The origin of the Hox and ParaHox loci and animal homeobox evolution
Abstract
The homeobox superfamily is one of the most significant gene families in the
evolution of developmental processes in animals. Within this superfamily the
ANTP class has expanded exclusively in animals and, therefore, the
reconstruction of its origin and diversification into the different ‘modern’
families have become prominent questions in the ‘evo-devo’ field. The current
burgeoning availability of animal genome sequences is improving the resolution
of these questions, putting them in a genome evolution context, as well as
providing the field with a large, detailed and diverse catalogue of animal
homeobox complements. Here I have contributed with a new hypothesis on the
origin and evolution of the Hox and ParaHox loci and the new term, ghost loci,
referring to homologous genome regions that have lost their homeobox genes.
This hypothesis proposes that the last common ancestor of all animals had a
much more complex genome (i.e. differentiated Hox, ParaHox and NK loci) that
underwent a simplification in the early animal lineages of sponges and
placozoans. In collaboration with the Adamska group I resolved the orthology of
the first ever ParaHox genes reported in calcareous sponges. This finding serves
as an independent confirmation of the ghost loci hypothesis and further resolves
the events of secondary simplification within the sponge lineage. Finally, I have
catalogued the homeobox complement of the newly sequenced arthropod, the
myriapod Strigamia maritima, and examined the linkage and clustering of these
genes. This has furthered our understanding of the evolution of the ANTP class.
The diversity of the homeobox complement and the retention in this myriapod
and the retention of some homeobox genes not previously described within
arthropods, in combination with the interesting phylogenetic position that this
lineage occupies relative to other arthropods, makes this complement an
important point of reference for comparison within the arthropods and in a
broader perspective in the ecdyzosoans. These findings have provided significant
further insights into the origin and evolution of the homeobox superfamily, with
important implications for animal evolution and the evolution of development.
Type
Thesis, PhD Doctor of Philosophy
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