Planktonic crustaceans belonging to the genus Daphnia have long been used as model organisms for the study of ecotoxicology, and show particular prowess in their ability to alter their phenotype in response to environmental stress. Now with the availability of the genome and genetic linkage map of Daphnia pulex, as well as the amenability of Daphnia for laboratory culture, these water fleas have shown themselves to also be of value in the field of evolutionary developmental biology. To maximise on the advantages offered by Daphnia to probe arthropod development and phylogeny, Carsten Wolff from the Humboldt University of Berlin, Germany, and colleagues present a detailed staging system of the embryonic development of Daphnia magna based on morphological landmarks, published in a recent study in EvoDevo. Wolff explains what makes Daphnia such a useful model organism and how their staging system contributes to current research efforts.
A female Daphnia magna carrying eggs in its brood pouch. Image source: Mittman et al, EvoDevo, 2014, 5:12
What is Daphnia magna, where does it live and what ecological function does it serve?
Daphnia magna is small planktonic branchiopod crustacean which is commonly called a ‘water flea’. Its distribution is worldwide and it plays a major role in aquatic food chains. In addition, the small crustaceans of the genus Daphnia are known for their high phenotypic adaptability. Daphnia is one of the oldest model organisms in ecotoxicology and ecology.
What qualities does Daphnia have that mean it has been adopted as a lab model?
There are plenty of important advantages to use Daphnia magna as a lab model. Daphnia is very easy to maintain in culture. The life cycle is relatively short and you can have access to large number of embryos easily. Furthermore, injecting embryos is possible and so enables the generation of transgenic animals and facilitates RNAi-experiments. It is made even more powerful by the fact that the Daphnia genome is sequenced and transcriptomic data is available.
What sort of questions are Daphnia researchers addressing?
The publication of the Daphnia pulex genome has facilitated the application of genomics and the development of genetic tools to long-standing questions in ecotoxicology, ecology and evolutionary biology. A particular focus is laid on understanding the genetic basis of the striking ability of daphnids to change their phenotype in response to environmental stressors. Furthermore, Daphnia have recently been developed into crustacean model organisms for EvoDevo research, contributing to the ongoing attempt of resolving arthropod phylogeny. These problems require the comparative analyses of gene expression and functional data.
6 e,f Daphnia magna development at stage 7.5. Nuclear stain in green, scanning electron micrograph in grey. (Left panels: a ventral view. Right panels: ventral view with a focus on thoracopod differentiation. Image source: Mittman et al, EvoDevo, 2014, 5:12
How does your staging system differ from standard staging approaches in other model organisms?
We provide a detailed staging system of the embryonic development of Daphnia magna based on morphological landmarks. The staging system does not rely on developmental hours and is therefore suitable for functional and ecological experiments, which often cause developmental delays in affected embryos and thus shifts in time reference points. We provide a detailed description of each stage and include schematic drawings of all stages showing relevant morphological landmarks in order to facilitate this application.
Scanning electron micrograph of a ventral view of Daphnia magna at stage 12. Image source: Mittman et al, EvoDevo, 2014, 5:14
How might your developmental staging be used by other researchers?
With our staging we provided an easy lab tool for all researchers working in Daphnia embryology regardless which field they particularly investigate. The schematic drawings make it easy to recognise the specific embryonic stages using uncomplicated methods.
How far can your results be extrapolated to other related taxa?
The staging system can be adopted for other daphnids with minor variations since the sequence of development is highly conserved during early stages and only minor heterochronic shifts occur in late embryonic stages.
What’s next for your research?
There are numerous questions about aspects in Daphnia development that still need attention. Our focus will be on early development, namely cleavages, gastrulation processes and germ band elongation. We also have a particular interest in investigating the development of the peripheral nervous system.