Holobiont
The holobiont concept was initially introduced by the German theoretical biologist Adolf Meyer-Abich in 1943,[3] and then apparently independently by Dr. Lynn Margulis in her 1991 book Symbiosis as a Source of Evolutionary Innovation.
However, a major shift away from holism occurred during the Age of Enlightenment when the dominant thought summarized as "dissection science" was to focus on the smallest component of a system as a means of understanding it.
[17] The idea of holism started to regain popularity in biology when the endosymbiosis theory was first proposed by Konstantin Mereschkowski in 1905 and further developed by Ivan Wallin in 1925.
It was coined independently by the German Adolf Meyer-Abich in 1943,[20][21] and by Lynn Margulis in 1990, who proposed that evolution has worked mainly through symbiosis-driven leaps that merged organisms into new forms, referred to as "holobionts", and only secondarily through gradual mutational changes.
First, the physicochemical properties of water result in higher chemical connectivity and signaling between macro- and micro-organisms in aquatic or moist environments.
In marine ecosystems, carbon fluxes also appear to be swifter and trophic modes more flexible, leading to higher plasticity of functional interactions across holobionts.
[4] Although most work on host-microbe interactions has been focused on animal systems such as corals, sponges, or humans, there is a substantial body of literature on plant holobionts.
[5] Arbuscular mycorrhizal fungi (Glomeromycota), for instance, are common across plant groups and provide improved nutrient acquisition, temperature and drought resistance, and reduced pathogen load.
Conversely, symbionts can buffer stressors via nutrient provision, physiological tolerance, and defense against host natural enemies.
[55] Holobiomics is the scientific analysis of a community of holobionts, which focuses on the interconnections between its components in the context of the prevailing environmental conditions rather than on the individual parts.
The term "holobiomics" is composed of Greek elements όλος (hólos), "all, whole, total", and βίος (bíos), "life", ending on -ome (biome); and the suffix -omics (-ομική, feminine), which identifies subfields of modern biology that aim at the characterisation and quantification of the entirety of similar individual elements in order to draw conclusions about the structure, function, and dynamics of a system.
[60] They argue that "the hologenome concept is unhelpful to the study of host interactions with resident microorganisms because it focuses on one level of selection (the holobiont), and as a result it is concerned with cooperative and integrative features of host-microbe systems to the exclusion of other kinds of interactions, including antagonism among microorganisms and conflicts between host and microbial partners.
"[60] The holobiont and by extension the hologenome concept remain controversial, particularly in regard to the host and its microbiome as a single evolutionary unit.
[61] In order to validate the holobiont concept from an evolutionary perspective, new theoretical approaches are needed that acknowledge the different levels at which natural selection can operate in the context of microbiome-host interactions.
For example, selection could occur at the level of the holobiont when a transgenerational association among specific host and symbiont genotypes can be maintained.
[61] Nevertheless, the holobiont concept has resulted in a shift from the focus on symbioses involving one microbial partner and a single host (squids and luminescent Aliivibrio, legumes and Rhizobium, aphids and Buchnera) toward a greater interest in symbioses in complex multi-partner consortia (animal gut systems, marine invertebrates, plant and seaweed epiphytes, microbe-microbe interactions in soil, aquatic biomes).
[61] Moreover, there is a realization that even the relatively well understood binary symbioses such as aphids and Buchnera are more complex with a number of diverse facultative symbionts contributing to resistance to parasites,[62] expanding host plant usage[63] and temperature adaptation.
