Introduction

A segmented body plan has been a key feature of arthropod diversification and evolution. Most arthropods build their segmented bodies during embryogenesis or early larval development, and do so by the sequential addition of segments from the posterior (reviewed in Sander, 1975; Davis and Patel, 2002; Peel et al„ 2005). The contrast between this mode of segmentation and the extensively studied model arthropod, Drosophila, has sparked numerous studies in diverse taxa examining the regulation of sequential segmentation (reviewed in Davis and Patel, 2002; Seaver, 2014; Minelli, 2004; Peel, 2004; Liu and Kaufman, 2005; Peel et ah, 2005; Damen, 2007; Blair, 2008; Couso, 2009; Chipman, 20Ю; also see Chapter 2). Studying segment patterning in arthropod model species outside of flies has brought focus to the cellular behaviors that elongate the body and accommodate the progressive addition of new segments (Freeman, 1995; Benton et ah, 2013; Nakamoto et ah, 2015; Cepeda et ah, 2017; Benton, 2018; Hemmi et ah, 2018). New insights into cell behaviors in the posterior have emerged from careful temporal analyses, fate mapping, live imaging, and functional genetic studies. It is clear that one variable feature of the evolution of arthropod segmentation has involved changing the degree to which embryos rely on cell division versus cell rearrangement to elongate.

Here we review our current understanding of the relative roles of cell division and cell rearrangement, discuss other possible contributors to elongation, and briefly discuss how these processes relate to cell fate determination. We summarize current findings about growth and elongation in sequentially segmenting arthropods. Key elements are a minor, but essential, role for posterior cell division that is both temporally and spatially controlled by the posterior organizer, as well as a role for convergent extension driven by diverse mechanisms.

Segmentation and Elongation: The Evolving Roles of Cell Division and Cell Rearrangement

Among arthropod embryos, the size of the initial embryonic anlage from which segments form is a highly variable feature. Consequently, the degree of growth needed to add new segments and elongate a fully segmented embryo or larva is also highly variable. This variability in initial size of the anlage is characterized in insects by the designation “long germband” versus “short germband” development (Krause, 1939; reviewed in Sander, 1975; Davis and Patel, 2002). While these categories were originally used for the comparative description of insects, embryos and larvae in other arthropod taxa also vary in the degree to which clusters of anterior segments are simultaneously specified as well as the number of total body segments formed prior to hatching (reviewed in Scholtz and Wolff, 2013). Despite this variability, all embryos or larvae typically show notable elongation during their early development. Elongation primarily depends on either increasing the number of cells or rearranging the relative positions of cells. Indeed, the relative degree of cell division versus cell rearrangement is a key variable in evolution. Oriented cell division or coordinated changes in cell shape can also influence elongation but may play more minor roles (e.g., see da Silva and Vincent, 2007, also Freeman, 1995), or may simply be less well known.

 
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