This revision of the classification of eukaryotes, which updates that of Adl et al. (2005), retains an emphasis on the protists and incorporates changes since 2005 that have resolved nodes and branches in phylogenetic trees. Whereas the previous revision was successful in re-introducing name stability to the classification, this revision provides a classification for lineages that were then still unresolved. The supergroups have withstood phylogenetic hypothesis testing with some modifications, but despite some progress, problematic nodes at the base of the eukaryotic tree still remain to be statistically resolved. Looking forward, subsequent transformations to our understanding of the diversity of life will be from the discovery of novel lineages in previously under-sampled areas and from environmental genomic information.

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The freshwater green algal family Hydrodictyaceae (Sphaeropleales, Chlorophyta) has traditionally consisted of four coenobial genera, Pediastrum Meyen 1829, Hydrodictyon Roth 1797, Sorastrum Kützing 1845, and Euastropsis Lagerheim1894. Two recent molecular phylogenetic studies demonstrated the need for reevaluation of the generic and species boundaries in this morphology-rich family. This study expands the previous work to include phylogenetic analyses of 103 ingroup isolates representing North America, Europe, and Australia, with an emphasis on the common and geographically widespread species Pediastrum duplex. Nucleotide sequence data were collected from the nuclear LSU (26S rDNA) and the chloroplast RUBISCO LSU (rbcL) genes, totaling >3,000 aligned characters. The 26S and rbcL data sets were analyzed using maximum-likelihood (ML) and Bayesian phylogenetic methods. In addition, SEM was used to examine the wall morphology of a majority of the isolates. The results supported previous indications that the P. duplex Meyen 1829 morphotype is nonmonophyletic and resolved some previously ambiguous relationships recovered in earlier phylogenetic estimations using fewer isolates. These new data allowed testing of the recent taxonomic revisions of the family that split Pediastrum into five genera. Some of the previous revisions by Buchheim et al. (2005) were well supported (erection of Stauridium and Monactinus), while others were not (Pediastrum, Pseudopediastrum, Parapediastrum).

All sexually reproducing eukaryotes have a life cycle consisting of a haploid and a diploid phase, marked by meiosis and syngamy (fertilization). Each phase is adapted to certain environmental conditions. In land plants, the recently reconstructed phylogeny indicates that the life cycle has evolved from a condition with a dominant free-living haploid gametophyte to one with a dominant free-living diploid sporophyte. The latter condition allows plants to produce more genotypic diversity by harnessing the diversity-generating power of meiosis and fertilization, and is selectively favored as more solar energy is fixed and fed into the biosystem on earth and the environment becomes more heterogeneous entropically. Liverworts occupy an important position for understanding the origin of the diploid generation in the life cycle of land plants. Hornworts and lycophytes represent critical extant transitional groups in the change from the gametophyte to the sporophyte as the independent free-living generation. Seed plants, with the most elaborate sporophyte and the most reduced gametophyte (except the megagametophyte in many gymnosperms), have the best developed sexual reproduction system that can be matched only by mammals among eukaryotes: an ancient and stable sex determination mechanism (heterospory) that enhances outcrossing, a highly bimodal and skewed distribution of sperm and egg numbers, a male-driven mutation system, female specialization in mutation selection and nourishment of the offspring, and well developed internal fertilization. The study of evolution of the land plant life cycle requires a multidisciplinary approach that considers morphology, development, genetics, phylogeny, ecology, and evolution in an integrated fashion, and will deepen our understanding of plant evolution.

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