No evidence that temperature-related fertility differences influence the distribution of a selfish genetic element

1. Selfish genetic elements (SGEs) are universal and can have major evolutionary and ecological impacts, but their ecology is not well understood. Most SGEs have stable population dynamics, but the reasons for this are rarely known. Several SGEs are distributed along latitudinal frequency clines, providing an opportunity to examine why these clines are stable. 2. Latitudinal temperature changes are a strong contender for an underlying environmental cause of such clines. Both SGEs and extreme temperatures commonly affect male fertility. If these factors interact to reduce male fertility, this could be the cause of the observed latitudinal clines. 3. The meiotic driving X-chromosome sex ratio (SR), found in the fruit fly Drosophila pseudoobscura, is distributed in a cline across North America, being absent in Canada and increasing in frequency southward to 30% at the Mexico/USA border. This cline has been stable for at least 50 years, despite considerable gene flow between populations. If SR males have decreased fertility in the colder Northern populations, this could undermine the success of SR and cause this cline. 4. Here, we examine the fertility of males reared at high and low temperatures. We find that both SR and non-SR males have reduced fertility at high temperatures. However, SR males show disproportionally lower fertility at high temperatures than non-SR males. 5. The geographical distribution of SR suggests the opposite should be true; SR males should be more successful at high temperatures than low ones, as SR is more common in the South, hence the observed temperature-related fertility differences of males cannot be the cause of the cline in SR in nature. 6. This study finds that the reduction in male fertility commonly caused by SGEs is temperature dependent. However, although these interactions are likely to be important for the ecology of SGEs, they do not necessarily provide the explanation for the observed latitudinal clines in SR.