Abstract

Many arthropods add body segments post-embryonically, including most of the myriapods. However, geophilomorph and scolopendromorph centipedes are epimorphic, i.e. they form all their segments during embryonic time, although this has never been demonstrated directly. Understanding the similarity between embryonic and post-embryonic segmentation is pivotal to understand the possible evolution from anamorphosis to epimorphosis. We have previously demonstrated that in the geophilomorph centipede Strigamia maritima most segments are produced by an oscillatory mechanism operating through waves of expression at double segment periodicity, but that the last-forming (posteriormost) segments are patterned with a different system which might be more similar to post-embryonic segmentation. With a careful analysis of a large number of specimens, I show that the first (“embryoid”) phase of post-embryonic development is clearly distinct from the following ones. It is characterized by more moults than previously reported, allowing me to define and name new stages. I describe these embryoid stages and the first free-leaving stage in detail, providing data on their duration and useful identification characters. At hatching, the prospective last leg-bearing segment is limbless and the genital segments are added in the following stages, indicating a residual anamorphosis in Strigamia segmentation. I demonstrate directly for the first time that at least the leg-bearing segments are in general produced during embryonic life, although in some individuals the external delineation of the last leg-bearing segment may be delayed to post-embryonic time, a possible further residual of anamorphic development. Additionally, I show that the development of the poison claws during this post-embryonic phase may have some element of recapitulation. The data presented in this paper show that the embryoid phase of post-embryonic development of geophilomorph centipedes may represent an extension of embryonic development, possibly in correlation with the evolution of epimorphic development from an anamorphic ancestor, accomplished without completely losing post-embryonic segmentation activity. This continuity in the segmentation process across the embryonic/postembryonic divide may concur to the evolvability of this developmental process.

Highlights

  • Metamerism is a key morphological character of arthropods

  • The first segments of those arthropods - and all segments in arthropods like the insects - are laid down embryonically through a molecular patterning process that can precede any morphological differentiation. This process extends to several segments or even to all segments. It is not clear whether this embryonic process is in continuity with the post-embryonic phase of segmentation, and if so to what extent, understanding this would be pivotal to understanding the evolutionary transition from anamorphosis to epimorphosis

  • Embryoid stages in Strigamia The following description of stages is based on the best conditions under which live embryoid specimens can be observed, i.e. immersed, mostly in mineral oil, with their exuviae artificially removed

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Summary

Introduction

Metamerism is a key morphological character of arthropods. To understand its evolution, it is fundamental to understand how this reiteration of modules along the anteroposterior (AP) axis is laid down during development. The first segments of those arthropods - and all segments in arthropods like the insects - are laid down embryonically through a molecular patterning process that can precede any morphological differentiation. This process extends to several segments (sequentially segmenting arthropods) or even to all segments (long germ arthropods like Drosophila). It is not clear whether this embryonic process is in continuity with the post-embryonic phase of segmentation, and if so to what extent, understanding this would be pivotal to understanding the evolutionary transition from anamorphosis to epimorphosis. We have previously demonstrated that in the geophilomorph centipede Strigamia maritima most segments are produced by an oscillatory mechanism operating through waves of expression at double segment periodicity, but that the last-forming (posteriormost) segments are patterned with a different system which might be more similar to post-embryonic segmentation

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