Concept, Definition, Prevalence, and Host‐Interactions of Xenosomes (Cytoplasmic and Nuclear Endosymbionts)1,2

Abstract

ABSTRACT.In this paper the concept of “xenosome” is greatly expanded from its current usage, which has been based on its application during the past 10 years by Soldo and co‐workers solely to certain bacterial invaders of the cytoplasm in species of a single genus of marine scuticociliales. The author proposes that the term now be considered to embrace all DNA‐containing, membrane‐bounded bodies or organelles—prokaryotic or eukaryotic in original nature—found within the cytoplasm or nucleus of eukaryotic cells of any or all kinds, whether the occupation (“colonization”) is temporary and transient or permanent and stable. Thus, virulent or pathogenically infectious organisms can be included as well as the commonly recognized cell endosymbionts sensu stricto, which are often mutualistic in nature. Of significance, such “normal” cell organelles as plastids, mitochondria, and even nuclei may also be embraced by this expanded definition of xenosome, based on the conjecture that these inclusions might have been “alien” or “foreign” extracellular, independent, free‐living organisms in their own past evolutionary histories. The author's enlarged concept and unifying principle allows more meaningful comparative consideration of the numerous and diverse kinds of xenosome‐host interrelationships, many of which involve species of protozoa and algae from a large number of the taxonomic groups comprising the kingdom Protista.