When an animal is exposed to a stress, it produces hormones that elevate its blood pressure, allowing it to quickly respond in a “fight-or-flight” scenario. When these hormones are produced in a pregnant female, they can be transferred to the foetus, changing the environment in which the embryo develops, therefore causing physiological and behavioural changes (Weinstock, 2008).
There is evidence that as well as stress hormones being transferred to foetuses in placental mammals, they can also be transferred to the developing eggs of birds (Saino et al., 2005) and fish (Giesing et al.,2011). The effects of maternal stress on offspring appear to vary greatly among fish. The offspring of predator exposed sticklebacks shoal more tightly (Giesing et al., 2011), which is a predator avoidance mechanism. However, a second study on sticklebacks showed that when the offspring of predator exposed mothers were individually introduced to a single predator (Esox niger) they survived for a shorter period of time than the offspring of unexposed fish. This lower survival was due to a reduction in predator avoidance behaviour (McGhee et al., 2012). The offspring of predator exposed females have also been found to have impaired learning when presented with simple foraging tasks (Roche et al., 2012). This suggests that maternal stress affects multiple traits in sticklebacks, which could result in either net fitness costs or benefits, but it is likely that increased concentrations of stress hormone in the egg results in fitness trade-offs.
Zebrafish (Danio rerio), a model organism used in the study of genetics, development, behaviour and disease, have been found to produce an alarm cue which is released when their skin has been damaged. Exposure to this alarm cue elicits a strong stress response, as it indicates that a predator is foraging in the area (Frisch, 1941, Oliveira et al., 2014).
For my master’s project, I undertook a study investigating how exposure to this natural alarm cue affects female zebrafish and their subsequent offspring. Although we know much about the direct effects of stress on zebrafish behaviour, we know little of how it affects reproductive output, or the effects of maternal stress on offspring. Therefore, the aim of this experiment was to determine how exposing female zebrafish to an acute chemical stress affects their fecundity and fertility, and how this prenatal stress affects offspring mortality, and the time taken for eggs to hatch.
Female zebrafish were exposed to either the alarm cue or a benign control. After exposure, they were each paired up with a randomly chosen male, with which they spawned the following morning. I then collected the eggs, counting the number of fertile and infertile eggs produced by each female to measure their fecundity and fertility. I then recorded the mortality and time taken for the eggs to hatch by counting deceased and hatched eggs twice a day for a week.
During this project I found that there was no difference in the number (Fig.1) or fertility (Fig.2) of eggs laid by stressed and non-stressed females. This was not particularly surprising, as the eggs had already been formed when the fish were exposed to the stress, so the stress had no time to affect egg development. A difference I did find between the stressed and non-stressed females was that the eggs laid by stressed females hatched earlier than those laid by control females (Fig.3). A similar result can be seen when the eggs of Ambon damselfish (Pomacentrus amboinensis) had elevated cortisol concentrations, with these eggs hatching earlier than control eggs (Gagliano and McCormick, 2009). The likely reason that eggs hatch earlier when exposed to a stress is that cortisol is a steroid hormone which speeds up the development of the embryos. However, unlike the study on Ambon damselfish, where embryo mortality increased when exposed to a stress hormone, my project found that zebrafish embryo mortality actually decreased when exposed to stress (Fig.4). This is an interesting and unexpected finding, and although it would seem to be beneficial to have a stressed mother, it is likely that there are other effects of stress exposure that are more harmful, as seen in other studies.
Although this project has left many answers about how and why prenatal stress affects offspring traits, what I can say is that it results in early-life fitness benefits for zebrafish, including reduced embryo mortality and earlier hatching. Whether this is an adaptive response to a stress exposure, or merely a byproduct of the stress response remains to be seen, and will hopefully be understood better with further study.
Frisch, K. V. 1941. Ueber eine Schreckstoff der Fischhaut und seine biologische Bedeutung. Zeitschr Vergleich Physiol, 29, 46-145.
Gagliano, M. & McCormick, M. I. 2009. Hormonally mediated maternal effects shape offspring survival potential in stressful environments. Oecologia, 160, 657-665.
Giesing, E. R., Suski, C. D., Warner, R. E. & Bell, A. M. 2011. Female sticklebacks transfer information via eggs: effects of maternal experience with predators on offspring. Proceedings of the Royal Society B-Biological Sciences, 278, 1753-1759.
McGhee, K. E., Pintor, L. M., Suhr, E. L. & Bell, A. M. 2012. Maternal exposure to predation risk decreases offspring antipredator behaviour and survival in threespined stickleback.Functional Ecology, 26, 932-940.
Oliveira, T. A., Koakoski, G., da Motta, A. C., Piato, A. L., Barreto, R. E., Volpato, G. L. & Gil Barcellos, L. J. 2014. Death-associated odors induce stress in zebrafish. Hormones and Behavior, 65,340-344.
Roche, D. P., McGhee, K. E. & Bell, A. M. 2012. Maternal predator-exposure has lifelong consequences for offspring learning in threespined sticklebacks. Biology Letters, 8, 932-935.
Saino, N., Romano, M., Ferrari, R. P., Martinelli, R. & Moller, A. P. 2005. Stressed mothers lay eggs with high corticosterone levels which produce low-quality offspring. Journal of Experimental Zoology Part a-Comparative Experimental Biology, 303A, 998-1006.
Weinstock, M. 2008. The long-term behavioural consequences of prenatal stress. Neuroscience and Biobehavioral Reviews, 32, 1073-1086.