'Vertition' of integumental organs in mites revisited: a case of fluctuating asymmetry.
Vertition in mites is defined as a meristic variation for a bilateral integumental organ with a separate genetic control for each body side. A prominent hypothesis expressed by Grandjean is the role of vertition in the evolutionary trend towards a reduced number of hair-like organs (mechano- and/or chemo-receptors) known to have occurred in many mite groups. Observations on leg setae in the two-spotted spider mite Tetranychus urticae do not support this hypothesis. Meristic variation for leg setae rather conforms to the notion of fluctuating asymmetry: the difference between the number of leg setae on the right and left sides of the body had a unimodal distribution with a mean of zero. Moreover, lack of heritability for left/right absences of leg setae in an inbred laboratory strain suggests that vertition could be purely environmental. It is therefore argued that meristic variation for hair-like organs in mites is caused by random developmental accidents not corrected by homeostatic mechanisms normally resulting in a perfect bilateral symmetry.
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Belgian Royal Institute of Natural Sciences, rue Vautier 29, B-1000 Brussels, Belgium.
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Bauchau
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Wauthy
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D000818:Animals; D002628:Chemoreceptor Cells; D005121:Extremities / Q000033:anatomy & histology / Q000294:innervation; D008465:Mechanoreceptors; D008925:Mites / Q000033:anatomy & histology*