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The family Hydrocharitaceae, with 15 genera and ca. 80 species, shows a remarkable morphological diversity which presumably
developed as an adaptation to their aquatic habitat. This is particularly true in the case of the many different kinds of
pollination mechanisms. To gather more basic information regarding the adaptive evolution of Hydrocharitaceae, we have carried
out a phylogenetic analysis based on the sequences of therbcL andmatK. Our resulting neighbor-joining distance tree provides the following insights: (1) none of the previous classification systems
were supported by molecular phylogenetic tree; (2) Najas (Najadaceae), which has never been included in Hydrocharitaceae except
in Shaffer-Fehre's (1991) system based on seed coat structures, is an ingroup of Hydrocharitaceae; (3) Limnocharitaceae and
Alismataceae are sister groups of Hydrocharitaceae; (4) the three marine genera,Halophila, Enhalus andThalassia, are monophyletic; and (5) a peculiar pollination mechanism specific to Hydrocharitaceae (Hydrocharitaceae-epihydrophily),
underwent a parallel evolution. 相似文献
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The pollen morphology of 11 genera and 11 species of the Hydrocharitaceae and one species of the Najadaceae was studied using scanning and transmission electron microscopies, and the exine structures and sculptures are discussed in relation to pollination mechanisms and the molecular phylogeny. The pollen grains of the Hydrocharitaceae are spherical, inaperturate, and form monads or tetrads, while those of the Najadaceae are elliptical, inaperturate, and form monads. The entomophilous genera Egeria, Blyxa, Ottelia, Stratiotes, and Hydrocharis share pollen grains that have projections like spines or bacula. The anemophilous genus Limnobium has reticulate pollen grains. The hypohydrophilous genera Thalassia and Najas are characterized by pollen grains with reduced exine structures. The pollen-epihydrophilous genera Elodea and Hydrilla have tightly arranged small spinous pollen grains, and the male flower-epihydrophilous genera Enhalus and Vallisneria have reduced reticulate or gemmate exines. Character state reconstruction of the exine structures and sculptures using a molecular phylogenetic tree suggests that variation in the exine is generally correlated with the pollination mechanism; the selective pressures acting on the pollination mechanisms have reduced the exine structure in hypohydrophilous plants and resulted in various exine sculptures that are adapted to the different pollination mechanisms in entomophilous, anemophilous, and pollen-epihydrophilous plants. 相似文献
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