Spatial structure imposes sex-specific costs but does not reduce interlocus sexual conflict

Spatial structure is a common feature of all naturally occurring populations. Theoretically, spatial structuring of a habitat could modulate the intensity of interlocus sexual conflict (ISC) in a population, possibly by modulating intersexual encounter rate. We tested this theory using laboratory populations of Drosophila melanogaster by measuring male-induced decline in female fitness in three-patch habitat systems with two alternative habitat types: structured–interconnected and unstructured. Our results on reproductive and survival costs in females suggested significant costs attributable to male presence (i.e. ISC) and to living on structured habitat. However, there was only weak evidence supporting the theory of modulation of ISC by habitat structuring only. Through a follow-up experiment, we also showed that the effect of habitat on ISC is robust to the alteration of female conditions. Therefore, it appears that spatial structuring per se is unlikely to modulate ISC but can impose additional survival costs. We also showed that this survival cost could be sex specific, possibly reflecting female-biased spontaneous dispersal.

Methods
Two randomized block design experiments were conducted on laboratory adapted populations of Drosophila melanogaster. The details of these populations were reported first in Nandy et al. 2016 (https://doi.org/10.1016/j.anbehav.2016.01.019). Laboratory structured habitat was set up by using interconnected plastic vials (for three-patch setups) or one vial and one bottle (for two-patch setups). All traits reported in this dataset were measured in the laboratory - close to the populations' usual natural holding condition (i.e., the laboratory conditions in which they are usually maintained). For full experimental details, see the pre-print - https://www.biorxiv.org/content/10.1101/2022.10.29.514349v3.