Reproductive biology of the arctic collembolan Hypogastrura tullbergi

Terrestrial organisms of the Arctic are faced with strong climatic fluctuat ions. Predictable seasonality with cold/long winters and short/cool summers are combined with unpredictable between and within year variation. This in dicates that various selection pressures act on the reproductive strategies of the populations. The arctic collembolan Hypogastrura tullbergi reproduc es in a short period following snow melt. Hatching occurs in late summer, t he animals grow to adult size within their second summer and reproduce for the first time in the beginning of their third summer. We performed several experiments to determine the reproductive investment and proximate mechani sm that regulate timing and duration of reproduction. We found that H. tull bergi entered a reproductive diapause when reared at constant temperature, a diapause that was terminated by a cold exposure (winter). Surprisingly, c old exposure of small juveniles may also prevent development of a reproduct ive diapause in adults. Thus, the life-cycle normally spanning 2 yr can pot entially be reduced to one year if the overwintering juveniles reach maturi ty before the end of the reproductive period in the field. After terminatio n of the diapause: the animals reproduced up to 3 times during a period of 6 weeks at 15 degrees C. This reproductive period was considerably longer ( measured in degree days) than the one observed in the field. Our results su ggested that temperature quiescence, i.e. the inability to reproduce under a certain temperature threshold, may adjust the termination of the reproduc tive period with current temperature before a new diapause occurs in late s ummer. The cost of reproduction was low and suggests that it can be adaptiv e to spread reproduction over more than one year. The results are discussed in relation to the arctic climate and strategies favoured by unpredictable and predictable (seasonal) variations in the environment. The present stud y forms part of a larger investigation on population dynamics and life hist ory strategies of H. tullbergi from the Arctic.