Evolution of the Stereotyped Division Pattern

As mentioned earlier, the regular gridlike arrangement of post-naupliar ectoderm and mesoderm cells and the elaborate stereotyped cell division pattern is restricted to mal- acostracan crustaceans. A closer look at some branchiopod, cirripede, and copepod species did not reveal a comparable pattern. Neither teloblasts nor the gridlike cell arrangement have been identified (Gerberding 1997; Dohle et al. 2004; Ponomarenko 2014; Hein and Scholtz 2018) (Figures 6.5A and 6.8). Furthermore, the figures of Stegner and Richter (2015) on the development of a cephalocarid species do not indicate the presence of regular cell arrangements and teloblasts. In addition, claims that cir- ripedes and anostracans possess teloblasts (Anderson 1967,1969) could not be substantiated by more recent investigations (Benesch 1969; Dohle et al. 2004; Ponomarenko

2014). However, in some non-malacostracan crustaceans (Anostraca, Copepoda) an unpaired midline cell population has been recognized (e.g., Dohle et al. 2004; Hein et al. 2019). Moreover, the images of nauplii of parasitic copepods suggest that some sort of regular cell arrangement occurs (McClendon 1907). In particular, in the light of recent phylogenetic hypotheses of Tetraconata/Pancrustacea (Regier et al. 2010; Schwentner et al. 2017), in which Copepoda are either the sister group of Malacostraca or the sister group of Thecostraca plus Malacostraca, this deserves a reinvestigation to reveal whether it shows similarities to the pattern found in malacostracans.

Comparison of relative cell size, cell numbers, and cell fate

FIGURE 6.8 Comparison of relative cell size, cell numbers, and cell fate. (A) The ventral aspect of an embryo of the cladoceran Leptodora kindtii. (B) The ventral aspect of the germ band of the amphipod Cryptorchestia garbinii. In both embryos (nuclei stained with the fluorescent dye: Bisbenzimide, Hoechst Blue) the labrum (lr), the antennae (al, a2), the mandibles (md), and corresponding thoracic segments (thl, th2) are labeled. It is obvious that in the non-malacostracan species (a) many more and relatively small cells form the corresponding structures compared with the malacostracan (B).

An invariant cell division pattern in the germ band has not been found in Chelicerata, Myriapoda, and Hexapoda (Anderson 1973). Thus, it appeared evolu- tionarily somewhere within the Pancrustacea/Tetraconata, most likely in the lineage leading to Malacostraca (Fischer et al. 2010). Furthermore, there are indications that some aspects of this elaborated pattern evolved only within malacostracans, namely in the Caridoida (Richter and Scholtz 2001). The leptostracans and stomatopods differentiate teloblasts with characteristic behavior (Manton 1934; Shiino 1942; Fischer et al. 2010). Yet, both groups reveal irregularities of the cell division pattern during later stages compared with those of decapods and peracarids (Fischer et al. 2010) (Figure 6.9).

The reasons for the evolution of the invariant cell division patterns of malacostracans are unknown. However, a possible explanation might be that malacostracans show a relatively low number of cells in their germ bands when compared to other crustaceans and arthropods (Figure 6.8). With a low number of cells, a stereotyped

Row formation in the ectoderm of Leptostraca and Stomatopoda

FIGURE 6.9 Row formation in the ectoderm of Leptostraca and Stomatopoda (surface rendering of confocal stacks; modified after Fischer et al. 2010, with permission from Elsevier). (A) Ventral side of the caudal papilla of the leptostracan Nebalia sp. (fluorescent dye; bis- benzimide, Hoechst Blue). An area with cells arranged in transverse rows and longitudinal columns in front of the telson ectoderm (te). The ectoteloblasts already finished their characteristic asymmetric divisions and are no longer recognizable. (B) Caudal papilla and ventral germ band of the stomatopod Gonodactylaceus falcatus (fluorescent dye: Sytox Green). In both species, the sister cells of the first wave of division of the cell row's show' an oblique orientation (arrowheads). This is different from the situation in the remaining malacostracans (Caridoida sensu Richter and Scholtz 2001) (see Figures 6.5 and 6.6). Hence, the pattern found in Leptostraca and Stomatopoda can be considered as ancestral among Malacostraca.

cell division is necessary to put the right cell in the right place at the right time to generate a specific determination of cell fate, as, for instance, a neuronal precursor or the tip of a limb bud. If there are numerous cells to form a limb or a ganglion anlage, an exact position or number of individualized cells is not required (Schnabel 1997).

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