Description

Like most species, Caenorhabditis elegans show a dramatic decrease in fertility when exposed to increasing temperatures (Harvey and Viney, 2007; Petrella, 2014; Poullet et al., 2015). In hermaphrodites, the susceptibility of sperm to temperature stress is the foremost factor defining the decline in fertility observed in several wild type strains of the species (Harvey and Viney, 2007; Petrella, 2014; Poullet et al., 2015). While some studies have looked directly at the impact of temperature stress on the males of the species (Nett et al., 2019; Petrella, 2014; Poullet et al., 2015), the conflation of temperature effects on male mating behavior, mating success, and male sperm function have made these studies more difficult. Previously, we showed that C. elegans males of several wild type strains have low mating success when exposed to moderate temperature stress (27°C), as measured by the percentage of males that had any viable cross progeny when crossed with unc-119 hermaphrodites (Petrella, 2014). However, we did not assess if those males that show some level of fertility had a decrease in the number of viable cross progeny that they could produce, as is seen in hermaphrodites exposed to moderate temperature stress (Harvey and Viney, 2007; Petrella, 2014; Poullet et al., 2015). These previous experiments were also done with males of each wild type strain crossed with the unc-119 mutant hermaphrodites in the N2 background, thus genetic incompatibilities could not be ruled out as playing a role in the decreased mating success. To avoid genetic incompatibilities and assess male fertility in a way that would more accurately recapitulate the most likely scenario in the natural habitat of C. elegans (Barrière and Félix, 2005; Richaud et al., 2018), we crossed males of three wild type strains of C. elegans: JU1171, LKC34, and N2 with strain matched wild type hermaphrodites. As genetic incompatibility may not be a strong factor, we also crossed wild type males with unc-24(e138) hermaphrodites in the N2 background because Unc hermaphrodites have limited motility, which would facilitate higher male mating success. Thus, we may be more likely to capture any viable progeny that could be produced by males of each strain because they are more likely to successfully mate.

We found that significantly fewer temperature stressed males of all three wild type strains were able to successfully mate at 27°C versus their unstressed counterparts at 20°C, whether crossed with strain matched wild type hermaphrodites or unc-24(e138) hermaphrodites (Figure 1A-C). In addition, the temperature stressed males that do achieve mating success at 27°C produced significantly fewer viable cross progeny versus their unstressed counterparts at 20°C, whether crossed with strain matched wild type hermaphrodites (hermaphrodites of their own strain) or unc-24(e138) hermaphrodites (Figure 1D-F). As we did not assess dead eggs or unfertilized oocytes this may be due to either an increase in embryonic lethality or fewer viable sperm being made or transferred. However, growth at stressful temperatures overall leads to a reduced number of viable cross progeny even when a male successfully mates with a hermaphrodite.

We predicted that more males from each wild type strain would successfully mate when crossed with unc-24(e138) hermaphrodites because these hermaphrodites have impaired motility and cannot exert mate choice behavior (Bahrami and Zhang, 2013). We found a significant increase only in the percentage of males that mated successfully with unc-24(e138) hermaphrodites versus strain matched hermaphrodites for N2 at 20°C and LKC34 at both 20°C and 27°C (Figure 1B-C). For both N2 and LKC34 there was no difference in the number viable progeny sired by those males (Figure 1E-F). This suggests that LKC34 hermaphrodites likely exercise mate choice as has been previously shown for N2 (Bahrami and Zhang, 2013) at both temperatures. However, hermaphrodite mate choice may be impacted by temperature stress in N2 hermaphrodites, such that N2 males are equally successful with strain matched or unc-24(e138) hermaphrodites at 27°C. Interestingly, there was no change in the percentage of JU1171 males that mated successfully with unc-24(e138) hermaphrodites versus strain matched hermaphrodites for JU1171 at 20°C, and there was a significant decrease in the number of males that could mate successfully at 27°C (Figure 1A). In addition, we saw a significant decrease in the number of viable cross progeny from JU1171 males at 20°C when crossed to unc-24(e138) hermaphrodites versus strain matched hermaphrodites (Figure 1D). These differences are likely due to a JU1171-N2 genetic incompatibility, which has been observed previously, and shown to limit viable progeny from this specific genetic cross (Seidel et al., 2008).

These data suggest that exposure to moderate temperature stress has a large and significant effect on the percentage of C. elegans males that can achieve mating success and the number of viable cross progeny they sire. In the context of climate change associated elevations in global temperature, our results underscore the overwhelming reproductive challenges faced by susceptible populations of sexually reproducing organisms.