The Kingdoms of Organisms

Abstract

Dodson, E. 0. (Dept. Biology, Univ. Ottawa, Ottawa, Ont. KIN6N5). 1971. The Kingdoms of Organisms. Syst. Zool. 20:265-281.-Traditionally, all organisms have been assigned to the Plant and Animal Kingdoms. Higher members of these assemblages differ so clearly in so many ways that no confusion arises even in thaose cases in which members of one kingdom show some characteristic of the other, for example, insectivorous plants, or cellulose producing tunicates. Among primitive organisms, especially among flagellates and fungi, however, application of the ordinary criteria for differentiation of plants and animals leads only to confusion. Most biologists have ignored this problem, but for over a century a few have tried to resolve it by erecting additional kingdoms for these primitive organisms. While such systems have expressed a widespread professional malaise, they have not achieved general acceptance because of logical diffictulties of these systems and the relative unfamiliarity of the key groups to most biologists. Recently, however, it has been shown that the bacteria and blue-green algae (procaryotes) contrast with all higher organisms (eucaryotes) so sharply in an extensive airay of nuclear and cytoplasmic characteristics that the procaryotes must be regarded as a separate kingdom of life, the Kingdom Mychota. The major cytoplasmic organelles of eucaryotes, i.e., chloroplasts, flagella and other 9 + 2 organelles, and mitochondria, are all missing from procaryotes. A long series of observations, culminating in exact biochemical comparisons, indicates that these organelles oliginated as intracellular symbionts. Thus, the eucaryotes arose from a large, amoeboid, anaerobic procaryote given to ingestion of smaller procaryotes, some of which becaiue established as intracellular symbionts. Aerobic symbionts were integrated into the host as mitochondria, while flagellate procaryotes with the 9 + 2 structure became flagella and a whole series of other 9 + 2 organelles. Mitosis and meiosis then evolved, and the eucaryote cell was complete. There is a nexus of relationships among the eucaryotes which makes it infeasible to separate the problematical groups of primitive eucaryotes into a kingdom apart. Thus, the present paper supports a basic taxonomy of three kingdoms Mychota, Animalia, and Plantae. The intergrades between the latter two are treated not as defects of the classification but rather as the inevitable result of common ancestry and separation by a process of evolution.