13. Hox Genes and Arthropod Diversity
205
establish homologies between body regions. However, once regional specialisations
were established, and linked to the expression of particular Hox (or other) genes,
we can define homologies and ask specific questions concerning the relation of one
body plan to another - for example, have the genes that define the post-genital
abdomen of the branchiopod crustaceans lost any such role in insects and higher
crustaceans, has their domain of function been collapsed to a single segment, or
have they acquired new and unrecognisable downstream meanings, as seems to
have occurred in the head.
4.3 Shifting Boundaries and Segment Specialisation
Within Tagma
Comparisons between such distantly related organisms as insects, chelicerates and
onychophorans provide little insight into the sequence of evolutionary intermediates that lead to these different body plans, or the functional consequences of those
changes. For this purpose, comparisons between more closely related species are
much more informative. Averof and Patel (1997) have used the cross-reacting Ubx/
abd-A antibody in this way, to show that the diversification of segments within the
crustacean thorax (pleon) has been accompanied by changes in the boundaries of
expression of Hox genes. They have examined Crustacea in which one or more of
the trunk segments have been recruited as auxiliary mouthparts, or maxillipeds.
The possession of a uniform array of thoracic segments is presumed to be a
basal characteristic of the crustaceans (Schram 1986). This pattern, seen in Artemia
and Triops among the branchiopod crustaceans, is also seen in some maxillopodans,
and in the basal branches of the malacostracans (e.g. Nebalia). However, maxillipeds have arisen repeatedly in many groups, providing an excellent test case to
correlate changes in gene expression with the evolution of new morphology. In the
case of the malacostracans, this change in segment patterning has occurred against
a background of conserved segment numbers in the thoracic tagma, making it particularly clear that the process involves a transformation in segment specialisation,
and not the intercalation of extra segments.
Averof and Patel found that the diverse species lacking maxillipeds express the
Ubx/abd-A class proteins throughout the thorax, with a boundary at the first thoracic segment. Thus for crustaceans, it is reasonable to assume that this is the ancestral pattern. Species with maxillipeds show a different pattern _. with the expression boundary for these Hox proteins being shifted more posteriorly. The extent of the shift correlates with the number of maxillipeds that will form (though
this correlation, established in the embryo, relates to the form of the animal at
hatching, and not directly to the adult morphology). Thus in this case we can infer
that repeated shifts in Hox expression boundaries have occurred concomitant with
the evolution of increasingly complex body plans.
205
establish homologies between body regions. However, once regional specialisations
were established, and linked to the expression of particular Hox (or other) genes,
we can define homologies and ask specific questions concerning the relation of one
body plan to another - for example, have the genes that define the post-genital
abdomen of the branchiopod crustaceans lost any such role in insects and higher
crustaceans, has their domain of function been collapsed to a single segment, or
have they acquired new and unrecognisable downstream meanings, as seems to
have occurred in the head.
4.3 Shifting Boundaries and Segment Specialisation
Within Tagma
Comparisons between such distantly related organisms as insects, chelicerates and
onychophorans provide little insight into the sequence of evolutionary intermediates that lead to these different body plans, or the functional consequences of those
changes. For this purpose, comparisons between more closely related species are
much more informative. Averof and Patel (1997) have used the cross-reacting Ubx/
abd-A antibody in this way, to show that the diversification of segments within the
crustacean thorax (pleon) has been accompanied by changes in the boundaries of
expression of Hox genes. They have examined Crustacea in which one or more of
the trunk segments have been recruited as auxiliary mouthparts, or maxillipeds.
The possession of a uniform array of thoracic segments is presumed to be a
basal characteristic of the crustaceans (Schram 1986). This pattern, seen in Artemia
and Triops among the branchiopod crustaceans, is also seen in some maxillopodans,
and in the basal branches of the malacostracans (e.g. Nebalia). However, maxillipeds have arisen repeatedly in many groups, providing an excellent test case to
correlate changes in gene expression with the evolution of new morphology. In the
case of the malacostracans, this change in segment patterning has occurred against
a background of conserved segment numbers in the thoracic tagma, making it particularly clear that the process involves a transformation in segment specialisation,
and not the intercalation of extra segments.
Averof and Patel found that the diverse species lacking maxillipeds express the
Ubx/abd-A class proteins throughout the thorax, with a boundary at the first thoracic segment. Thus for crustaceans, it is reasonable to assume that this is the ancestral pattern. Species with maxillipeds show a different pattern _. with the expression boundary for these Hox proteins being shifted more posteriorly. The extent of the shift correlates with the number of maxillipeds that will form (though
this correlation, established in the embryo, relates to the form of the animal at
hatching, and not directly to the adult morphology). Thus in this case we can infer
that repeated shifts in Hox expression boundaries have occurred concomitant with
the evolution of increasingly complex body plans.