Periodic Reporting for period 1 - ESCAPE (Abandoning ship – sex and dormancy strategies in Daphnia)
Reporting period: 2019-10-01 to 2021-09-30
When environments change, the organisms that live in them must adapt to those changes, otherwise they become extinct. In the face of environmental variability, some organisms have evolved the ability to switch between sexual and asexual reproduction. For these organisms, sex is often associated with harsh conditions, typically found at the end of the growing season; sexual reproduction here commonly results in the formation of dormant embryos that are housed in a protective case (so-called ephippium) that shields them from adverse environmental conditions and allows them to hatch as soon as environmental conditions improve. While we know that some organisms that are able to switch to sexual reproduction do specialize by producing males to fertilize eggs or sexual females that can produce ephippia (but not necessarily both), we do not know whether such ‘gender specialization’ is due to the genetic background, environmental impacts or potentially a mixture of both.
The overall objective of this project was thus to assess if genetic variation in the production of males and sexual females exists, and whether this variation is strongly environment dependent. We further aimed to identify the molecular mechanisms that drive the production of males and sexual females. For our work, we used a classic ecological and genomic model organism, the microcrustation Daphnia. Daphnia are an excellent system to address fundamental ideas of why animals can and do switch between sexual and asexual reproduction: They are capable of both sexual and asexual reproduction. Under favourable environmental conditions, daphnids reproduce clonally, generating genetically identical daughters. Sexual reproduction, on the other hand, is triggered by high population density, low food conditions and shifting photoperiod associated with end of summer.
The overall objective of this project was thus to assess if genetic variation in the production of males and sexual females exists, and whether this variation is strongly environment dependent. We further aimed to identify the molecular mechanisms that drive the production of males and sexual females. For our work, we used a classic ecological and genomic model organism, the microcrustation Daphnia. Daphnia are an excellent system to address fundamental ideas of why animals can and do switch between sexual and asexual reproduction: They are capable of both sexual and asexual reproduction. Under favourable environmental conditions, daphnids reproduce clonally, generating genetically identical daughters. Sexual reproduction, on the other hand, is triggered by high population density, low food conditions and shifting photoperiod associated with end of summer.
One way to assess if genetic variation in the production of males and sexual females exists, and whether this variation is strongly environment dependent, is to expose a number of different individuals to several, different environmental stressors, and document respective responses of each of the individuals in response to the applied conditions(s).
Here, we exposed different clones of Daphnia pulex that either originated from ephemeral and permanent ponds in the UK (see Figure) to different environments that varied in the amount of food availability (i.e. different algae concentrations), different day length (mimicking either winter or summer conditions), and different population densities, i.e. using different numbers of individuals that share the same space. We then assessed the timing and total number of males or ephippia produced in the different environmental conditions.
Our phenotypic assessment suggests that a contribution of both genetic and environmental factors contribute to the switch from asexual to sexual reproduction in the investigated Daphnia clones.
In order to identify the molecular mechanisms that may drive the production of males and sexual females, we then looked at how the DNA is expressed in different clones under different conditions. Expression of genes is one of the first steps in turning an environmental signal into a distinct phenotype like the production of males or sexual females. Since we identified clonal clusters with clones that showed i) no sex investment, ii) male investment only, iii) sexual female investment only, and iv) investment in males and sexual females. While we expect that the genes that are expressed in these clonal clusters differ among the four groups, we are currently awaiting confirmation from our molecular analyses to proof this. In a final step, we will be looking at which genes are expressed together when the environment changes. Such co-expression generates networks of genes, somewhat like the network of connections among people on social networks. Ultimately, this analysis, we allow us to then look at whether there are certain genes that control expression patterns in other parts of the network.
Here, we exposed different clones of Daphnia pulex that either originated from ephemeral and permanent ponds in the UK (see Figure) to different environments that varied in the amount of food availability (i.e. different algae concentrations), different day length (mimicking either winter or summer conditions), and different population densities, i.e. using different numbers of individuals that share the same space. We then assessed the timing and total number of males or ephippia produced in the different environmental conditions.
Our phenotypic assessment suggests that a contribution of both genetic and environmental factors contribute to the switch from asexual to sexual reproduction in the investigated Daphnia clones.
In order to identify the molecular mechanisms that may drive the production of males and sexual females, we then looked at how the DNA is expressed in different clones under different conditions. Expression of genes is one of the first steps in turning an environmental signal into a distinct phenotype like the production of males or sexual females. Since we identified clonal clusters with clones that showed i) no sex investment, ii) male investment only, iii) sexual female investment only, and iv) investment in males and sexual females. While we expect that the genes that are expressed in these clonal clusters differ among the four groups, we are currently awaiting confirmation from our molecular analyses to proof this. In a final step, we will be looking at which genes are expressed together when the environment changes. Such co-expression generates networks of genes, somewhat like the network of connections among people on social networks. Ultimately, this analysis, we allow us to then look at whether there are certain genes that control expression patterns in other parts of the network.
So far, our data indicate that both genetic and environmental factors drive the shift to sexual reproduction in Daphnia, allowing a unique insight into our understanding of why and how sexual reproduction is maintained in natural populations. While our findings are informative for the field of evolutionary ecology, our results are relevant for many medical and agricultural problems as well where a switch from asexual to sexual reproduction is common (e.g. plant pathogens or malaria).