Many insects rely on symbiotic microbes for survival, growth, or reproduction. Over evolutionary timescales, the association with intracellular symbionts is stabilized by partner fidelity through strictly vertical symbiont transmission, resulting in congruent host and symbiont phylogenies. However, little is known about how symbioses with extracellular symbionts, representing the majority of insect-associated microorganisms, evolve and remain stable despite opportunities for horizontal exchange and de novo acquisition of symbionts from the environment. Here we demonstrate that host control over symbiont transmission (partner choice) reinforces partner fidelity between solitary wasps and antibiotic-producing bacteria and thereby stabilizes this Cretaceous-age defensive mutualism. Phylogenetic analyses show that three genera of beewolf wasps (Philanthus, Trachypus, and Philanthinus) cultivate a distinct clade of Streptomyces bacteria for protection against pathogenic fungi. The symbionts were acquired from a soil-dwelling ancestor at least 68 million years ago, and vertical transmission via the brood cell and the cocoon surface resulted in host-symbiont codiversification. However, the external mode of transmission also provides opportunities for horizontal transfer, and beewolf species have indeed exchanged symbiont strains, possibly through predation or nest reuse. Experimental infection with nonnative bacteria reveals that-despite successful colonization of the antennal gland reservoirs-transmission to the cocoon is selectively blocked. Thus, partner choice can play an important role even in predominantly vertically transmitted symbioses by stabilizing the cooperative association over evolutionary timescales.protective symbiosis | cospeciation | mutualism stability | Hymenoptera | Crabronidae

Insects engage in mutualistic relationships with a wide variety of microorganisms that are usually transmitted vertically to the next generation. During transmission, the symbiont populations often suffer significant bottlenecks that may entail major genetic and genomic consequences. Here we investigated the life-cycle and the severity of transmission bottlenecks in a symbiotic system with an unusual way of post-hatch vertical transmission by using quantitative PCRs and morphological 3D-reconstructions. European beewolves (Philanthus triangulum, Hymenoptera: Crabronidae) harbor symbiotic bacteria ('Candidatus Streptomyces philanthi') in specialized antennal gland reservoirs and secrete them into their subterranean brood cells. The symbionts are later taken up by the beewolf larva and incorporated into the cocoon material to provide protection against pathogenic microorganisms. Even after months of hibernation, the symbiont population on the cocoon is estimated to encompass around 1.4 9 10 5 cells. However, our results indicate that only few of these bacterial cells (about 9.7 9 10 2 ) are taken up from the cocoon by the emerging female. The symbiont population subsequently undergoes logistic growth within the antennal gland reservoirs and reaches a maximum of about 1.5 9 10 7 cells 3-4 days after emergence. The maximum specific growth rate is estimated to be 0.084-0.105 h -1 . With a total reduction in cell numbers of about 6.7 9 10 -5 during vertical transmission, the symbiont population experiences one of the most severe bottlenecks known for any symbiotic system to date. This extreme bottleneck may have significantly affected the evolution of the beewolf-Streptomyces symbiosis by increased genetic drift, an accumulation of mildly deleterious mutations and genome erosion.

Among horizontally acquired symbioses, the mechanisms underlying microbial strain- and species-level specificity remain poorly understood. Here, confocal-microscopy analyses and genetic manipulation of the squid-vibrio association revealed quantitative differences in a symbiont's capacity to interact with the host during initial engagement. Specifically, dominant strains of Vibrio fischeri, 'D-type', previously named for their dominant, single-strain colonization of the squid's bioluminescent organ, were compared with 'S-type', or 'sharing', strains, which can co-colonize the organ. These D-type strains typically: (i) formed aggregations of 100s-1000s of cells on the light-organ surface, up to 3 orders of magnitude larger than those of S-type strains; (ii) showed dominance in co-aggregation experiments, independent of inoculum size or strain proportion; (iii) perturbed larger areas of the organ's ciliated surface; and, (iv) appeared at the pore of the organ approximately 4×s more quickly than S-type strains. At least in part, genes responsible for biofilm synthesis control the hyperaggregation phenotype of a D-type strain. Other marine vibrios produced relatively small aggregations, while an array of marine Gram-positive and -negative species outside of the Vibrionaceae did not attach to the organ's surface. These studies provide insight into the impact of strain variation on early events leading to establishment of an environmentally acquired symbiosis.