Donelson, Jennifer M., and Philip L. Munday. “Transgenerational plasticity mitigates the impact of global warming to offspring sex ratios.” Global change biology (2015). DOI: 10.1111/gcb.12912
A population of fish is a group composed of a single species that lives in the same area and interbreeds (reproduces within itself). The size of a population is determined by how many fish are added to the population through reproduction and immigration relative to how many are subtracted through death (natural mortality, predation, and fishing) and emigration (Fig. 1). The ratio of sexually mature males to females is an important part of successful reproduction within a population. The optimal gender ratio can vary based on many characteristics specific to a certain species, population, and environmental attributes. Changes to population gender ratios may be influenced by the environment even after fertilization occurs in a process termed environmental sex determination. There is growing evidence that changing environmental conditions, typically caused by humans (including the hormone pollution in waste water), may negatively impact gender ratios and reproductive success. Temperature is the most widely studied environmental factor influencing gender ratios in offspring. Temperature-dependent sex determination (TSD) is of concern as changing temperatures are a robustly predicted impact of climate change progression.
Many species exhibit temperature dependent gender within the range of temperatures currently experienced in the environment. This demonstrates potential for adaptation to future temperature change scenarios. It is important to determine what the limits are for TSD adaptation as this may predict the fate of populations under future climate change. There are many pathways for modifying gender ratios. Some non-genetic means include nesting site selection but others are considered “plastic” traits meaning they are adaptable but still not genetically coded into the animal’s DNA. In vertebrates, temperature affects the expression of an enzyme complex which affects hormone production gender and ratios in offspring.
This study used tropical damselfish (Acanthochromis polycanthus) to test whether thermal effects experienced by parent fish would affect the gender ratios of their offspring. They did this by attempting to answer three questions.
- Do future temperatures (+1.5°C and +3.0°C) affect offspring ratios? And is there a critical period in development during which those temperatures would need to be experienced?
- Does exposing adult pairs to warmer temperatures affect the gender ratios of offspring?
- Would a lifetime at warmer temperatures influence the effect of developmental temperature?
The tropical damselfish was chosen for this study because they are monogamous and have nearly equal proportions of males to females.
Results and Interpretations
Offspring gender ratios were approximately 0.5 when raised at present day temperatures but the proportion of females lowered significantly when raised at warmer temperatures. These effects did not occur in the fish transferred to warmer temperatures 3 months post hatching (Fig. 3). This suggests that the critical age for thermal exposure affecting gender ratios happens before the 3 month point.
Offspring sex ratios were also affected by the temperatures their parents were reared in (Fig. 4). Even with just +1.5°C above mean temperatures, the proportion of females in offspring was significantly lower. However, offspring of parent fish that were reared at high temperatures for one or two generations produced offspring at normal gender ratios. This is good news fish under climate change. Adults that experience warm temperatures can influence their offspring’s reaction norms such that gender ratios are maintained. Pretty cool huh? However, the scope of this ability remains untested, future warming may outpace this species’ ability to adapt and fundamentally alter normal gender ratios.