentation, and the presence or absence of aperture-like regions.
The pollen of terrestrial species, as well as that of C. marginata, had
well-developed exines with thick sculptured and basal layers. In general,
amphibious taxa produced pollen with distinct, but thinner, exines than that
of terrestrial taxa. Pollen of the amphibious taxa with internal geitonogamy
had a thicker basal layer than species without internal geitonogamy, whereas
the overall exine was reduced in C. hamulata and absent in C. brutia and
C. lusitanica. Pollen of the obligately submersed C. truncata also lacked an
exine. These palynological data were correlated with growth habits and related
pollination biologies, as well as with phylogenetic interpretations of Callitrichaceae.
Exine reduction or loss has evolved at least twice in the family, and it is
associated with aneuploid reduction in chromosome number.
aquatic plants; Callitrichaceae;
Callitriche; hydrophily; morphology; pollen; pollination; ultrastructure.