One of Kate McLean’s dissertation chapters just appeared in Evolutionary Ecology! The paper explored the roles of sexual recombination and temporal gene flow (via the egg bank) in maintaining host resistance and genetic diversity. Many studies (including some from our lab) have focused on how host resistance changes within a growing season. Others have looked at long term trends in resistance evolution across many years. Kate’s study looks at the neglected middle ground: how the resistance distribution that exists at the end of one growing season changes by the start of the next growing season as a result of sexual recombination and temporal gene flow from the egg bank.
To study this, Kate and colleagues tracked resistance to Metschnikowia and genetic diversity in Daphnia dentifera in two lakes. This involved collecting sexual females in the autumn, getting them to release their resting eggs, and hatching those resting eggs. Comparing the resistance phenotypes of the moms vs. the hatchlings let us uncover the effect of sexual recombination on resistance, and also to estimate the heritability of resistance. In addition, we sampled the populations early the following spring to characterize the populations shortly after they were reestablished from the egg bank. Since the hatchlings told us about what had gone in to the egg bank the prior year, comparing them with the spring population allowed us to determine the effect of temporal gene flow via the egg bank. Resistance was quantified using infection assays, and genetic diversity was quantified using microsatellites; the diversity component of this project was the foundation of the undergraduate Honors Thesis of Haniyeh Zamani, who is a coauthor on the study.
Because we know that resistance and fecundity trade off in the Daphnia dentifera–Metschnikowia system, we expected that populations would evolve toward higher susceptibility (due to its fecundity advantages) unless an epidemic had recently selected for resistance. Moreover, if an epidemic did occur, we expected resistance to increase temporarily but that sexual recombination and temporal gene flow would then shift the population back towards susceptibility. This is not what we found! Instead, susceptibility was the transient state, with recombination and gene flow restoring and/or maintaining high resistance.
For genetic diversity, we expected that fall offspring would show greater genotypic diversity than their parents due to the effects of sexual recombination; this was observed in one lake (Hackberry) but not in the other (Midland), where genotypic diversity of parents was already very high. We also predicted that the egg bank clones would have higher diversity than the fall offspring, since we anticipated hatching of individuals produced across multiple years; again, this was observed in Hackberry but not in Midland.
In short, sexual recombination and temporal gene flow are both important players in determining inter-annual variation in host resistance in this study system, but not always in the ways we predicted!
In addition to lead author Kate McLean, the authors on this paper are former Duffy Lab grad student Camden Gowler, lab postdoc Marcin Dziuba, former lab undergrad Haniyeh Zamani, long term collaborator Spencer Hall, and Meghan.
Congrats to Kate on their new publication!