ABSTRACT
The impact of rising global temperatures on survival and reproduction is putting many species at risk of extinction. In particular, it has recently been shown that thermal effects on reproduction, especially limits to male fertility, can underpin species distributions in insects. However, the physiological factors influencing fertility at high temperatures are poorly understood. Key factors that affect somatic thermal tolerance such as hardening, the ability to phenotypically increase thermal tolerance after a mild heat shock, and the differential impact of temperature on different life stages are largely unexplored for thermal fertility tolerance. Here, we examine the impact of high temperatures on male fertility in the cosmopolitan fruit fly Drosophila virilis. We first determined whether temperature stress at either the pupal or adult life history stage impacts fertility. We then tested the capacity for heat-hardening to mitigate heat-induced sterility. We found that thermal stress reduces fertility in different ways in pupae and adults. Pupal heat stress delays sexual maturity, whereas males heated as adults can reproduce initially following heat stress, but become sterile within seven days. We also found evidence that while heat-hardening in D. virilis can improve high temperature survival, there is no significant protective impact of this same hardening treatment on fertility. These results suggest that males may be unable to prevent the costs of high temperature stress on fertility through heat-hardening, which limits a species' ability to quickly and effectively reduce fertility loss in the face of short-term high temperature events.
ABSTRACT
Meiotically driving sex chromosomes manipulate gametogenesis to increase their transmission at a cost to the rest of the genome. The intragenomic conflicts they produce have major impacts on the ecology and evolution of their host species. However, their ecological dynamics remain poorly understood. Simple population genetic models predict meiotic drivers will rapidly reach fixation in populations and spread across landscapes. In contrast, natural populations commonly show spatial variation in the frequency of drivers, with drive present in clines or mosaics across species ranges. For example, Drosophila subobscura harbors a sex ratio distorting drive chromosome (SRs) at 15-25% frequency in North Africa, present at less than 2% frequency in adjacent southern Spain, and absent in other European populations. Here, we investigate the forces preventing the spread of the driver northward. We show that SRs has remained at a constant frequency in North Africa, and failed to spread in Spain. We find strong evidence that spread is impeded by genetic incompatibility between SRs and Spanish autosomal backgrounds. When we cross SRs from North Africa onto Spanish genetic backgrounds we observe strong incompatibilities specific to hybrids bearing SRs. The incompatibilities increase in severity in F2 male hybrids, leading to almost complete infertility. We find no evidence supporting an alternative hypothesis, that there is resistance to drive in Spanish populations. We conclude that the source of the stepped frequency variation is genetic incompatibility between the SRs chromosome and the genetic backgrounds of the adjacent population, preventing SRs spreading northward. The low frequency of SRs in South Spain is consistent with recurrent gene flow across the Strait of Gibraltar combined with selection against the SRs element through genetic incompatibility. This demonstrates that incompatibilities between drive chromosomes and naïve populations can prevent the spread of drive between populations, at a continental scale.
