The Development of Arthropod Segmentation Across the Embryonic/Post-embryonic Divide – An Evolutionary Perspective

Abstract

In many arthropods, the appearance of new segments and their differentiation are not completed by the end of embryogenesis but continue, in different form and degree, well after hatching, in some cases up to the last post-embryonic molt. Focusing on the segmentation process currently described as post-embryonic segment addition (or, anamorphosis), we revise here the current knowledge and discuss it in an evolutionary framework which involves data from fossils, comparative morphology of extant taxa and gene expression. We advise that for a better understanding of the developmental changes underlying the evolution of arthropod segmentation, some key concepts should be applied in a critical way. These include the notion of the segment as a body block and the idea that hatching represents a well-defined divide, shared by all arthropods, between two contrasting developmental phases, embryonic vs. post-embryonic. This eventually reveals the complexity of the developmental processes occurring across hatching, which can evolve in different directions and with a different pace, creating the observed vagueness of the embryonic/post-embryonic divide.

Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis

“…We used lnCCS as the main logsize variable, because it is considered to be relatively unaffected by anamorphosis (the postembryonic addition of body segments; Fusco and Minelli 2021), as cephalic segment number remained constant throughout postembryonic ontogeny.…”

Revisiting kinorhynch segmentation: variation of segmental patterns in the nervous system of three aberrant species

“…Similarly, arthropods, annelids and chordates can generate segments in a different way between body regions, and with different temporal sequences [ 51 , 52 , 62 ]. Particularly, in arthropods with anamorphic development, segment differentiation has been reported to continue in different form and degree post-embryonically (summarized in [ 63 ]).…”

Serial Homology

“…Assessment of serial homology can be a research target, but also the starting point for investigating other evolutionary questions. For instance, one can be interested in (1) the morphological evolution of the elements of a series, including the tendency toward increasing or decreasing similarity among them (e.g., Siomava et al 2020), (2) the evolution of individuation of some or all the elements of the series (e.g., Monteiro 2008), (3) the degree of morphological integration between the elements of the series (e.g., Billet and Bardin 2019), (4) the evolutionary trends in the number of elements of the series (e.g., Fusco 2005), or (5) their ontogeny (e.g., Fusco and Minelli 2021).…”