Diversity in Segmentation Mechanisms

Ariel D. Chipman

CONTENTS

  • 2.1 Introduction....................................................................................................27
  • 2.2 The Sequence of Segment Formation.............................................................28
  • 2.2.1 Embryonic or Post-Embryonic Segmentation....................................28
  • 2.2.2 Simultaneous or Sequential Segmentation.........................................29
  • 2.3 Cyclical Processes in Segmentation...............................................................29
  • 2.4 Proliferation versus Cell Rearrangement As Drivers of

Segment Formation.........................................................................................30

  • 2.5 Types of Cells in Segmentation Processes.....................................................31
  • 2.6 Commonalities in Segmentation Processes: Are There Any?........................32

References................................................................................................................33

Introduction

Segmented body plans are extremely common when measured in terms of the number of species that display them. However, in terms of their phylogenetic distribution, fully segmented body plans are only found in three phyla: arthropods, chordates, and annelids (Minelli and Fusco, 2004; Chipman, 2010). Of course, various degrees of repetitive units and metameric organization are found in numerous other taxa (see Chapter 9 for a detailed discussion). Nonetheless, regardless of the specific definition used for “truly” segmented organisms or for a segment (see Chapter 1), almost all authors would agree that the aforementioned three phyla display the most complete and consistent examples of segmented body plans.

This chapter serves as a general introduction to the themes discussed in detail in the subsequent chapters. In it, I will discuss the many different ways segments are generated and segmented body plans are arranged throughout the developmental process, and point out where in the book these points are elaborated upon. But, before doing so, it is worth asking what—if anything—segmented body plans have in common. If they have little or nothing in common, it is not surprising that they display a great diversity in developmental mechanisms. However, if there are common organizational principles to segments across taxa, and these are achieved through a diversity of mechanisms, we have an interesting phenomenon that is worth exploring, providing justification for the current book.

So what do segmented body plans have in common? Segmented body plans are made up of repeated body units, and these units contain components from a number of different organ systems, with coelomic sacs often considered to be the most important repeated units. This is the accepted definition of “true” segments (Scholtz, 2002; Minelli and Fusco, 2004; Hannibal and Patel, 2013). Beyond this, one of the hallmarks of segmented body plans, and the one that is most pertinent to the current discussion, is that during their development, segments pass through a transient stage wherein they are already morphologically distinct from segments anterior or posterior to them, but have not yet differentiated to give rise to their constituent organ systems (Scholtz, 2002; Chipman, 2019). Thus, individual segments are specified in ontogeny (embryonic or post-embryonic) as undifferentiated but distinct units, which undergo subsequent differentiation to the repeated elements of the different organ systems that make up the mature segment. The diverse mechanisms described in this chapter all give rise to such an undifferentiated transient segment.

 
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