Homology Issues

The observed evolutionary alterations of the ectoteloblast patterns raise some interesting questions concerning homology. The very detailed correspondence between the row arrangement and the subsequent division patterns of ectodermal cells in the germ band in amphipods and other malacostracans strongly suggests homology (Dohle and Scholtz 1988; Scholtz and Dohle 1996; Dohle et al. 2004). Yet, this homologous pattern is generated by different processes. In the ancestral case, ectoteloblasts form the rows by specific asymmetric mitoses in a certain sequence. By contrast, in the lineage leading to amphipods, these ectoteloblasts are lost and the migration of scattered ectodermal cells leads to row formation (see Figures 6.5 and 6.6). This means that homologous structures are not necessarily formed via the same developmental pathway. There are numerous examples of this phenomenon (see Scholtz 2005). Yet, because it is counterintuitive, many discussions of homology stress similar ontogeny as necessary or at least indicative for inferring homology (see Scholtz 2005 for discussion). Moreover, different developmental pathways are often used to negate assumptions of homology of similar structures or to show their convergence based on this argument.

The alteration of the ancestral ring of 19 ectoteloblasts to about 40 in the freshwater crayfish bears on the problem of serial homology (see Scholtz 1993). Are the ectotelo- blast rings as such homologous despite the different numbers of cells? Alternatively, are just the inner (ventral) 19 ectoteloblasts of the crayfish homologous to the 19 ectoteloblasts of other malacostracans? But what about the 20 more dorsal ectoteloblasts of crayfish? These form the dorsal side of the posterior thoracic and the pleonic segments. By contrast, in other malacostracans these dorsal parts are formed by the dorsal ectoteloblasts within the ring of 19 ectoteloblast cells (see Figure 6.2). No one questions the homology of the tergites of a lobster (19 ectoteloblasts) and a crayfish (about 40 ectoteloblasts). In any case, the cell division pattern and the fate of the ventral ectoderm cells in crayfish correspond to those of other malacostracans. This means that, again, the question of the homology of teloblasts has to be seen as independent of the question of the homology of the cell division pattern in the germ band.

Yet, this example touches the more general problem of the homology of serial structures, since similar problems occur with the number of cervical vertebrae among tetrapods, the number of segments in arthropod tagmata, or the number of vertebrate teeth. In all these examples there is a defined entity or frame (neck, a tagma such as thorax, mouth dentition) that is considered homologous but the number of serial elements that constitute this entity varies (see Goodrich 1913; Scholtz 1993; Muller and Wagner 1996; Bohmer et al. 2018).

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