Abstract
In some species of myxobacteria, adjacent cells sufficiently similar at the adhesin protein TraA can exchange components of their outer membranes. The primary benefits of such outer membrane exchange (OME) in natural populations are unclear, but in some OME interactions, transferred OM content can include SitA toxins that kill OME participants lacking an appropriate immunity gene. Such OME-dependent toxin transfer across Myxococcus xanthus strains that differ only in their sitBAI toxin/antitoxin cassette can mediate inter-strain killing and generate colony-merger incompatibilities (CMIs)–inter-colony border phenotypes between distinct genotypes that differ from respective self-self colony interfaces. Here we ask whether OME-dependent toxin transfer is a common cause of prevalent CMIs and antagonisms between M. xanthus natural isolates identical at TraA. We disrupted traA in eleven isolates from a cm-scale soil population and assayed whether traA disruption eliminated or reduced CMIs between swarming colonies or antagonisms between strains in mixed cultures. Among 33 isolate pairs identical at traA that form clear CMIs, in no case did functional disruption of traA in one partner detectably alter CMI phenotypes. Further, traA disruption did not alleviate strong antagonisms observed during starvation-induced fruiting-body development in seven pairs of strains identical at traA. Collectively, our results suggest that most mechanisms of interference competition and inter-colony kin discrimination in natural populations of myxobacteria do not require OME. Finally, our experiments also indicate that several closely related laboratory reference strains kill some natural isolates by toxins delivered by a shared, OME-independent type VI secretion system (T6SS), suggesting that some antagonisms between sympatric natural isolates may also involve T6SS toxins.
Highlights
Kin discrimination among microbes can be defined phenomenologically as differential expression of behaviour across interactants as a function of genetic relatedness [1,2,3]
Because i) strain DK1622 is killed by DK101 due to outer membrane exchange (OME)-mediated toxin delivery [46], ii) the genome sequence of GJV1 is identical to the published DK1622 sequence excepting five mutations [51], and iii) disruption of both traA and traB alleviate DK101 inhibition of GJV1 swarming (Fig 1), we infer that DK101 inhibits GJV1 swarming by the same OME-delivered toxin that inhibited swarming by the version of DK1622 used by Dey et al (2016) [46]
The occurrence of five predicted TraA compatibility groups in a cm-scale M. xanthus population [3] suggests that a large fraction of randomly selected isolate pairs will be incompatible for OME due to functional dissimilarity at the PA14 domain of TraA
Summary
Kin discrimination among microbes can be defined phenomenologically as differential expression (or effects) of behaviour across interactants as a function of genetic relatedness [1,2,3]. Kin discrimination and outer membrane exchange in Myxococcus xanthus to which victim cells are not [4, 5] and preferential co-aggregation with genetically similar cells [1, 6]. Because evolutionary forces other than kin selection can cause kin-discriminatory behaviors to evolve [1, 17], the ultimate causes of such behaviors among naturally-evolved microbes are often unclear
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