Recently, I posted about one of my favorite symbiont-host systems: mites that are sexually transmitted among their lady beetle hosts. Because that system is so empirically tractable, studies on mites and lady beetles are increasing our understanding of STIs by leaps and bounds. Today I want to talk about another really cool thing that we just learned by studying lady beetle sex.
In order for symbionts to be maintained in host populations over the long term, there needs to be transmission of the symbiont from one host generation to the next. In temperate regions, many animals have seasonal population cycles where adults from multiple generations don’t overlap and/or adults and their offspring don’t temporally overlap. For instance, many dragonflies lay eggs in the summer/fall and then die. The eggs hatch into aquatic larvae, which then overwinter and don’t emerge until the next year. In that case, adult dragonflies from the first generation never see the adult dragonflies from the second generation, so those dragonflies wouldn’t be able to pass on STIs between generations.
Coccipolipus hippodamiae lady beetles overwinter as adults, not larvae. They emerge in the spring, and in May-June they have lots of sex and lay lots of eggs. Eventually, the overwintered adults die. Also, at some point, the new cohort emerges from the eggs and individuals develop from larvae to sexually mature adults.
Interestingly, not all Coccipolipus hippodamiae lady beetle populations have mites, and the presence or absence of mites in a population is consistent over time (Pastok et al. 2015). Populations north of 61°N tend to be mite-free, and populations south of 61°N tend to have mites. But lady beetles in populations north of 61°N can become infected by mites and transmit them in the laboratory, so it doesn’t seem like there is a physiological/biological reason why northern and southern populations differ in mite infection (Pastok et al. 2015).
Instead, that division by latitude suggests that ecology (specifically, phenology) might play an important role in intergenerational mite transmission. And sure enough, in August, southern mite populations have adults from both the overwintered lady beetle generation and the new lady beetle generation (Pastok et al. 2015). And at least some of the new adults have already had sex when the old generation is still present, which means that sexual contacts could be happening between the generations. In contrast, in the northern populations, the overwintered lady beetles die sooner and the new lady beetles mature later, so there is no overlap among generations (Pastok et al. 2015). It looks like that generation gap prevents sexually transmitted mites from persisting in those northern populations! SO. COOL.
So, phenology plays a really important role in symbiont transmission when symbionts are sexually transmitted. But that’s not all! For instance, symbionts that require multiple host species may also be very sensitive to host phenology, especially when there are mismatches in the phenologies of various host species (here and here). As host phenologies continue to change in response to the changing global climate, the role of host phenology in symbiont transmission will remain a huge area for future research.
Pastok, D, M-J Hoare, J.J. Ryder, M. Boots, R.J. Knell, D. Atkinson, and D.D. Hurst. 2015. The role of host phenology in determining the incidence of an insect sexually transmitted infection. Oikos.