About the blog author: Murielle Ålund is a researcher interested in how environmental change affects interactions between species, particularly in scenarios where previously separated species suddenly come into contact, leading to new scenarios of interspecific competition, predator-prey, or host-parasite interactions. Currently a postdoctoral researcher at Uppsala university, Sweden, she studies reproduction, parasitism and long-term interactions between two hybridizing species of Ficedula flycatchers. When she received this award, Murielle was a postdoctoral researcher at the department of Integrative Biology at Michigan State University, USA, studying adaptation to new sensory environments in Icelandic threespine sticklebacks. Murielle tweets about her research here.
The factors influencing biodiversity, and, in particular, how quickly new adaptations to novel environmental conditions might evolve, have fascinated biologists for centuries. Human activities have caused changes to natural habitats at an increasing pace these last few decades, nonetheless via changes in global temperatures affecting all kinds of environments. Understanding how organisms are affected by such changes, and what conditions might be required for them to adapt quickly enough to new conditions is essential for us to predict how biodiversity might look like in the near future.
The threespine stickleback, Gasterosteus aculeatus, is a small fish inhabiting most waters of the Northern hemisphere. It is a popular model system in evolutionary studies, notably because of its impressive capacity at repeatedly invading freshwater systems from its original marine distribution (McKinnon and Rundle 2002). Colonizing freshwater from the sea comes with a whole set of new physiological and ecological conditions. Stickleback populations have been shown to develop a whole range of characteristics allowing them to deal with changes in salinity, nutrients, and predation pressures over just a few generations (Kitano et al. 2008). The group of Janette Boughman at Michigan State University studies threespine sticklebacks in Iceland, where unique geological conditions provide an exceptional opportunity to compare multiple populations exposed to similar changes in environments. Iceland has a complex system of freshwater lakes, that sticklebacks have colonized 10,000 years ago at the earliest, as the country was covered in ice until then. The lakes differ widely in their ecology, as some are fed by glaciers and very turbid (Figure 1), and others are crystal clear and fed by springs. These lakes also vary greatly in nutrient content, and in parasite prevalence.
One of the best studied, and arguably very damaging parasites affecting sticklebacks is the cestode Schistocephalus solidus (Figure 2). This parasitic worm has a complex life cycle, and the fish are only an intermediate host as they need to infect avian blood to be able to reproduce (Barber and Scharsack 2010). Infected sticklebacks are known to greatly modify their behavior, which is thought to result from manipulation from the parasite in order to increase its chances at reaching its final host (Poulin 2013). Infected fish are indeed swimming to the surface and do not seem to be afraid of birds, who can thus easily catch them and eat them, allowing the parasite to then infect the birds. The exact molecular pathways leading to these well-described changes in behavior are however still largely unknown.
In this project, we first performed a set of behavioral experiments confirming behavior alteration in infected fish. On top of previously documented lack of fear reaction to predators, an experiment led by Elizabeth Phillips, now PhD candidate in the Netherlands, and a former undergraduate student in our lab, McKain Williams, showed that infected fish either have troubles detecting, or possibly lack motivation, to catch prey. All of these behavioral alterations point towards parasite-induced changes in the brains of infected fish. One of the goals of this project was thus to further investigate potential changes in gene expression in the brains of parasitized, compared to healthy fish.
Thanks to the 2019 EECG award, and in collaboration with Becca Young, Mariana Rodriguez Santiago, and Hans Hoffman at the University of Texas in Austin, I was able to use TAG sequencing to sequence expressed genes in two sub-regions of the brains of 74 different individuals from eight Icelandic populations, including both parasitized and unparasitized fish. These brain regions are the dorsolateral (dl) and dorsomedial (dm) regions of the pallium, that perform hippocampus- and amigdala-like functions in fish (Silva et al. 2015). They are known to be involved in the fear pathway and stress reactions, two pathways likely targeted by S. solidus given the resulting lack of predator escape behavior and reduced foraging efficiency in parasitized fish. The brains were first dissected, fixed and frozen, and then sectioned using a cryostat, allowing to keep samples frozen throughout the process to preserve RNA quality (Figure 3). Micro-punches were then used to dissect the micro-regions of interest. Gene expression datasets from these samples are being analyzed at the moment.
Exploring several known pathways of fear reactions in the brains of infected and non-infected fish will shed light on detailed potential mechanisms underlying this fascinating phenomenon by which parasites seem to completely highjack the brains of their intermediate hosts, and manipulate their behavior to their own benefit, with detrimental consequences for the fish.
Barber, I., & Scharsack, J. (2010). The three-spined stickleback-Schistocephalus solidus system: An experimental model for investigating host-parasite interactions in fish. Parasitology, 137(3), 411-424.