The perpetuation of symbioses through host generations relies on symbiont transmission. Horizontally transmitted symbionts are taken up from the environment anew by each host generation, and vertically transmitted symbionts are most often transferred through the female germ line. Mixed modes also exist. In this Review we describe the journey of symbionts from the initial contact to their final residence. We provide an overview of the molecular mechanisms that mediate symbiont attraction and accumulation, interpartner recognition and selection, as well as symbiont confrontation with the host immune system. We also discuss how the two main transmission modes shape the evolution of the symbiotic partners.Symbiont transmission maintains symbioses through host generations and has a pivotal role in their evolution 1 -5 . Despite growing evidence that microbial associations are present in diverse animals and plants in virtually every environment, our knowledge of the mechanisms of symbiont transmission is restricted to a few model systems. Most of these involve bacteria and some archaea 6 , 7 (Supplementary information S1 (table)). Two fundamentally different modes of transmission can be distinguished: horizontal (that is, from an environmental, free-living symbiont source) and vertical (that is, inheritance of the symbiont from the mother or, more rarely, from both parents). However, there is great variation, and transmission can also be mixed, involving both vertical and horizontal transfers from the environment and intraspecific or interspecific host switching (FIG. 1).This Review focuses on those associations that maintain protracted physical contact and involve most of the host population. We use de Bary's definition of symbiosis: living together of differently named organisms 8 , irrespective of the effects of the interaction on the fitness of the partners 9 . The key question is how the symbiont is transferred to the host progeny, regardless of the type of symbiosis. Reviews are already available for a range of symbiotic systems: plants 10 -16 , sponges 17 , 18 , chemosynthetic bacteria-marine invertebrates 19 , 20 , entomopathogenic nematodes 21 , 22 , annelids 23 , 24 , insects 25 -30 , squid 31 , 32 © 2010 Macmillan Publishers Limited. All rights reserved monika.bright@univie.ac.at; silvia.bulgheresi@univie.ac.at. Competing interests statementThe authors declare no competing financial interests. 33 -36 . Here, we review how the conversation between partners, which ultimately integrates the symbiont into the host's life cycle, is initiated. By identifying the similarities and differences between the two principal types of symbiont transmission, we explore their evolutionary implications. DATABASES Horizontal transmissionThe cyclic occurrence of aposymbiotic (that is, before the symbiont is acquired) and symbiotic phases in the host's life cycle is intrinsic to horizontally transmitted symbionts (Boxes 1,2; FIG. 2) (Supplementary information S1 (table)). In animal hosts, the aposymbiotic phase inc...

Transmission of obligate bacterial symbionts between generations is vital for the survival of the host. Although the larvae of certain hydrothermal vent tubeworms (Vestimentifera, Siboglinidae) are symbiont-free and possess a transient digestive system, these structures are lost during development, resulting in adult animals that are nutritionally dependent on their bacterial symbionts. Thus, each generation of tubeworms must be newly colonized with its specific symbiont. Here we present a model for tubeworm symbiont acquisition and the development of the symbiont-housing organ, the trophosome. Our data indicate that the bacterial symbionts colonize the developing tube of the settled larvae and enter the host through the skin, a process that continues through the early juvenile stages during which the trophosome is established from mesodermal tissue. In later juvenile stages we observed massive apoptosis of host epidermis, muscles and undifferentiated mesodermal tissue, which was coincident with the cessation of the colonization process. Characterizing the symbiont transmission process in this finely tuned mutualistic symbiosis provides another model of symbiont acquisition and additional insights into underlying mechanisms common to both pathogenic infections and beneficial host-symbiont interactions.

BackgroundWe studied the meiofauna community at deep-sea hydrothermal vents along a gradient of vent fluid emissions in the axial summit trought (AST) of the East Pacific Rise 9°50′N region. The gradient ranged from extreme high temperatures, high sulfide concentrations, and low pH at sulfide chimneys to ambient deep-sea water conditions on bare basalt. We explore meiofauna diversity and abundance, and discuss its possible underlying ecological and evolutionary processes.Methodology/Principal FindingsAfter sampling in five physico-chemically different habitats, the meiofauna was sorted, counted and classified. Abundances were low at all sites. A total of 52 species were identified at vent habitats. The vent community was dominated by hard substrate generalists that also lived on bare basalt at ambient deep-sea temperature in the axial summit trough (AST generalists). Some vent species were restricted to a specific vent habitat (vent specialists), but others occurred over a wide range of physico-chemical conditions (vent generalists). Additionally, 35 species were only found on cold bare basalt (basalt specialists). At vent sites, species richness and diversity clearly increased with decreasing influence of vent fluid emissions from extreme flow sulfide chimney (no fauna), high flow pompei worm (S: 4–7, H'loge: 0.11–0.45), vigorous flow tubeworm (S: 8–23; H'loge: 0.44–2.00) to low flow mussel habitats (S: 28–31; H'loge: 2.34–2.60).Conclusions/SignificanceOur data suggest that with increasing temperature and toxic hydrogen sulfide concentrations and increasing amplitude of variation of these factors, fewer species are able to cope with these extreme conditions. This results in less diverse communities in more extreme habitats. The finding of many species being present at sites with and without vent fluid emissions points to a non endemic deep-sea hydrothermal vent meiofaunal community. This is in contrast to a mostly endemic macrofauna but similar to what is known for meiofauna from shallow-water vents.

Zoothamnium niveum is a giant, colonial marine ciliate from sulfide-rich habitats obligatorily covered with chemoautotrophic, sulfide-oxidizing bacteria which appear as coccoid rods and rods with a series of intermediate shapes. Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization showed that the ectosymbiont of Z. niveum belongs to only one pleomorphic phylotype. The Z. niveum ectosymbiont is only moderately related to previously identified groups of thiotrophic symbionts within the Gammaproteobacteria, and shows highest 16S rRNA sequence similarity with the free-living sulfur-oxidizing bacterial strain ODIII6 from shallowwater hydrothermal vents of the Mediterranean Sea (94.5%) and an endosymbiont from a deep-sea hydrothermal vent gastropod of the Indian Ocean Ridge (93.1%). A replacement of this specific ectosymbiont by a variety of other bacteria was observed only for senescent basal parts of the host colonies. The taxonomic status "Candidatus Thiobios zoothamnicoli" is proposed for the ectosymbiont of Z. niveum based on its ultrastructure, its 16S rRNA gene, the intergenic spacer region, and its partial 23S rRNA gene sequence.

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