18S rDNA phylogeny and evolution of cap development in Polyphysaceae (formerly Acetabulariaceae; Dasycladales, Chlorophyta)

Abstract Cells of the members of the Dasycladales have a unique body plan well known from fossils. They persist today in 38 recognized species. This study investigates in detail the development of reproductive structures in 17 Polyphysaceae (= Acetabulariaceae) species and provides a molecular phylogenetic analysis of 18S ribosomal DNA sequence data of 23 species of the order Dasycladales, including 17 of the 19 extant members of the family Polyphysaceae. Reproductive cap development is documented by scanning electron microscopy in 17 species, by histological sections in five species, and by growth measurements. Other morphometric data are also provided for most species. Bayesian analysis of DNA data reveals three well-supported clades for which morphological synapomorphies exist, but which are not completely in accordance with previous generic concepts. An early-branching clade comprising the monotypic Chalmasia and two species of Halicoryne is characterized by the formation of buds by the pointed cap ra...