Serially homologous neurones as concomitants of functional specialisation

Abstract

MOST higher invertebrates and all vertebrates are metamerically segmented and their evolution has depended on the progressive functional specialisation of serially repeated, originally similar segments. In structurally similar body segments, serially homologous neurones have been shown to have remained largely unchanged^1–4. Is this also the case for serially homologous neurones in segments which show important morphological and functional differences? We report here that we have identified motoneurones innervating muscles in the first two pairs of legs of the locust Schistocerca gregaria, Forskal (=S. americana, Dirsch). These legs are similar and represent the primitive unspecialised form, functioning especially in posture and walking. The hind legs are greatly enlarged and specialised for jumping, and motoneurones innervating muscles in these legs have been identified⁵. In walking, the legs in a segment move alternatively, whereas in jumping, both hind legs act almost synchronously⁶. We have found that serially homologous motoneurones have evolved new functional properties for jumping while retaining their primeval morphologies.