24.1 Introduction
24.1.1 The Holobiont Concept
Multicellular organisms are a complex association of different organisms with
diverse degrees of complexity, formed by the aggregation of a host organism plus
its microbiome. This complexity has been shaped, in part, through the formation of
beneficial associations with symbiotic microbes. The term symbiosis describes any
long-term relationship between organisms of different species, no matter whether
they are mutualists, commensalists or parasitic (Zilber-Rosenberg and Rosenberg
2008; Theis et al. 2016; Roughgarden et al. 2017; Suárez 2018). Permanent symbiotic associations form the so-called holobiont, which necessarily includes either a
microbe or macrobe host (animal or plant) and its set of symbiotic microbes
(bacteria, fungi, viruses, etc.). The sum of the genetic information of the host and
its symbiotic microorganisms define the hologenome (Zilber-Rosenberg and Rosenberg 2008). The hologenome theory assumes the holobiont as a unit element for
selection and evolution, thus natural selection can act to remove deleterious mutations in the host and the associated microbes, while selecting advantageous genetic
variations (|Rosenberg and Zilber-Rosenberg 2016). In evolving holobionts,
symbiogenesis confers cellular-tissue and organ-level development and morphological complexity. In many organisms, microbially produced signals trigger the development of organs (Douglas 2010), as occurs with development of the light organ in
the bobtail squid (McFall-Ngai et al. 2012) as an example. Therefore, host-microbe
interaction within the hologenome context is a key factor for evolution, supported by
an entangled network of communications between holobiont components that are
founded on metabolic and genetic signals (Collens et al. 2019).
Recently, a new perspective has been introduced in which the environmental
microbiome would play an important role in the formation and structure of the hostmicrobial complex (Singh et al. 2020). This eco-holobiont concept proposed that
both biotic (animal and plants) and environmental (soil, water, air, etc.) microbiomes
form a microbial loop that may impact the assembly of holobionts within an
ecosystem (Fig. 24.1). For example, bivalve mollusc associated microbiota are
highly impacted by the microbiome composition of the marine ecosystem. Indeed,
their powerful feeding activity links the molluscs to the surrounding environment,
allowing them to filter large volumes of water while concentrating on different
microorganisms (Kueh and Chan 1985; Prieur et al. 1990), many of which are
ingested as food.
In this chapter, we recapitulate and explore the most relevant symbiotic associations between marine molluscs and microbes and the coevolution of the hostmicrobe complex.
514
A. Lasa and J. L. Romalde
24.1.1 The Holobiont Concept
Multicellular organisms are a complex association of different organisms with
diverse degrees of complexity, formed by the aggregation of a host organism plus
its microbiome. This complexity has been shaped, in part, through the formation of
beneficial associations with symbiotic microbes. The term symbiosis describes any
long-term relationship between organisms of different species, no matter whether
they are mutualists, commensalists or parasitic (Zilber-Rosenberg and Rosenberg
2008; Theis et al. 2016; Roughgarden et al. 2017; Suárez 2018). Permanent symbiotic associations form the so-called holobiont, which necessarily includes either a
microbe or macrobe host (animal or plant) and its set of symbiotic microbes
(bacteria, fungi, viruses, etc.). The sum of the genetic information of the host and
its symbiotic microorganisms define the hologenome (Zilber-Rosenberg and Rosenberg 2008). The hologenome theory assumes the holobiont as a unit element for
selection and evolution, thus natural selection can act to remove deleterious mutations in the host and the associated microbes, while selecting advantageous genetic
variations (|Rosenberg and Zilber-Rosenberg 2016). In evolving holobionts,
symbiogenesis confers cellular-tissue and organ-level development and morphological complexity. In many organisms, microbially produced signals trigger the development of organs (Douglas 2010), as occurs with development of the light organ in
the bobtail squid (McFall-Ngai et al. 2012) as an example. Therefore, host-microbe
interaction within the hologenome context is a key factor for evolution, supported by
an entangled network of communications between holobiont components that are
founded on metabolic and genetic signals (Collens et al. 2019).
Recently, a new perspective has been introduced in which the environmental
microbiome would play an important role in the formation and structure of the hostmicrobial complex (Singh et al. 2020). This eco-holobiont concept proposed that
both biotic (animal and plants) and environmental (soil, water, air, etc.) microbiomes
form a microbial loop that may impact the assembly of holobionts within an
ecosystem (Fig. 24.1). For example, bivalve mollusc associated microbiota are
highly impacted by the microbiome composition of the marine ecosystem. Indeed,
their powerful feeding activity links the molluscs to the surrounding environment,
allowing them to filter large volumes of water while concentrating on different
microorganisms (Kueh and Chan 1985; Prieur et al. 1990), many of which are
ingested as food.
In this chapter, we recapitulate and explore the most relevant symbiotic associations between marine molluscs and microbes and the coevolution of the hostmicrobe complex.
514
A. Lasa and J. L. Romalde