Latitudinal cline in segment number in an arthropod species,Strigamia maritima

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Abstract
Arthropods vary more than 30–fold in segment number. The evolutionary origins of differences in segment number among species must ultimately lie in intraspecific variation. Yet paradoxically, in most groups of arthropods, the number of segments is fixed for each species and shows no intra– or interpopulation variation at all. Geophilomorph centipedes are an exception to this general rule, and exhibit intraspecific variation in segment number, with differences between individuals being determined during embryonic development and hence independent of population age structure. Significant differences in segment number between different geographical populations of the same species have been previously reported, but insufficient sampling has been conducted to reveal any particular geographical pattern. Here, we reveal a latitudinal cline in segment number in the geophilomorph species Strigamia maritima: segment number in British populations decreases with distance north. This is the first such cline to be reported for any centipede species; indeed as far as we are aware it is the first such cline reported for any arthropod species. In vertebrates, fish are known to exhibit a latitudinal cline in segment number, but interestingly, this is in the opposite direction; fish add segments with increasing latitude, centipedes subtract them.