Entities inside one another ‐ a matryoshka doll in biology?

Abstract

Matryoshka dolls, more commonly known as Russian or babushka dolls, are a set of wooden figures nested into each other. The outer doll often displays a female wearing a safaran, a long, traditional Russian folk costume likely dating back to the 14th century. Inside it, the dolls may be male or female, with the smallest of them typically being a baby made out of solid wood. One cannot help but think about matryoshka dolls when looking at biology, and in particular, oddities such as the mealybug symbiosis (Husnik and McCutcheon, 2016; Szabo et al., 2017), where a symbiont is contained within a symbiont residing in yet another cell. Previous Crystal Balls have nicely discussed symbiosis from different angles and we do not intend to rehash any of these fascinating discussions and viewpoints. Instead, we provide a perspective on the biological entities (cells and/or particles) physically contained within single microbial cells. Endosymbiosis, in the sense of endocytobiosis, is the most intimate form of symbiosis, as defined by one symbiotic partner (endosymbiont) being housed intracellularly within the second symbiotic partner (host). It is theorized that symbiogenesis (endosymbiont theory) was a prominent event in the evolution of the eukaryotic cell through the acquisition of an alphaproteobacterium by a proto-eukaryote, leading to the evolution of the endosymbiont mitochondria (Zimorski et al., 2014; Archibald, 2015; Martin et al., 2015; Roger et al., 2017; Eme et al., 2018). Later on in a mitochondrion-containing eukaryote, cyanobacteria were engulfed and evolved to form chloroplasts (Archibald, 2015; Martin et al., 2015). This engulfment of one cell by another cell, or so-called primary endosymbiosis, is not the end of the story and that is where the matryoshka dolls come into play again. In another theory termed secondary endosymbiosis, resulting products of primary endosymbiosis are then engulfed by a free-living eukaryotic cell. Such engulfment has occurred several times during the evolution of eukaryotes (Zimorski et al., 2014; Archibald, 2015; Martin et al., 2015; Gentil et al., 2017). Within both green and red algae, for example, secondary endosymbiosis gave rise to secondary plastids. Tertiary endosymbioses, the engulfment of an alga containing a secondary plastid, has been shown in dinoflagellates (Bhattacharya et al., 2004). In addition to these rather ancient engulfment events, there are more recent endosymbiotic associations across the tree of life. In protists, endosymbiosis has facilitated the host’s acquisition of expanded metabolic functions for increased biochemical versatility (Nowack and Melkonian, 2010). As such, protist hosts have acquired functions including photosynthesis, nitrogen fixation, methanogenesis and sulfide oxidation through their symbiotic partnerships. A prominent example is the cercozoan amoeba Paulinella chromatophore, which associated with cyanobacterial ancestors to sustain its phototrophic lifestyle (Bhattacharya et al., 2004). However, the nature of most endosymbiotic interactions remains elusive. The intracellular niches that are occupied by endosymbionts are manifold; some examples include the nucleus (Schulz and Horn, 2015), the perinuclear space (Schulz et al., 2015), the endoplasmic reticulum (Vogt, 1992), chloroplasts (Wilcox, 1986) and mitochondria (Sassera et al., 2006; Deeg et al., 2018). In particular, the latter two are great examples for the Matryoshka doll principle, as symbionts acquired relatively recently replicate inside symbionts acquired hundreds of millions of years ago (Fig. 1). A fascinating example of an endosymbiont retained inside of a bacterium is the parasite of Thiotrix (Larkin et al., 1990). In the early 1990s, imaging data enabled the discovery of bacterial parasitic endosymbionts of the sulfideoxidizing gammaproteobacterium Thiothrix; these endosymbionts were contained within either the periplasm or cytoplasm. Surprisingly, nearly 30 years after this discovery, very little information and evidence exists for modern endosymbioses between two non-eukaryotic cells. The few examples that do exist indicate that bacteria are able to successfully enter another bacterial host cell. Considering billions of years of evolution of bacterial cells, which would provide ample time to form stable associations through cell engulfment, this apparent lack of endosymbioses without eukaryotic involvement is astonishing. While the terminology of endosymbiosis applies to nonviral associations only, we also want to discuss other nucleic acid containing intracellular entities, specifically *For correspondence. E-mail [email protected]; Tel. 925-296-5839; Fax 925-927-2554.