Transposable Elements Mediate Adaptive Debilitation of Flagella in Experimental Escherichia coli Populations.

Although insertion sequence (IS) elements are generally considered genomic parasites, they can mediate adaptive genetic changes in bacterial genomes. We discovered that among 12 laboratory-evolved Escherichia coli populations, three had experienced at least six different IS1-mediated deletions of flagellar genes. These deletions all involved the master flagellar regulator flhDC, and as such completely incapacitate motility.

The fitness effects of a point mutation in Escherichia coli change with founding population density.

Although intraspecific competition plays a seminal role in organismal evolution, little is known about the fitness effects of mutations at different population densities. We identified a point mutation in the cyclic AMP receptor protein (CRP) gene in Escherichia coli that confers significantly higher fitness than the wildtype at low founding population density, but significantly lower fitness at high founding density. Because CRP is a transcription factor that regulates the expression of nearly 500 genes, we compared global gene expression profiles of the mutant and wildtype strains.

Insertion sequence distribution bias in Archaea.

Insertion sequences (IS) are common transposable elements in Archaea. Intergenic IS elements are usually less harmful than intragenic ISs, simply because they are less likely to disrupt host gene function. However, because regulatory sequences are intergenic and upstream of genes, we hypothesized that not all intergenic regions are selectively equivalent for IS insertion.

Relaxed natural selection alone does not permit transposable element expansion within 4,000 generations in Escherichia coli.

Insertion sequences (ISs) are transposable genetic elements in bacterial genomes. IS elements are common among bacteria but are generally rare within free-living species, probably because of the negative fitness effects they have on their hosts. Conversely, ISs frequently proliferate in intracellular symbionts and pathogens that recently transitioned from a free-living lifestyle.

Intergenic transposable elements are not randomly distributed in bacteria.

Insertion sequences (ISs) are mobile genetic elements in bacterial genomes. In general, intergenic IS elements are probably less deleterious for their hosts than intragenic ISs, simply because they have a lower likelihood of disrupting native genes. However, since promoters, Shine-Dalgarno sequences, and transcription factor binding sites are intergenic and upstream of genes, I hypothesized that not all neighboring gene orientations (NGOs) are selectively equivalent for IS insertion.

Transposable element loads in a bacterial symbiont of weevils are extremely variable.

Not only are transposable elements profuse in the bacterial endosymbiont of maize weevils, but we found that their quantities also vary approximately 10-fold among individual weevils. Because multicopy elements can facilitate homologous recombination, this insertion sequence (IS) load variability suggests that these essentially asexual bacteria may exhibit substantial intraspecific genomic variation.

Extensive proliferation of transposable elements in heritable bacterial symbionts.

We found that insertion sequence (IS) elements are unusually abundant in the relatively recently evolved bacterial endosymbionts of maize weevils. Because multicopy elements can facilitate genomic recombination and deletion, this IS expansion may represent an early stage in the genomic reduction that is common in most ancient endosymbionts.

Genomic changes following host restriction in bacteria.

Many genomic sequences have been recently published for bacteria that can replicate only within eukaryotic hosts. Comparisons of genomic features with those of closely related bacteria retaining free-living stages indicate that rapid evolutionary change often occurs immediately after host restriction. Typical changes include a large increase in the frequency of mobile elements in the genome, chromosomal rearrangements mediated by recombination among these elements, pseudogene formation, and deletions of varying size.

Hatching asynchrony, survival, and the fitness of alternative adult morphs in Ambystoma talpoideum.

The mole salamander, Ambystoma talpoideum, exhibits both aquatic (gilled) and terrestrial (metamorphosed) adult morphologies. Previous studies have shown the existence of body-size advantages associated with the terrestrial morph in A. talpoideum and other polymorphic salamanders (e.

Low and homogeneous copy number of plasmid-borne symbiont genes affecting host nutrition in Buchnera aphidicola of the aphid Uroleucon ambrosiae.

The bacterial endosymbiont of aphids, Buchnera aphidicola, often provides amino acids to its hosts. Plasmid amplification of leucine (leuABCD) and tryptophan (trpEG) biosynthesis genes may be a mechanism by which some Buchnera over-produce these nutrients. We used quantitative polymerase chain reaction to assess the leuABCD/trpEG copy variability within Uroleucon ambrosiae, an aphid with a wide diet breadth and range.

Type III secretion systems and the evolution of mutualistic endosymbiosis.

The view that parasites can develop cooperative symbiotic relationships with their hosts is both appealing and widely held; however, there is no molecular genetic evidence of such a transition. Here we demonstrate that a mutualistic bacterial endosymbiont of grain weevils maintains and expresses inv/spa genes encoding a type III secretion system homologous to that used for invasion by bacterial pathogens. Phylogenetic analyses indicate that inv/spa genes were present in presymbiotic ancestors of the weevil endosymbionts, occurring at least 50 million years ago.