Jeremy M. Chacón

Density-dependent intraguild predation of an aphid parasitoid

A growing body of research has examined the effect of shared resource density on intraguild predation (IGP) over relatively short time frames. Most of this work has led to the conclusion that when the shared resource density is high, the strength of IGP should be lower, due to prey dilution. However, experiments addressing this topic have been done using micro- or mesocosms that excluded the possibility of intraguild predator aggregation. We examined the effect of shared resource density on IGP of an aphid parasitoid in an open field setting where the effects of prey dilution and predator aggregation could occur simultaneously. We brought potted soybean plants with 2, 20, or 200 soybean aphids (Aphis glycines) and 20 pupae (‘mummies’) of the soybean aphid parasitoid Binodoxys communis into soybean fields in Minnesota, USA. We monitored predator aggregation onto the potted plants, predation of parasitoid mummies, and successful adult emergence of B. communis. We found that predator aggregation was higher at the higher aphid densities on our experimental plants and that this coincided with lower adult emergence of B. communis, indicating that even if a prey dilution effect occurred in our study, it was overcome by short-term predator aggregation. Our results suggest that the effect of shared resource density on IGP may be more nuanced in a field setting than in microcosms due to predator aggregation.

Pericentromere tension is self-regulated by spindle structure in metaphase

Pericentromere tension in yeast is substantial and is tightly self-regulated by the metaphase mitotic spindle through adjustments in spindle structure.

A noncatalytic function of the topoisomerase II CTD in Aurora B recruitment to inner centromeres during mitosis

The C-terminal domain (CTD) of Topo II is dispensable for its catalytic activity yet essential for Topo II function in chromosome segregation during mitosis. Here, Edgerton et al. resolve the role of the Topo II CTD during mitosis in yeast, showing that it functions noncatalytically via the Haspin-H3 T3-Phos pathway to recruit Ipl1/Aurora B to mitotic inner centromeres.

Monopolin recruits condensin to organize centromere DNA and repetitive DNA sequences.

Higher-order structure of chromatin is essential for chromosome segregation and repetitive DNA stability. Monopolin recruits condensin to organize centromere DNA irrespective of the number of kinetochore–microtubule attachments. In addition, the role of monopolin in stabilizing repeat tracts observed in budding yeast is conserved in Candida albicans.

Engineering species-like barriers to sexual reproduction

Controlling the exchange of genetic information between sexually reproducing populations has applications in agriculture, eradication of disease vectors, control of invasive species, and the safe study of emerging biotechnology applications. Here we introduce an approach to engineer a genetic barrier to sexual reproduction between otherwise compatible populations. Programmable transcription factors drive lethal gene expression in hybrid offspring following undesired mating events. As a proof of concept, we target the ACT1 promoter of the model organism Saccharomyces cerevisiae using a dCas9-based transcriptional activator. Lethal overexpression of actin results from mating this engineered strain with a strain containing the wild-type ACT1 promoter.Genetic isolation of a genetically modified organism represents a useful strategy for biocontainment. Here the authors use dCas9-VP64-driven gene expression to construct a ‘species-like’ barrier to reproduction between two otherwise compatible populations.

Sex-specific dispersal by a parasitoid wasp in the field

Although parasitoid wasps are important regulators of herbivorous insect populations, very little is known regarding their movement under field conditions. Here, we examined the vertical and cardinal directionality of dispersal by Binodoxys communis (Gahan) (Hymenoptera: Braconidae: Aphidiinae), an exotic Asian parasitoid released as a biological control agent of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae: Aphidinae), in North American soybeans, Glycine max (L.) Merr. (Fabaceae). The patterns are consistent with time‐dependent, sex‐specific dispersal strategies; whereas males show flight that is devoid of vertical or cardinal directionality, females consistently move at or above soybean canopy height toward the east and north. Male capture rates also appear to be more correlated with local conditions than those of females. These results suggest that females actively cross the flight boundary layer (the space above which insect flight is largely wind‐driven as opposed to self‐directed) and follow air currents away from soybean fields, whereas males engage in more localized movement. The data also suggest differing response potentials of the sexes to changing local resource conditions (host availability, female availability, host plant cultivar). We discuss the implications for sex‐specific movement on the basic biology of introduced parasitoid species, and their applied role as potential agents in importation biological control programs.

