Abstract:
Background: Hydrocharitaceae is a fully aquatic monocot family, consists of 18 genera with approximately 120
species. The family includes both fresh and marine aquatics and exhibits great diversity in form and habit including
annual and perennial life histories; submersed, partially submersed and floating leaf habits and linear to orbicular
leaf shapes. The family has a cosmopolitan distribution and is well represented in the Tertiary fossil record in
Europe. At present, the historical biogeography of the family is not well understood and the generic relationships
remain controversial. In this study we investigated the phylogeny and biogeography of Hydrocharitaceae by
integrating fossils and DNA sequences from eight genes. We also conducted ancestral state reconstruction for
three morphological characters.
Results: Phylogenetic analyses produced a phylogeny with most branches strongly supported by bootstrap values
greater than 95 and Bayesian posterior probability values of 1.0. Stratiotes is the first diverging lineage with the
remaining genera in two clades, one clade consists of Lagarosiphon, Ottelia, Blyxa, Apalanthe, Elodea and Egeria;
and the other consists of Hydrocharis-Limnobium, Thalassia, Enhalus, Halophila, Najas, Hydrilla, Vallisneria,
Nechamandra and Maidenia. Biogeographic analyses (DIVA, Mesquite) and divergence time estimates (BEAST)
resolved the most recent common ancestor of Hydrocharitaceae as being in Asia during the Late Cretaceous and
Palaeocene (54.7-72.6 Ma). Dispersals (including long-distance dispersal and migrations through Tethys seaway and
land bridges) probably played major roles in the intercontinental distribution of this family. Ancestral state
reconstruction suggested that in Hydrocharitaceae evolution of dioecy is bidirectional, viz., from dioecy to
hermaphroditism, and from hermaphroditism to dioecy, and that the aerial-submerged leaf habit and short-linear
leaf shape are the ancestral states.
Conclusions: Our study has shed light on the previously controversial generic phylogeny of Hydrocharitaceae. The
study has resolved the historical biogeography of this family and supported dispersal as the most likely explanation
for the intercontinental distribution. We have also provided valuable information for understanding the evolution
of breeding system and leaf phenotype in aquatic monocots.
Keywords: Hydrocharitaceae, Phylogeny, Historical biogeography, Dispersal, Vicariance, Morphological character
evolution