Deleterious mutations are of fundamental importance to all aspects of organismal biology. Evolutionary geneticists have expended tremendous effort to estimate the genome-wide rate of mutation and the effects of new mutations on fitness, but the degree to which genomic mutational properties vary within and between taxa is largely unknown, particularly in multicellular organisms. Beginning with two highly inbred strains from each of three species in the nematode family Rhabditidae (Caenorhabditis briggsae, Caenorhabditis elegans, and Oscheius myriophila), we allowed mutations to accumulate in the relative absence of natural selection for 200 generations. We document significant variation in the rate of decay of fitness because of new mutations between strains and between species. Estimates of the per-generation mutational decay of fitness were very consistent within strains between assays 100 generations apart. Rate of mutational decay in fitness was positively associated with genomic mutation rate and negatively associated with average mutational effect. These results provide unambiguous experimental evidence for substantial variation in genome-wide properties of mutation both within and between species and reinforce conclusions from previous experiments that the cumulative effects on fitness of new mutations can differ markedly among related taxa.Caenorhabditis briggsae ͉ Caenorhabditis elegans ͉ deleterious mutation ͉ mutation accumulation ͉ Oscheius myriophila F ew topics in evolutionary biology have generated as much controversy in recent years as spontaneous mutation, considered at the level of the entire genome (1-4). It is widely accepted that the great majority of new mutations are either neutral or deleterious (refs. 1 and 5-7; also see refs. 3 and 8-13), and the importance of deleterious mutations to a wide variety of biological processes and phenomena is well appreciated (7). The source of the controversy stems from uncertainty in the rate at which new mutations accrue in the genome and the distribution of effects of those mutations on fitness.Beginning in the early 1960s, Mukai, Ohnishi, and their colleagues (14-18) conducted several large mutation accumulation (MA) experiments with Drosophila melanogaster that were designed to estimate the rate and average effect of new mutations on fitness. By the early 1970s, the results seemed clear: averaged over experiments, egg-to-adult viability declined rapidly, on the order of 1% per genome per generation, with the implication that the diploid genomic mutation rate (U) was on the order of 0.6 per generation or greater and the average homozygous effect on fitness of a new mutation (2a ) was Ϸ0.05 or less. The typical newborn fly was thus expected to harbor one new mutation that could be expected to reduce its viability by Ϸ5% when homozygous.Several additional lines of evidence supported the results from the early f ly MA experiments. First, the considerable inbreeding depression in Drosophila implied either a high rate of deleterious mutation or considerable...

Summary The pathogenic Yersinia species share a conserved type III secretion system, which delivers cytotoxic effectors known as Yops into target mammalian cells. In all three species, YopK (also called YopQ) plays an important role in regulating this process. In cell culture infections, yopK mutants inject higher levels of Yops, leading to increase cytotoxicity; however, in vivo the same mutants are highly attenuated. In this work, we investigate the mechanism behind this paradox. Using a β-lactamase reporter assay to directly measure the effect of YopK on translocation, we demonstrated that YopK controls the rate of Yop injection. Furthermore, we find that YopK cannot regulate effector Yop translocation from within the bacterial cytosol. YopE is also injected into host cells and was previously shown to contribute to regulation of the injectisome. In this work we show that YopK and YopE work at different steps to regulate Yop injection, with YopK functioning independently of YopE. Finally, by expressing YopK within tissue culture cells, we confirm that YopK regulates translocation from inside the host cell, and we show that cells pre-loaded with YopK are resistant to Yop injection. These results suggest a novel role for YopK in controlling the Yersinia type III secretion system.

Yersinia pestis, the causative agent of bubonic and pneumonic plague, is typically a zoonotic vector-borne disease of wild rodents. Bacterial biofilm formation in the proventriculus of the flea contributes to chronic infection of fleas and facilitates efficient disease transmission. However prior to biofilm formation, ingested bacteria must survive within the flea midgut, and yet little is known about vector-pathogen interactions that are required for flea gut colonization. Here we establish a Drosophila melanogaster model system to gain insight into Y. pestis colonization of the insect vector. We show that Y. pestis establishes a stable infection in the anterior midgut of fly larvae, and we used this model system to study the roles of genes involved in biofilm production and/or resistance to gut immunity stressors. We find that PhoP and GmhA both contribute to colonization and resistance to antimicrobial peptides in flies, and furthermore, the data suggest biofilm formation may afford protection against antimicrobial peptides. Production of reactive oxygen species in the fly gut, as in fleas, also serves to limit bacterial infection, and OxyR mediates Y. pestis survival in both insect models. Overall, our data establish the fruit fly as an informative model to elucidate the relationship between Y. pestis and its flea vector.