1150 THE STRUCTURE OF EVOLUTIONARY THEORY
like" genes named Post1 and Post2. In addition, ecdysozoan genomes include Ubx in
the central cluster, whereas lophotrochozonas share the different, but closely related,
Lox2.
Although De Rosa et al. (1999) cite 7 as a minimum for the common bilaterian
ancestor (lab/Hox1, pb/Hox2, Hox3, Dfd/Hox4, Scr/Hox5, one additional central gene
and one posterior gene), Figure 10-24 also indicates at least 10 shared Hox genes for
the stem lophotrochozoan and 8 for the stem ecdysozoan. Using a more generous
estimate based on a hypothesis that "most or all of the Hox genes that are present in
extant bilaterians may have been present in the common ancestor, but that some
orthology relationships have become obscured" (De Rosa et al., 1999, p. 775), the
protostome common ancestor might have possessed ten Hox genes (the 7 listed above
plus 2 central and 1 posterior), or even more if the deuterostome situation of multiple
posterior Hox genes is primitive rather than derived. In any case, either a minimum of
7 or a maximum of 10 or more provides ample support for the key conclusion that a
full Hox complex had already evolved before the establishment of distinctive features
of the major bilaterian Bauplan. De Rosa et al. (1999, p. 775) conclude their article
by stating: "The subsequent bilaterian history of Hox genes would have been
primarily one of functional divergence and gene loss, rather than gene duplication.
Regardless of the exact number of Hox genes in the bilaterian ancestor, the major
period of progressive expansion of the Hox cluster due to tandem duplication events
predated the radiation that generated the bilaterian crown phyla, concurrent with
radical evolutionary changes in body architecture and development."
As a fascinating footnote to the rich phyletic information contained in the
conservation of Hox genes, the Mesozoa have long presented a deep puzzle in the
study of animal phylogeny. These creatures lack body cavities and effectively all the
characteristic organs of animals, including a gut or a nervous system. Their
maximally simplified development even proceeds without gastrulation or the
differentiation of germ layers. Many zoologists have therefore considered their
organization as primitive, and have even regarded the Mesozoa as a surviving key to
the phyletic transition between unicellularity and the evolution of truly multicellular
organization with differentiation of tissues and organs—hence their name, Mesozoa
(from the Greek meso, meaning "middle"), as a potential intermediate between the
protistans, formerly called Protozoa, and the true Metazoa. But the mesozoans are
parasites of metazoans, and parasites often become extremely simplified in
phenotype. Thus, the opposite interpretation of descent from an ordinary and complex
metazoan ancestor has remained entirely plausible. Unfortunately, the highly
simplified and autapomorphic anatomy of mesozoans has provided no clues about
ancestry, despite more than a century of extensive study.
But Kobayashi et al. (1999) have isolated a Hox gene, DoxC, from a dicyemid
mesozoan (parasites of cephalopod renal sacs). PCR analysis shows that DoxC is an
ortholog of the "middle group" Hox series. The middle group Hox genes have only
been found in triploblasts, and do not exist in Cnidaria. Hence, these data would seem
to validate the hypothesis that mesozoans