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Review
. 2016 Sep 8:70:143-60.
doi: 10.1146/annurev-micro-102215-095325. Epub 2016 Jun 17.

Kin Recognition in Bacteria

Daniel Wall  1
Affiliations
Review

Kin Recognition in Bacteria

Daniel Wall. Annu Rev Microbiol. .

Abstract

The ability of bacteria to recognize kin provides a means to form social groups. In turn these groups can lead to cooperative behaviors that surpass the ability of the individual. Kin recognition involves specific biochemical interactions between a receptor(s) and an identification molecule(s). Recognition specificity, ensuring that nonkin are excluded and kin are included, is critical and depends on the number of loci and polymorphisms involved. After recognition and biochemical perception, the common ensuing cooperative behaviors include biofilm formation, quorum responses, development, and swarming motility. Although kin recognition is a fundamental mechanism through which cells might interact, microbiologists are only beginning to explore the topic. This review considers both molecular and theoretical aspects of bacterial kin recognition. Consideration is also given to bacterial diversity, genetic relatedness, kin selection theory, and mechanisms of recognition.

Keywords: bacteriocin; greenbeard; kin recognition; kin selection; relatedness.

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Figures

Figure 1
Figure 1
Bacterial kin recognition involves three steps.
Figure 2
Figure 2
Core genomes and pan genomes among conspecific strains. (a) Plot of 61 completed Escherichia coli genomes showing a cumulative count of common (core; red) and total (pan; blue) gene families as a function of the number of analyzed genomes. The bars represent the number of unique (new) gene families as genomes are added along the x-axis. Genomes 62–64 are phylogenetically distant strains. Adapted with permission from Reference . (b) Schematic illustration comparing types of genes. The core genome consists of genes shared by conspecific isolates that are presumably important for the survival of the species. Accessory genes provide adaptation advantages for growth under different conditions or in different niches. Strains that share accessory gene sets belong to the same genovar. The social genome (gray) can allow kin recognition and cooperative behaviors; it includes genes that can belong to the core or accessory genome.
Figure 3
Figure 3
Two paths that lead to kin enrichment. The recognition model is based on binding affinities between related cells. The discrimination model is based on antagonism (bacteriocin production). Colors indicate distinct genovars/strains; dashed borders represent cells inhibited by blue cells.
Figure 4
Figure 4
The recognize-and-verify model for kin recognition. The first stage involves receptor-ligand binding. During the second stage (verification), a polymorphic toxin (orange) is delivered to the recognized cell. Clonemates will express a cognate immunity factor and survive, whereas a cell that is not a clonemate will lack immunity and die.
Figure 5
Figure 5
Kin recognition and outer membrane exchange (OME) in myxobacteria. A) A model of OME. Two cells have compatible TraA proteins (red) for recognition, which leads to membrane fusion and bi-directional cell component exchange. Here, the ‘recipient’ lacks a particular outer membrane motility protein and can move only once it receives that protein (blue) from the donor. (b) A phylogenetic tree showing the relatedness of traA alleles from different isolates. The tree contains six functionally distinct recognition groups that are color coded; those in red boxes pertain to panel c, which shows phenotypic assays demonstrating kin (allele)-specific recognition by TraA. Colony edges contain 1:1 mixtures of isogenic donor and recipient strains that contain the indicated traA alleles. All strains contain mutations that block motility; however, motility in the recipients can be restored by OME (see panel a). OME occurs only between identical traA alleles (red boxes). M. fulvus = Myxococcus fulvus. Panels b and c adapted with permission from Reference .
Figure 6
Figure 6
(a) Larger kin groups are less stable. A single individual cannot be socially exploited, whereas a large social group can. Exploitation mechanisms include social cheats and the propagation of a phage infection in a social group. (b) A model suggesting that large social groups are under environmental and exploitation pressures that tend to lead to diversification. In contrast, a collection of smaller groups provides overall population resiliency for two reasons. First, if an exploitation phenotype develops, it is limited to one social group. Second, different groups (genovars; see accessory genes, Figure 2) expand the genetic diversity, which in turn increases the potential, of the entire population, to adapt when faced with changing environmental conditions and competitors. The relative size of each group is dynamic and will temporally fluctuate depending on the conditions. For ecological examples of niche diversity and diversification, see References , , , and .
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