200
M. Akam
I illustrate this below with reference to the diversity of arthropods. These studies are in their infancy, with few taxa sampled, and incomplete data available for
even those few taxa. However, my preliminary conclusion from these studies is that
patterning of the arthropod head and trunk have been evolving in rather different
ways. In the head, the expression domains of Hox genes appear to have become
fixed in one particular relation to segment formation before the major surviving
clades of arthropods diverged, whereas in the trunk, changes in the relationship
between segment formation and Hox gene expression have played, and probably
continue to play, a major role in the diversification of arthropods.
4.1 The Arthropod Head
Segmentation of the head is one of the key characters used to define the four major
subphyla of the arthropods (Brusca and Brusca 1990). Crustacea are defined by a
head containing six segments - the eye bearing segment innervated from the first
ganglion of the brain, followed by two pairs of segments bearing antennae, and
three pairs of segments making up the jaws - a biting mandible, and two pairs of
maxillae. Insects look superficially very different. They have only one pair of antennae, and the mouthparts may be highly modified. It has been one of the triumphs
of comparative morphology to recognise that the diversity of insect mouthparts
may be related to a common plan, and that this common plan is recognisably similar to that of Crustacea (Snodgrass 1931; Rogers and Kaufman 1997). The primary
difference is that the appendage of the second antennal segment has been lost,
though the segment which should carry it, the so-called intercalary segment, is
clearly visible in the embryo. Both insects and crustaceans have a biting mandible
as the first mouthpart segment, and so these arthropods are termed mandibulates.
Myriapods- millipedes, centipedes and their kin, have traditionally been viewed
as closely allied to the insects within the mandibulate arthropods, but the phylogenetic position of the myriapods is now in doubt (Averof and Akam 1995b; Freidrich
and Tautz 1995), and the similarity of their mouthparts to those in insects is questionable (Popadic et al. 1996). There arc no data yet available on the expression of
Hox genes in the head of myriapods, so I will leave them out of this discussion.
The organisation of anterior segments in chelicerates is indisputably very different from that of the mandibulates. These animals (spiders, scorpions, mites and
horseshoe crabs) have no antennae, and only two pairs of specialised segments
behind the eye - the chelicerae and the pedipalps (Brusca and Brusca 1990). The
pedipalps are followed immediately by the walking legs, which are born on the
same body region as the mouthparts, termed the prosoma.
The split between chelicerates and mandibulates has traditionally been seen as
one of the most basal among living arthropods, and this is supported by molecular
phylogenies, problems with the myriapods not withstanding. Therefore, if we are to
have some idea of the ancestral morphology of the arthropods, and the evolution of
appendage specialisation in each group, we need to be able to say how the structures of the mandibulate and chelicerate heads relate to one another. Unfortunately,
M. Akam
I illustrate this below with reference to the diversity of arthropods. These studies are in their infancy, with few taxa sampled, and incomplete data available for
even those few taxa. However, my preliminary conclusion from these studies is that
patterning of the arthropod head and trunk have been evolving in rather different
ways. In the head, the expression domains of Hox genes appear to have become
fixed in one particular relation to segment formation before the major surviving
clades of arthropods diverged, whereas in the trunk, changes in the relationship
between segment formation and Hox gene expression have played, and probably
continue to play, a major role in the diversification of arthropods.
4.1 The Arthropod Head
Segmentation of the head is one of the key characters used to define the four major
subphyla of the arthropods (Brusca and Brusca 1990). Crustacea are defined by a
head containing six segments - the eye bearing segment innervated from the first
ganglion of the brain, followed by two pairs of segments bearing antennae, and
three pairs of segments making up the jaws - a biting mandible, and two pairs of
maxillae. Insects look superficially very different. They have only one pair of antennae, and the mouthparts may be highly modified. It has been one of the triumphs
of comparative morphology to recognise that the diversity of insect mouthparts
may be related to a common plan, and that this common plan is recognisably similar to that of Crustacea (Snodgrass 1931; Rogers and Kaufman 1997). The primary
difference is that the appendage of the second antennal segment has been lost,
though the segment which should carry it, the so-called intercalary segment, is
clearly visible in the embryo. Both insects and crustaceans have a biting mandible
as the first mouthpart segment, and so these arthropods are termed mandibulates.
Myriapods- millipedes, centipedes and their kin, have traditionally been viewed
as closely allied to the insects within the mandibulate arthropods, but the phylogenetic position of the myriapods is now in doubt (Averof and Akam 1995b; Freidrich
and Tautz 1995), and the similarity of their mouthparts to those in insects is questionable (Popadic et al. 1996). There arc no data yet available on the expression of
Hox genes in the head of myriapods, so I will leave them out of this discussion.
The organisation of anterior segments in chelicerates is indisputably very different from that of the mandibulates. These animals (spiders, scorpions, mites and
horseshoe crabs) have no antennae, and only two pairs of specialised segments
behind the eye - the chelicerae and the pedipalps (Brusca and Brusca 1990). The
pedipalps are followed immediately by the walking legs, which are born on the
same body region as the mouthparts, termed the prosoma.
The split between chelicerates and mandibulates has traditionally been seen as
one of the most basal among living arthropods, and this is supported by molecular
phylogenies, problems with the myriapods not withstanding. Therefore, if we are to
have some idea of the ancestral morphology of the arthropods, and the evolution of
appendage specialisation in each group, we need to be able to say how the structures of the mandibulate and chelicerate heads relate to one another. Unfortunately,