Duncan, A. B. | bioRxiv Science

bioRxiv2021

Virulence constrains transmission even in the absence of a genetic trade-off

The virulence-transmission trade-off predicts that parasite fitness peaks at intermediate virulence. However, whether this relationship is driven by the environment or genetically determined and if it depends on transmission opportunities remains unclear. We tackled these issues using inbred lines of the macro-parasitic spider-mite Tetranychus urticae. When transmission was not possible during the infection period, we observed a hump-shaped relationship between virulence and parasite fitness, as predicted by theory. This was environmentally driven, as no genetic correlation between traits was detected. However, when transmission to uninfected hosts occurred during the infection period, virulence was positively, environmentally and genetically correlated with parasite fitness. Therefore, the virulence-transmission trade-off depends on within-host dynamics and on the timing of transmission, rather than on a genetic correlation. This fundamental correlation may thus be easier to manipulate than previously thought.

evolutionary biology↗

bioRxiv2020

A sexual dimorphism in the spatial vision of band-winged grasshoppers

Visual acuity (VA) --- a measurement of the fineness or coarseness of vision --- correlates with the size of an animal, with larger species often possessing sharper vision. However, it is unknown whether the same relationship between visual acuity and size holds within a species when individuals differ consistently and substantially in size, such as through a sexual size dimorphism. Here we examine the visual acuity of three species of sexually dimorphic band-winged grasshoppers, in which females are the larger sex (Arphia pseudonietana, Dissosteira carolina, and Spharagemon equale; total n = 98). Using a radius of curvature estimation method, we find that females have [~]21% finer vision in the most acute region and axis of the eye than do males. Further explorations of the eyes of the species showing the greatest size dimorphism (D. carolina) suggest that this VA dimorphism is driven by females having larger eyes with more ommatidia. In contrast to many flying insects where males have finer vision to acquire mates, our study is one of the first to demonstrate a female-biased sexual dimorphism in acuity. Given the number of species in which females are larger than males, our results suggest that differences in VA between the sexes may be more common than currently appreciated.

physiology↗