Chapter 5 : Are Species Cohesive?—A View from Bacteriology

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This chapter briefly summarizes the most important differences in genetic exchange. In bacteria, genetic exchange is unidirectional, where a usually small chromosomal segment transfers from a donor to a recipient. Bacterial recombination can occur across vastly more divergent organisms than is possible in animals and plants. The chapter also talks about the challenge of identifying ecotypes, or bacterial species subjected to intrapopulation cohesion provided by periodic selection and/or drift. The Recurrent Niche Invasion model takes into account the role of mobile genetic elements, such as plasmids or phage, in determining bacterial niches. Recombination does not seem likely to be a cohesive force that quashes speciation in either the macroorganisms or bacteria. Most clearly in bacteria, recombination is not sufficient to prevent adaptive divergence in niche-specifying genes, and sexual isolation is not required for bacterial speciation. Studies of speciation in bacteria have focused on the origins of niche-specifying adaptations that distinguish newly divergent species, by investigating the ecological dimensions of speciation and the roles of horizontal genetic transfer and homologous recombination in bacterial speciation. This emphasis on the origins of ecological divergence was forced on bacteriology because bacteria can acquire niche-transcending genes potentially from any organism; so it would be futile to study the end of sharing niche-transcending adaptations in bacteria. It appears that, fortuitously, bacteriology has produced a paradigm of value for species studies in macroorganisms as well as bacteria—that our focus should be on the origins of ecological diversity and not on barriers to recombination.

Citation: Cohan F. 2011. Are Species Cohesive?—A View from Bacteriology, p 43-65. In Walk S, Feng P (ed), Population Genetics of Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555817114.ch5
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Image of Figure 1.
Figure 1.

Three classes of mutation and recombination events that determine ecotype diversity in bacteria. The circles and triangles represent individuals within ecotypes 1 and 2, respectively; the asterisks represent adaptive mutations. (A) Niche-invasion mutations. Here a mutation changes the ecological niche of the cell, such that it can now escape periodic selection events in its former ecotype. This founds a new ecotype. (B) Periodic-selection mutations. These improve the fitness of an individual such that the mutant and its descendants outcompete all other cells within the ecotype; periodic selection events precipitated by these mutations generally do not affect the diversity within other ecotypes, owing to the differences in ecological niche. Periodic selection enhances the distinctness of ecotypes by purging the divergence within but not between ecotypes. (C) Speciation-quashing mutations in the Nano-Niche model. Even if two ecotypes have sustained a history of separate periodic selection events, an extraordinarily adaptive genotype may outcompete another ecotype to extinction. Competitive extinction of another ecotype (ecotype 2) is possible only if all of ecotype 2’s resources are also used by ecotype 1. Used with permission from Landes Bioscience ( ).

Citation: Cohan F. 2011. Are Species Cohesive?—A View from Bacteriology, p 43-65. In Walk S, Feng P (ed), Population Genetics of Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555817114.ch5
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Image of Figure 2.
Figure 2.

The Species-less model of bacterial diversification. In the Species-less model, the diversity within an ecotype is not limited by periodic selection but instead by the short time from the ecotype’s invention as a single mutant until its extinction. Each ecotype is represented in the figure by a unique line style; the origination and extinction of each ecotype are indicated by and , respectively. (A) In the absence of periodic selection, each extant ecotype that has given rise to another ecotype is a paraphyletic group, and each recent ecotype that has not yet given rise to another ecotype is monophyletic. If two closely related ecotypes represent a monophyletic-paraphyletic pair (as in the case of ecotypes D and E, in bold), then we may conclude that a periodic selection event has not occurred in the parental ecotype since the origin of the daughter ecotype. (B) If instead a periodic selection event has occurred in the parental ecotype since the founding of the daughter ecotype, then the parent and daughter ecotypes will be sister monophyletic groups. Observing that pairs of most-closely-related ecotypes usually form monophyletic-paraphyletic pairs would indicate that the origin of new ecotypes is more frequent than periodic selection events in established ecotypes.

Citation: Cohan F. 2011. Are Species Cohesive?—A View from Bacteriology, p 43-65. In Walk S, Feng P (ed), Population Genetics of Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555817114.ch5
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Image of Figure 3.
Figure 3.

Ecological conversion from one plasmid-defined ecotype to another. Each cell is represented by a large circle, and its chromosomal genotype is represented as or . Ecotype 1 is determined by a plasmid represented by a small solid-line circle, and ecotype 2 is determined by a plasmid represented by a small dashed circle. (A) A transfer of an ecotype 1-determining plasmid into a recipient member of ecotype 2 (indicated by the bold circle), along with subsequent loss of the ecotype 2-determining plasmid. (B) The recipient is converted to become a member of ecotype 1, so all of its chromosomal genes are effectively transferred into ecotype 1.

Citation: Cohan F. 2011. Are Species Cohesive?—A View from Bacteriology, p 43-65. In Walk S, Feng P (ed), Population Genetics of Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555817114.ch5
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