Phadke, S. | Search bioRxiv

bioRxiv2024

Live-tracking of replisomes reveals nutrient-dependent regulation of replication elongation rates in Caulobacter crescentus

In bacteria, commitment to genome replication (initiation) is intricately linked to nutrient availability. Whether growth conditions affect other stages of replication beyond initiation, remains to be systematically studied. To address this, we assess the replication dynamics of Caulobacter crescentus, a bacterium that undergoes only single round of replication per cell cycle, by tracking the replisome across various growth phases and nutrient conditions. We find that the replication elongation rates slowdown as cells transition from exponential (high-nutrient) to stationary (low-nutrient) phase, and this contributes significantly to the overall cell cycle delay. While elongation rates are correlated with growth rates, both properties appear to be differentially influenced by nutrient status. This slowdown in replication progression is not associated with increased mutagenesis or upregulation of the DNA damage responses. We conclude that the growth conditions not only dictate the commitment to replication but also the rates of genome duplication. Such regulation appears to be distinct from stress-induced replication slowdown and likely serves as an adaptive mechanism to cope with fluctuations in nutrient availability in the environment.

microbiology↗

bioRxiv2017

Understanding the evolution of anisogamy in the early diverging fungus, Allomyces

Gamete size dimorphism between sexes (anisogamy) is predicted to have evolved from an isogamous system in which sexes have equal-sized, monomorphic gametes. Although adaptive explanations for the evolution of anisogamy abound, we lack comparable insights into molecular changes that bring about the transition from monomorphism to dimorphism. The basal fungal clade Allomyces provides unique opportunities to investigate genomic changes that are associated with this transition in closely related species that show either isogamous or anisogamous mating systems. The anisogamous species show sexual dimorphism in gamete size, number, pigmentation and motility. We sequenced transcriptomes of five Allomyces isolates representing the two mating systems, including both male and female phenotypes in the anisogamous species. Maximum likelihood ancestral character state reconstruction performed in MESQUITE using the de-novo assembled transcriptomes indicated that anisogamy likely evolved once in Allomyces, and is a derived character as predicted in theory. We found that sexual stages of Allomyces express homologs of several genes known to be involved in sex determination in model organisms including Drosophila and humans. Furthermore, expression of CatSper homologs in male- and female-biased samples in our analysis support the hypothesis that gamete interaction in the anisogamous species of Allomyces may involve similar molecular events as the egg-sperm interaction in animals, including humans. Although the strains representing either mating system shared much of the transcriptome, supporting recent common ancestry, the analysis of rate of evolution using individual gene trees indicates high substitution rates and divergence between the strains. In summary, we find that anisogamy likely evolved once in Allomyces, using convergent mechanisms to those in other taxa.

evolutionary biology↗