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M.Akam
tent pattern, they may provide powerful discrimination between alternative hypotheses.
Comparisons of Hox gene expression in the arthropod trunk suggest a more
flexible relationship between the processes of segment formation and Hox gene
regulation. Changes in this relationship have led to changes in the pattern of
tagmosis, which underlie the functional specialisation and evolutionary radiation
of the arthropods.
1 Introduction
Diversity of form is one of the most striking characteristics of life on earth. This
diversity depends on genetic instructions, acting through the mechanisms of development. Until recently we knew little of these mechanisms and even less about how
changes in genes could generate diversity of form. Different phyla were considered
to be so fundamentally different that comparisons between, for example, insects
and vertebrates were thought unlikely to reveal many similarities in the molecular
mechanisms of development.
Research in developmental genetics and genomics has radically changed that
view. For a few model organisms it is now possible to outline how genes define the
body plan. Moreover, it is clear that many, and perhaps most of the developmental
mechanisms in metazoa depend on a "tool kit" of genes that arose before the major
phyla of animals diversified. These conserved tools are used, sometimes in remarkably similar ways, to build the diversity of modern organisms.
Paradoxically, it is only because we can recognise these molecular similarities
that we can now begin to study the mechanisms that generate the diversity of animal forms. We can use what we know from well studied model systems to inform
comparative studies of other animals. In what foHows, I explore this approach in
the context of studies on the Hox genes of arthropods.
The Hox genes provided one of the first and still one of the most dramatic
examples of universality in developmental mechanisms. Below, I introduce their
role in development, largely as we understand it from studies in Drosophila. I then
consider the antiquity and diversity of this gene family and, as a slight digression,
mention some recent results using Hox gene sequences that address the relationships of the metazoan phyla. Finally, I return to the question of how studies of the
Hox genes help us to understand the diversity of body plans among arthropods.
2 The Hox Genes
The Hox genes were first identified in Drosophila over 80 years ago, by a class of
striking mutations that transform one part of the body into another (Bridges and
Morgan 1923). Sporadic individuals showing similar phenotypes had previously
M.Akam
tent pattern, they may provide powerful discrimination between alternative hypotheses.
Comparisons of Hox gene expression in the arthropod trunk suggest a more
flexible relationship between the processes of segment formation and Hox gene
regulation. Changes in this relationship have led to changes in the pattern of
tagmosis, which underlie the functional specialisation and evolutionary radiation
of the arthropods.
1 Introduction
Diversity of form is one of the most striking characteristics of life on earth. This
diversity depends on genetic instructions, acting through the mechanisms of development. Until recently we knew little of these mechanisms and even less about how
changes in genes could generate diversity of form. Different phyla were considered
to be so fundamentally different that comparisons between, for example, insects
and vertebrates were thought unlikely to reveal many similarities in the molecular
mechanisms of development.
Research in developmental genetics and genomics has radically changed that
view. For a few model organisms it is now possible to outline how genes define the
body plan. Moreover, it is clear that many, and perhaps most of the developmental
mechanisms in metazoa depend on a "tool kit" of genes that arose before the major
phyla of animals diversified. These conserved tools are used, sometimes in remarkably similar ways, to build the diversity of modern organisms.
Paradoxically, it is only because we can recognise these molecular similarities
that we can now begin to study the mechanisms that generate the diversity of animal forms. We can use what we know from well studied model systems to inform
comparative studies of other animals. In what foHows, I explore this approach in
the context of studies on the Hox genes of arthropods.
The Hox genes provided one of the first and still one of the most dramatic
examples of universality in developmental mechanisms. Below, I introduce their
role in development, largely as we understand it from studies in Drosophila. I then
consider the antiquity and diversity of this gene family and, as a slight digression,
mention some recent results using Hox gene sequences that address the relationships of the metazoan phyla. Finally, I return to the question of how studies of the
Hox genes help us to understand the diversity of body plans among arthropods.
2 The Hox Genes
The Hox genes were first identified in Drosophila over 80 years ago, by a class of
striking mutations that transform one part of the body into another (Bridges and
Morgan 1923). Sporadic individuals showing similar phenotypes had previously