It's getting hot in here: Temperature as a driver of symbiotic polymorphisms in an agricultural spider

Maternally-inherited bacterial symbionts often manipulate the reproduction of their hosts, to promote the spread of infection throughout the host population. Theory predicts that symbionts will often spread to near fixation, based on several parameters including penetrance of the manipulative phenotype, fitness costs to the host, and efficiency of vertical transmission from mother to offspring. Host populations that are polymorphic, consisting of individuals with different symbiont complements and phenotypes, have rarely been investigated in nature, and we know relatively little about the mechanisms that maintain such variation. Our group has now been studying the agricultural spider Mermessus fradeorum for over a decade, and we have documented that our local population has maintained a mixture of spiders with different symbiotypes and different phenotypes throughout this time. Temperature likely plays a critical role in maintaining this polymorphism, through effects on all the parameters that affect symbiont maintenance and spread. This leads us to several concrete hypotheses about geographic variation in infection that we are currently testing. Our results contribute to the growing body of literature that shows temperature may be a driving but underappreciated force in determining the distribution and persistence of host-symbiont polymorphisms.