Infections by parasitic nematodes inflict a huge burden on the health of humans and livestock throughout the world. Anthelmintic drugs are the first line of defense against these infections. Unfortunately, resistance to these drugs is rampant and continues to spread. To improve treatment strategies, we must understand the genetics and molecular mechanisms that underlie resistance. Studies of the fungus Aspergillus nidulans and the free-living nematode Caenorhabditis elegans discovered that a beta-tubulin gene is mutated in benzimidazole (BZ) resistant strains. In parasitic nematode populations, three beta-tubulin alleles, F167Y, E198A, and F200Y, have long been correlated with resistance. Additionally, improvements in sequencing technologies have identified new alleles - E198V, E198L, E198K, E198I, and E198Stop - also correlated with BZ resistance. However, none of these alleles have been proven to cause resistance. To empirically demonstrate this point, we independently introduced the F167Y, E198A, and F200Y alleles as well as two of the newly identified alleles, E198V and E198L, into the BZ susceptible C. elegans N2 genetic background using the CRISPR-Cas9 system. These genome-edited strains were exposed to both albendazole and fenbendazole to quantitatively measure animal responses to BZs. We used a range of concentrations for each BZ compound to define response curves and found that all five of the alleles conferred resistance to BZ compounds equal to a loss of the entire beta-tubulin gene. These results prove that the parasite beta-tubulin alleles cause resistance. The E198V allele is found at low frequencies along with the E198L allele in natural parasite populations, suggesting that it could affect fitness. We performed competitive fitness assays and demonstrated that the E198V allele reduces animal health, supporting the hypothesis that this allele might be less fit in field populations. Overall, we present a powerful platform to quantitatively assess anthelmintic resistance and effects of specific resistance alleles on organismal fitness in the presence or absence of the drug.

Evolutionary life history theory seeks to explain how reproductive and survival traits are shaped by selection through allocations of an individual’s resources to competing life functions. Although life-history traits evolve rapidly, little is known about the genetic and cellular mechanisms that control and couple these tradeoffs. Here, we find that two laboratory-adapted strains of C. elegans descended from a single common ancestor that lived in the 1950s have differences in a number of life-history traits, including reproductive timing, lifespan, dauer formation, growth rate, and offspring number. We identified a quantitative trait locus (QTL) of large effect that controls 24%–75% of the total trait variance in reproductive timing at various timepoints. Using CRISPR/Cas9-induced genome editing, we show this QTL is due in part to a 60 bp deletion in the 3’ end of the nurf-1 gene, which is orthologous to the human gene encoding the BPTF component of the NURF chromatin remodeling complex. Besides reproduction, nurf-1 also regulates growth rate, lifespan, and dauer formation. The fitness consequences of this deletion are environment specific—it increases fitness in the growth conditions where it was fixed but decreases fitness in alternative laboratory growth conditions. We propose that chromatin remodeling, acting through nurf-1, is a pleiotropic regulator of life history trade-offs underlying the evolution of multiple traits across different species.

The standard reference Caenorhabditis elegans strain, N2, has evolved marked behavioral changes in social feeding behavior since its isolation from the wild. We show that the causal, laboratory-derived mutations in two genes, npr-1 and glb-5, confer large fitness advantages in standard laboratory conditions. Using environmental manipulations that suppress social/solitary behavior differences, we show the fitness advantages of the derived alleles remained unchanged, suggesting selection on these alleles acted through pleiotropic traits. Transcriptomics, developmental timing, and food consumption assays showed that N2 animals mature faster, produce more sperm, and consume more food than a strain containing ancestral alleles of these genes regardless of behavioral strategies. Our data suggest that the pleiotropic effects of glb-5 and npr-1 are a consequence of changes to O2 -sensing neurons that regulate both aerotaxis and energy homeostasis. Our results demonstrate how pleiotropy can lead to profound behavioral changes in a popular laboratory model.

Dear Editor,CRISPR interference (CRISPRi) is a recently developed tool used to study single guide RNA (gRNA)-mediated sequence-specific repression of transcription in both prokaryotic and eukaryotic cells [1][2][3][4][5]. Transcription initiation and elongation of a gene can be interfered by the presence of gRNA:DNA hetero-duplex/dCas9 (a catalytically inactive form of Cas9) complex in its promoter and exons. If Krüppel associated box (KRAB) domain is fused to dCas9, repression of target gene is more efficient. Likewise, fusion of a transcription activator such as VP16 (CRISPR-on) can increase target gene expression through three to four gene-specific gRNAs (up to seven) that recognize the proximal promoter of a target gene in cultured cells and in vivo [2]. We applied both CRISPRi and CRISPR-on tools in worm and zebrafish, and demonstrated here that our dCas9 fusion systems modify gene expression at or near their endogenous expression location(s) through target-specific gRNAs (ts-gRNAs).We fused dCas9 to the KRAB domain (repressor) or the VP160 domain containing 10 tandem VP16 units (activator; Figure 1A). In C. elegans, dCas9 constructs were driven by tissue-specific promoters while ts-gRNAs were under control of a U6-type RNA polymerase III promoter ( Figure 1B). Different combinations of the DNA constructs were injected into adult hermaphrodite gonads to generate transgenic worms. For zebrafish experiments, dCas9-KRAB and dCas9-VP160 mRNAs and ts-gRNAs were synthesized in vitro and injected into one-cell stage embryos ( Figure 1C), and the resulted embryos were analyzed by real-time RT-PCR and in situ hybridization. Flanking sequences, including the nuclear localization sequences (NLSs) and UTRs, were indicated in Figure 1B and 1C or described previously [6].Decrease in dpy-5 expression levels produces short body and dumpy phenotype, known as Dpy. Dbl-1 is a TGF-β family member expressed primarily in neurons and regulates lon-1 to adjust body length. When dbl-1 is overexpressed, worm body length is elongated (Lon phenotype). We targeted dpy-5 by selecting seven ts-gRNAs that recognize its non-template DNA strand of