The Sequence of Segment Formation

The plesiomorphic mode of segment addition is sequential addition from a subterminal domain in the posterior of the organism (Scholtz, 2002). This is true for all three of the fully segmented phyla, but there are significant differences among them and within them.

Embryonic or Post-Embryonic Segmentation

The most noteworthy variability is in whether some or all segment addition occurs during embryonic development or post-embryonically. Within annelids, the primitive and most common pattern is embryonic development to a trochophore larva, which is essentially unsegmented but includes two to three ciliary bands. A posterior region of this larva, the area between the metatroch and the telotroch (the middle and posterior ciliary bands, respectively) becomes the segment addition zone, and segments are patterned sequentially from the posterior in a post-embryonic stage (Bleidorn etal., 2015). In many cases (e.g., in the model species Platynereis dumeri- lii; Fischer et al., 2010), several anterior segments form during the early larval stage. This gives rise to a nectochaete larva, which grows through posterior segment addition (see Chapter 4 for more details on the diversity of annelid segmentation modes). Clitellates (leeches, earthworms, and their relatives), unlike most other annelids, are direct developers and generate all segments during embryogenesis through specialized stem cells known as teloblasts (see Chapter 7).

Segmentation in chordates is almost exclusively embryonic (cephalochordates being an exception). Segments are generated from an embryonic domain, known as the pre-somitic mesoderm (PSM), through a cellular oscillator that leads to the organization of distinct mesodermal units, known as somites (Palmeirim et al., 1997). All trunk and tail segments are formed using the same mechanism, during embryo- genesis, with no evidence of unusual anterior segments.

Arthropods present a much more complex picture. There is some debate as to whether direct development is the plesiomorphic state in arthropods or not, and there have been arguments raised both for embryonic generation of all segments as primitive (Chipman, 2015) and for a biphasic life cycle being primitive (Wolfe, 2017). Either way, both direct development and indirect development are found in most clades within arthropods. Insects and arachnids are almost exclusively direct developers, with all segments formed in embryogenesis (but one pair of legs differentiating only post-hatching in mites, and three segments added post-embryonically in proturans). Crustaceans are mixed, with many taxa developing into a nauplius larva, which adds segments sequentially post-embryonically, and some taxa (e.g., isopods, amphipods) generating all segments during embryonic development. In myriapods again both modes are found. Millipedes add segments post-embryonically, whereas in centipedes, post-embryonic segmentation is plesiomorphic and generation of all segments embryonically is derived—including in the geophilomorphs, which have the highest segment numbers among centipedes (Minelli, 2001).

Among fossil arthropods, there are highly detailed descriptions of post-embryonic segment addition in trilobites (Hughes et al., 2006; Hopkins, 2017). Recently, there have been reports of post-embryonic segment addition in the stem group arthropod Fuxianhuia (Fu et al., 2018).

Simultaneous or Sequential Segmentation

A second variability in mode of segment formation is the extent to which segments are patterned sequentially or simultaneously. In chordates, again the picture is the simplest. All chordates generate all segments sequentially. In annelids, the formation of the nectochaete segments is almost simultaneous, but all other segments are formed sequentially, whether they are embryonic or post-embryonic (Bleidorn et al., 2015).

Arthropods present great diversity in this aspect as well. While most segments in most species appear sequentially, even in direct developing species, this is not true of all segments. The anteriormost segments (the pre-gnathal segments, or naupliar segments) often appear separately from other segments, through a distinct mechanism about which we know relatively little (Scholtz and Edgecombe, 2006; Posnien et al., 2010; Janssen et al., 2011). Segments of the post-gnathal head and sometimes of the trunk/thorax are often patterned simultaneously (Stahi and Chipman, 2016). The most extreme cases of simultaneous segmentation are found in many holometabo- lous insects, including the best studied model species, Drosophila melanogaster, where all segments are patterned simultaneously, and there is no posterior addition of segments at all (Hartenstein and Chipman, 2015; Stahi and Chipman, 2016). Amphipod crustaceans are unusual among crustaceans in having an almost simultaneous assembly of the segments, without a phase of posterior addition (Scholtz and Wolff, 2002; Hannibal et al., 2012).

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