Local climate is an important source of selection on life histories that has been well investigated in cline studies where populations from different climates are compared. Biotic factors depending on climate vary with it but are rarely considered as agents of selection in these studies. A previous study suggested that resource distribution that depends on climate may be more important than climate itself in selection of life histories. To confirm this result, we compared life histories of populations of a Drosophila parasitoid, Leptopilina boulardi, originating from two different climates and from two environments with different hosts availability for each climate.
We sampled four populations of L. boulardi near the France-Spain border, two in the French Pyrenean Mountains where winters are cold and snowy and summers are mild, and two in the North of Spain where winters are mild and summers are very hot and very dry. Two different environments were sampled in each area: orchard and forest. Orchards offer lots of laying opportunities over small distances for parasitoids while forests offer less and more dispersed opportunities.
We measured egg load, longevity, lipid content, wing loading, body size and development time on parasitoids reared at 25°C.
Females from forests had different life histories than females from orchards, independently of the original climate. At 25°C, females from forests invested more in reproduction, lived fewer days and emerged later with smaller lipid content than females from orchards.
In relatively poor and stochastic habitats such as forests, the increased fecundity allows the exploitation of above-average reproductive opportunities, while relatively few opportunities are lost by a shorter life. Thus, shifting resources to eggs covers a greater part of the variation in reproductive opportunities about the expected value. Analogously, a lot of laying opportunities would be lost by a shorter life in rich habitats such as orchard and a shift from eggs to lifespan may be adaptive there.
Our results show the strong importance of resource distribution in selection of life histories, that is never considered in cline studies. We are now processing AFLP analyses to understand if observed differences are the result of genetic differentiation between populations from orchards and forests or migration of individuals with a particular genotype from an environment to the other one. Plasticity of these traits in response to different rearing temperatures has also been investigated. The results will be discussed during the meeting.