The limits of subkingdom Protozoa in unicellular Eukarya

Abstract

According to the five-kingdom system, all unicellular eukaryotes are classified under the kingdom Protista, and multicellular eukaryotes contain only Animals, Plants, and Fungi. This classification is generally not accepted by zoologists. Many zoological textbooks still classify Animals into two large groups: Protozoa and Metazoa. However, the criterion for selecting Protozoa is the specific digestive system of heterotrophic nutrition for animals, thus, the range of Protozoa is often wide, includes many different, far related groups in unicellular eukaryotes. The entire eukaryotic phylogenetic tree (both unicellular and multicellular) built on molecular sequence comparisons of recent genetics indicates that only a few single-celled eukaryotic groups have phylogenetic relationship with the ancestor of multicellular eukaryotes in the animal kingdom. This situation forces a narrowing of the range of protozoan groups. According to principle (1) Kingdom taxa must include all groups of organisms that are monophyletic, i.e. have the same root in the phylogenetic tree of eukaryotes. This principle holds for all taxa above species. (2) Inheriting the traditional perception of considering Animals, Fungi and Plants (including unicellular and multicellular) as kingdom taxons. Protozoa includes 3 groups Choanoflagellata, Filasterea and Ichthyosporea. The article also updates the morphological, biological and taxonomic characteristics of these three groups.