title: How reversible is development? Contrast between developmentally plastic gain and loss of segments in barnacle feeding legs.
abstract: Segmented organisms and structures have fascinated biologists since William Bateson first described homeotic transformation and recognized the fundamental evolutionary significance of segmental organization. On evolutionary time scales, segments may be lost or gained during major morphological transitions. But how segment loss compares to gain on developmental time scales remains mysterious. Here, we examine the ease of reverse development (opposite to normal growth) by comparing developmentally plastic leg segment loss versus gain in individual barnacles transplanted between different water flow conditions. Plastic segment addition occurred rapidly (one to two molts) and exclusively near the limb base. In contrast, developmentally plastic segment loss-the first observation in any arthropod-took much longer (>10 molts) and, remarkably, occurred throughout the leg (23% of losses occurred mid-limb). Segment loss was not a simple reversal of segment addition. Intersegmental membranes fused first, followed by elimination of duplicate tendons and gradual shortening (but not loss) of duplicate setae. Setal loss, in particular, may impose a severe developmental constraint on arthropod segment fusion. This asymmetric developmental potential (time lag of phenotypic response)-plastic segment addition (amplified normal development) is faster and more orderly than segment loss (reverse development)-adds a new dimension to models of developmental plasticity because the cost of making a developmental mistake in one direction will be greater than in the other.
authors.affiliation: Systematics and Evolution Group, Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada.|Bamfield Marine Sciences Centre, Bamfield, British Columbia, V0R 1B0, Canada.
authors.forename: Tomonari
authors.initials: T
authors.lastname: Kaji
authors.affiliation: Systematics and Evolution Group, Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada.|Bamfield Marine Sciences Centre, Bamfield, British Columbia, V0R 1B0, Canada.
authors.forename: A Richard
authors.initials: AR
authors.lastname: Palmer
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005121:Extremities / Q000033:anatomy & histology; D001468:Thoracica / Q000235:genetics / Q000254:growth & development*
keywords: Appendage; Cirripedia; arthropod; asymmetry of development; developmental constraint; developmental plasticity; evo-devo; limb; segmentation; time lag