Table 2. A summary of modes of symbiont transmission, examples of some bacterial species and the functions they perform for animal hosts, and trends in the reduction of symbiont genomes.

Transmission	Description	Symbiont and function	Host	Genome	References
Environmental	Acquired from free-living bacteria	Luminescence Aliivibrio fischeri Photobacterium leiognathi Photobacterium kishitanii Nutrition "Candidatus Endoriftia persephone" Various Gammaproteobacteria Burkholderia spp.	Fish and squid Fish Fish Tubeworms Mussels Insects	Comprable to free-living relatives	Dunlap and Urbanczyk, 2013; Gyllborg et al., 2012 Urbanczyk et al., 2011; Ast et al., 2007 Li et al., 2018; Kleiner et al., 2012 Ponnudurai et al., 2017 Kikuchi et al., 2005; Kikuchi et al., 2007
Proposed Environmental	Environmentally persistant cells	Luminescence "Candidatus Enterovibrio escacola" "Candidatus Enterovibrio luxaltus"	Anglerfish Anglerfish	Ongoing reduction	Hendry et al., 2018 Hendry et al., 2018
Mixed	Pseudovertical or surface transmission	Luminescence "Candidatus Photodesmus blepharus" "Candidatus Photodesmus katoptron" Nutrition Various Gammaproteobacteria “Candidatus Ishikawaella capsulata”	Flashlight fish Flashlight fish Clams Stink bug	Moderate toextreme reduction	Hendry et al., 2014; Hendry and Dunlap, 2014 Hendry et al., 2014; Hendry and Dunlap, 2014 Kuwahara et al., 2007
Inherited	Direct passage from parent to offspring on egg or sperm	Nutrition Buchnera aphidicola Carsonella ruddii Portiera aleyrodidarum Varied "Candidatus Synechococcus spongiarum"	Aphids Psyllids Whiteflies Sponges	Greatly reduced	Moran et al., 2008; Fisher et al., 2017 Moran et al., 2008; Fisher et al., 2017 Moran et al., 2008; Fisher et al., 2017 Gao et al., 2014; Burgsdorf et al., 2015