Many short-lived organisms survive temporal variation in reproductive success by incorporating prolonged dormancy into their life cycle. Although selection can shape optimal dormancy patterns, variation in the environment to which dormant eggs are exposed also influences observed hatching rates. Questions remain, however, regarding the relative importance of environmental and genetic influences in different habitats. Previous work on lake dwelling Daphnia has demonstrated a lack of genetic differentiation among populations for this trait. I predict that species from shallow ponds should experience greater access to hatching cues, and thus, be more likely to show genetic divergence or population by environment interactions for hatching rates. To test this prediction, I measured variation in prolonged dormancy and egg survival for Daphnia from 22 shallow fishless ponds in the Midwestern USA. I then used reciprocal transplant experiments to quantify genetic and environmental influences on these traits.
Although all eggs were incubated at a water depth of 0.75 m or less in their natal pond, hatching rates varied between 5 – 95% and survival rates ranged from 0 - 80%. There was no apparent relationship between hatching and environmental cues such as light, oxygen content or conductivity, although a negative relationship with depth was observed. The reciprocal transplant experiments showed a strong population by host environment interaction for both traits. Thus, plasticity to environmental cues and genetic or maternal effects likely interact to determine hatching and survival rates in the field.