Spatial structuring in trait variation in Polyommatus icarus in a functional context
The Common blue butterfly, Polyommatus icarus (Rott,) , is widespread throughout its Palaearctic distribution and persists in areas with differing climatic conditions. It is known to be a highly variable butterfly with marked within and between population variation in morphology, thermal biology, and voltinism. These traits together with allozymes and a neutral DNA marker (AFLP) variation are studied here to understand how geographic trait variation is related to environmental variation. The approach adopted here is to study this along a latitudinal cline of temperature and photoperiod, using four populations from south to north within mainland Britain. AFLP differences, but not allozyme variation, indicate genetic structuring, with an isolation by distance effect. Enzyme diversity of P. icarus butterflies in the British Isles is lower than on mainland Europe, indicative of a past bottleneck. This, combined with selection on, or drift in, the allozymes could cause for a lack of population structure in this marker. Despite high levels of gene flow between populations, local adaptation is possible, as differentiation in certain allozyme loci was found (PGM and PGI). Populations differed in their response to developmental cues. Northern populations have an obligate diapause strategy and southern populations' development times differ in response to temperature, indicating local variation in response to environmental conditions. Populations differed in wing morphology (size, shape and melanisation) but this was not related to latitude. Experimental determination of heating rates in different basking positions and thoracic temperature at take-off revealed no strong relationships of the morphological characteristics with heating or cooling rates and an indication of relationship with PGI alleles. It is suggested that m comparison to larger butterflies morphological variation IS unconstrained by thermal requirements. The persistence and widespread occurrence of this butterfly may be a consequence of the variability of traits within this butterfly.
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