Historical Constraints and the Evolution of Development 1149
diversity of arthropods evolved without an increase in Hox gene number. The
evolution of arthropod segmental diversity must therefore have involved regulatory
changes in Hox genes and/or their targets."
- Phylogenetic reconstruction affirms a full Hox complement for the bilaterian
common ancestor, with restriction and occasional unemployment far more prominent
than addition in subsequent evolution. The analysis of De Rosa et al. (1999) indicates
at least 8 Hox genes for the protostome common ancestor, and at least 7 for the
bilaterian progenitor (see Fig. 10-24). Moreover, comparisons at greater detail
support the growing consensus (see Halanych et al., 1995; and Aguinaldo et al., 1997,
based on 18S ribosomal RNA) that the protostome phyla split into two great
genealogical groups, the ecdysozoans or molting phyla (including arthropods,
nematodes and priapulids), and the lophotrochozoans (including annelids, mollusks,
brachiopods, platyhelminths, and nemerteans). The ecdysozoan genome includes the
posterior Hox gene Abd-B, whereas lophotochozoans have two "Abd-B-
10 - 24. By this analysis and cladogram, the protostome common ancestor must have possessed
at least eight Hox genes, whereas the bilaterian progenitor must have had seven. From De
Rosa et al., 1999.