Physiological differentiation during symbiosis leads to division of labor between smaller and larger cells in an uncultured bacterial tubeworm symbiont population and results in remarkable metabolic diversity and complexity.

Beneficial symbiotic bacteria encode an exceptional number of toxin-related genes that are all expressed by the symbionts in the host, supporting their key role in host-microbe interactions.

Environmental transmission is atypical of symbionts that have undergone genome degradation, yet genetically reduced deep-sea anglerfish symbionts likely persist in the deep sea biome in search of a new host.

Genes for adhesion, interbacterial competition, and secondary metabolite production are important colonization factors of the honey bee gut symbiont Frischella perrara and are regulated by the conserved histone-like DNA-binding protein integration host factor.

Morphological and fitness defects imposed on amoebae hosts by Burkholderia symbionts demonstrates symbiont species-specific effects and provides evidence of host adaptation to naturally acquired symbionts.

The colonization of corals and their relatives by intracellular microalgae is facilitated by immunity proteins in the animal that contain thrombospondin-type-1 repeats, elucidating the inter-partner recognition processes required for the establishment of this ecologically important symbiosis.

Insect symbiotic bacteria regulate expression of an odorant binding protein that mediates an evolutionarily conserved hematopoietic pathway crucial for host immune system development and function.

Bleaching-resistant corals are widespread across Palau and concentrated in warmer regions, but this trait is associated with decreased growth which could lead to reduced fitness if it is the only priority of conservation efforts.

A cellular map of a deep-sea mussel gill unveils how distinct cell types collaborate to cultivate intracellular symbiotic bacteria, providing insights into host–symbiont interactions and environmental adaptation in bivalves.

This review serves as a gateway to experimental, conceptual, and quantitative themes of cytoplasmic incompatibility (CI) and outlines significant gaps in the understanding of CI's mechanism that are ripe for investigation.