Placing the forgotten slime molds (Sappinia, Copromyxa, Fonticula, Acrasis, and Pocheina), using molecular phylogenetics.

摘要

Multicellularity has many different independent origins during the evolutionary history of the eukaryotes. For most people, plants, animals, and fungi are considered to represent the extent of multicellularity. The cells that comprise plants, animals and fungi are conjoined and arranged within organisms very differently in each lineage. Contemporary phylogenetic studies are making it increasingly clear that each of the above lineages evolved from different unicellular ancestors. However, there is much more than first meets the eye. One type of multicellularity, the cellular slime mold habit, appears to have evolved independently, on numerous occasions, throughout the Eukaryote Tree of Life. Cellular slime molds are amoeboid protists that live their vegetative lives as independent cells, but at some point can assemble into a group that works together to form a propagule dispersing structure called a fruiting body, in which each cell maintains its individuality. Because these fruiting bodies are "fungus-like," mycologists have traditionally studied them, and they often have been overlooked by protistologists. One exception is the group known as dictyostelids, which have become model organisms in developmental biology for the study of cellular sociality, cell differentiation and multicellularity. Dictyostelids are usually not considered to be closely related to any of the other cellular slime molds. Other than the dictyostelids, the cellular slime molds have not been extensively studied at the molecular level. In fact, prior to this study there were, with one exception, no molecular data for any of them.;The wide variety of described cellular slime molds are often considered as closely related due to the superficial similarities in their life cycles. These organisms have been placed into various taxa, commonly named with the root Acras-, often informally called the "acrasid cellular slime molds." This study attempted to examine the phylogenetic affinities of the cellular slime molds as a whole, using both morphological and molecular data. The work embodied in this disseration illustrates the systematic positions and evolutionary history of cellular slime molds. The approach taken herein is one of a holistic method where data from morphological observations of the whole life cycle, molecular analyses, and previously published literature are combined in an effort to better understand the evolution of the cellular slime mold habit. These data provide grounds for accurate species concepts and rigorous revision of systematics and taxonomy. This study added taxonomic clarity to cellular slime molds as well as robustly demonstrating that cellular slime molds are scattered on the Eukaryotic Tree of Life. In all, there are five genera that have been included in this study. Herein, the first molecular data are presented from Sappinia pedata, Copromyxa protea, Fonticula alba, Pocheina rosea , and several novel species of Acrasis. Gene sequences were also obtained from the cellular slime mold Guttinulopsis and its amoeba' sister taxon, Rosculus. These data illustrate that the cellular slime mold habit has been converged upon numerous times during the evolution of eukaryotes.;Small subunit rRNA gene and actin gene sequences from Sappinia pedata show that it branches as a sister group to Sappinia diploidea; together, they are in the Thecamoebidae of the supergroup Amoebozoa. Small subunit rRNA gene sequences from Copromyxa protea showed that the taxon branches as sister group to the amoeba Hartmannella cantabrigiensis, together branching robustly in the Tubulinea of Amoebozoa. Herein the genus Copromyxa is expanded to include H. cantabrigiensis, as C. cantabrigiensis . Five gene phylogenetic analyses of Fonticula alba show that Fonticula branches sister to the nucleariid amoebae in the supergroup Opisthokonta. Small subunit rRNA gene sequences form Acrasis and Pocheina show that they are congeneric and should be tentatively considered within the single taxon Acrasis . These phylogenetic analyses suggest the desciption of three new species of Acrasis. Acrasis branches as sister to the amoebae in Allovahlkampfiia, in the supergroup Excavata. Large subunit rRNA gene and actin gene sequences from Guttulinopsis and Rosculus preliminarily show that they are in the Cercozoa of the supergroup Rhizaria. These data illustrate a novel home for the cellular slime mold habit in Opisthkonta, Rhizaria, and Tubulinea.;Given the broad distribution of the habit, this suggests that there is an evolutionary advantage for cooperation among cells to form a multicellular, propagule dispersal structure. Further, the data presented herein on cellular slime molds should prove to be an excellent framework to begin to examine the molecular underpinnings of multicellularity and to examine the evolution of convergence.