Status lab-led for Varble 2014. McCune 2020 is collaborative and led by the Pfeiffer laboratory at the University of Texas Southwestern, where all mouse work was done, and Muñoz-Moreno 2019 is collaborative and led by the García-Sastre laboratory, with the tenOever contribution in both being the barcoded library method and supervision Areas Viral Populations, Evolution and Transmission, Influenza Genome Regulation and Replication
Bottleneck size had mostly been inferred from consensus sequence divergence or from surveys of natural infection, which cannot separate a restriction on which genomes enter from one acting later, and cannot say whether survivors survive because of what they encode or by chance. Varble 2014 solved the measurement problem by building more than one hundred influenza A viruses differing only by a 22 nucleotide barcode carried in the engineered intergenic region of a split NS segment, and by testing the neutrality assumption against wild type growth curves, individually amplified clones and duplicate sequencing before drawing conclusions from it.
Passing that library through a graded series of settings gave directly comparable numbers. Canine and human cell monolayers imposed no detectable bottleneck. Embryonated eggs collapsed the library to between five and thirteen clones. Guinea pig recipients infected across cages carried two to five clones, ferret contact recipients carried seven to twenty four against seventy one to one hundred in donors, and airborne transmission reduced the recovered population to as few as two barcodes. Two further results carry more weight than the counts. Egg passage was accompanied by convergent hemagglutinin change at residues associated with avian receptor specificity while mammalian transmission showed no such convergence, which separates genetically driven selection during host adaptation from sequence-independent sampling. And three guinea pigs cocaged with one donor all became infected and all carried different barcode sets despite identical exposure, which places the restriction at the recipient. Transmission probability correlated with a clone's abundance in donor nasal wash and not in donor bronchus.
The consequence for risk assessment is stated in the paper. If airborne transmission can be founded by as few as two genomes, a phenotype requiring several mutations and present at low frequency in a donor is unlikely to be carried intact into a new host, which Varble 2014 connects to why an airborne transmissible H5N1 has not emerged despite repeated human infections. Two extensions were led elsewhere. McCune 2020 recovered all 135 barcodes of a coxsackievirus B3 library from the upper gastrointestinal tract at 7.5 and 19 hours and found three or fewer barcodes dominating every tissue by 48 to 72 hours, using neutral red labelling to show that diverse members had genuinely replicated first, so the loss of diversity in that system is a post-replication event rather than a gate at the barrier. Muñoz-Moreno 2019 competed 107 barcoded viruses carrying 56 natural NS1 sequences in a common backbone across dog cells, human cells, eggs and mice and found allele B overrepresented in every substrate, clusters sharing fitness profiles despite substantial amino acid divergence, and Stat1 deficiency flattening much of the spread where Rag1 deficiency did not, so phylogenetic position predicts phenotype poorly for this gene and the selective filter sits in early innate signalling.
What is not established is the barrier in the recipient that performs the sampling, which no publication in this corpus identifies, and Varble 2014 leaves open whether an additional selection step occurs at secretion. How fitness is maintained across repeated severe bottlenecks is unresolved. Muñoz-Moreno 2019 establishes no molecular mechanism for any individual fitness difference. Every measurement rests on engineered viruses carrying inserted sequence in cultured cells and animal models, so generalisation to unmodified viruses in natural infection is an extrapolation. Within this laboratory the transmission bottleneck work was not returned to, and the honest description is that the method travelled further than the question did.
Substantiated by - Influenza A Virus Transmission Bottlenecks Are Defined by Infection Route and Recipient Host, measures bottleneck size across routes and recipients and locates the restriction in the recipient rather than in viral genetics - Rapid Dissemination and Monopolization of Viral Populations in Mice Revealed Using a Panel of Barcoded Viruses, led by the Pfeiffer laboratory, shows that diversity in an enteric infection collapses after replication rather than at the barrier - Viral Fitness Landscapes in Diverse Host Species Reveal Multiple Evolutionary Lines for the NS1 Gene of Influenza A Viruses, led by the García-Sastre laboratory, shows that phylogenetic proximity predicts NS1 phenotype poorly and places the selective filter in innate signalling
A library of 135 barcoded coxsackievirus B3 clones shows that orally inoculated virus reaches systemic tissues within 20 minutes and replicates as a diverse population, after which fewer than three variants come to dominate every tissue in the animal without any detectable adaptive mutation.
A library of 107 barcoded influenza A viruses differing only in their NS1 sequence, competed in dog cells, human cells, chicken eggs and mice, resolves NS1-driven fitness as a set of divergent and partly convergent evolutionary trajectories rather than a single ordered adaptation gradient.
Genetically barcoded influenza A virus libraries tracked through cell culture, embryonated eggs, guinea pigs, ferrets and mice show that transmission bottlenecks differ by route and recipient, with airborne transmission reducing a diverse inoculum to as few as two founder clones.