BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

First Subkingdom: Acrasiobionta

This subkingdom includes slime Molds that lack flagellated Cells at all stages of their life cycle¹. Their aggregation plasmodia are formed by the aggregation of individual cells without fusion.

Sole division: Acrasiomycota

In Acrasiobionta, which comprise a single division and a single Class, Acrasiomycetes², myxamoebae aggregate into an aggregation plasmodium (also called a pseudoplasmodium; see Fig. 5.3) without fusing. Flagellated myxoflagellates are absent. The Cell walls consist of Cellulose.

¹ Flagellated stages are present in some Representatives of the Acrasiales proper (Acrasiales in this book). — Transl. note.

² It is now almost universally accepted that the Acrasiomycetes ("cellular slime molds") in the sense used here are an extremely heterogeneous group that cannot be considered a natural taxon: the Acrasiales proper (here, Acrasiales) are a highly distinct group showing a distant relationship to euglenids, whereas the dictyostelids (here, Dictyosteliales) are related to myxomycetes (Myxomycota). — Transl. note.

Life cycle. During the asexual reproduction phase, amoebae divide until food resources are depleted. Like myxomycetes, their Nutrition is phagotrophic, consisting mainly of Bacteria. When food is no longer sufficient, a specific group of amoebae begins to aggregate toward a certain center (see Fig. 5.3). The amoebae aggregating into an aggregation plasmodium without fusing are attracted to each other by the substance acrasin (see 8.2.1.1; Fig. 8.5).

The aggregation plasmodium can migrate across the substrate in various directions until the culmination phase begins, during which it rises upward as a Column. Even at this stage, the individuality of the single-celled amoebae is preserved, although certain differentiation processes become apparent during The formation of the fruiting bodies. In the central part of the developing fruiting body (see Fig. 5.3), the amoebae soon stop, secrete a rigid Cell wall, and form a stalk of individual cells; this is the Water/144.html">Origin of the name "cellular slime molds" given to the Acrasiomycetes. As more amoebae are added from below, the stalk elongates. New amoebae crawl up the stalk, forming the sporocarp ("sporangium") proper at the top. In the HEAD, the peripheral cells become the cortex, while the inner cells round up and transform into haploid spores (cysts); a columella is also formed as an extension of the stalk. After sporulation, the stalk and cortical cells die.

The question of a sexual process involving the fusion of amoebae into diploid macrocysts followed by Meiosis remained controversial for a long time. Sexual reproduction is now considered established, at least in the aggregation plasmodium of Polysphondylium (Dictyosteliales). A widely known subject of laboratory research is Dictyostelium. In Acrasis (Acrasiales), in contrast to representatives of the former genus, the amoebae do not form streams within the aggregation plasmodium toward the center of fruiting body formation.