The spatial and metabolic basis of colony size variation

Spatial structure impacts microbial growth and interactions, with ecological and evolutionary consequences. It is therefore important to quantitatively understand how spatial proximity affects interactions in different environments. We tested how proximity influences colony size when either Escherichia coli or Salmonella enterica are grown on various carbon sources. The importance of colony location changed with species and carbon source. Spatially explicit, genome-scale metabolic modeling recapitulated observed colony size variation. Competitors that determine territory size, according to Voronoi diagrams, were the most important drivers of variation in colony size. However, the relative importance of different competitors changed through time. Further, the effect of location increased when colonies took up resources quickly relative to the diffusion of limiting resources. These analyses made it apparent that the importance of location was smaller than expected for experiments with S. enterica growing on glucose. The accumulation of toxic byproducts appeared to limit the growth of large colonies and reduced variation in colony size. Our work provides an experimentally and theoretically grounded understanding of how location interacts with metabolism and diffusion to influence microbial interactions.

A shared limiting resource leads to competitive exclusion in a cross-feeding system

Species interactions and coexistence are often highly dependent upon environmental conditions. This is especially true for cross-feeding bacteria that rely on one another for essential nutrients. The addition of a cross-fed nutrient to the environment can release one species from its dependence on another, thereby altering the species’ interaction and potentially affecting coexistence. Using invasion-from-rare experiments with cross-feeding bacteria, genome-scale metabolic modeling, and classical ecological models, we explored the potential for coexistence when one cross-feeding mutualist becomes independent. We show that whether nutrient addition shifts an interaction from mutualism to commensalism or parasitism depends on whether the limiting nutrient can be metabolized by only one species or by both species. Furthermore, we show that coexistence is only lost when the interaction becomes parasitism, and the obligate species has a slower maximum growth rate. Surprisingly, models suggest that rates of cross-fed nutrient production have a negligible effect. These results contribute to an understanding of how resource changes, whether intentional or not, will manipulate interactions and coexistence in microbial communities.

Evolution of bidirectional costly mutualism from byproduct consumption

Significance Organisms frequently exchange costly resources with other species. Theory suggests that this paradoxical cooperation between species might have its origins in waste consumption. When a species benefits from the waste of another, the recipient can evolve to aid the waste producer. The waste producer could then be selected to provide costly resources in return. We previously demonstrated the first step of this theorized process: Salmonella enterica evolved to secrete a costly amino acid to increase access to a byproduct generated by Escherichia coli. Here, we provide demonstration of a waste producer switching to costly cooperation. E. coli repeatedly evolved novel secretion of sugar to feed S. enterica. The results validate long-standing theory about the evolutionary origins of costly mutualism. Mutualisms are essential for life, yet it is unclear how they arise. A two-stage process has been proposed for the evolution of mutualisms that involve exchanges of two costly resources. First, costly provisioning by one species may be selected for if that species gains a benefit from costless byproducts generated by a second species, and cooperators get disproportionate access to byproducts. Selection could then drive the second species to provide costly resources in return. Previously, a synthetic consortium evolved the first stage of this scenario: Salmonella enterica evolved costly production of methionine in exchange for costless carbon byproducts generated by an auxotrophic Escherichia coli. Growth on agar plates localized the benefits of cooperation around methionine-secreting S. enterica. Here, we report that further evolution of these partners on plates led to hypercooperative E. coli that secrete the sugar galactose. Sugar secretion arose repeatedly across replicate communities and is costly to E. coli producers, but enhances the growth of S. enterica. The tradeoff between individual costs and group benefits led to maintenance of both cooperative and efficient E. coli genotypes in this spatially structured environment. This study provides an experimental example of de novo, bidirectional costly mutualism evolving from byproduct consumption. The results validate the plausibility of costly cooperation emerging from initially costless exchange, a scenario widely used to explain the origin of the mutualistic species interactions that are central to life on Earth.

Antimicrobials: Constraints on microbial warfare

Microorganisms produce antibiotics, which can exclude competitors, but bacteria typically only synthesize modest amounts of these compounds. New work suggests this may be an evolutionary strategy to balance the benefits of antimicrobial warfare against inadvertently providing help to resistant free-loaders.