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Howard Hughes Medical Institute, University of Wisconsin, 1525 Linden Drive, Madison, WI 53706
Edited by Michael S. Levine, University of California, Berkeley,
CA, and approved October 23, 2001 (received for review September 10, 2001)
The evolution of the Metazoa from protozoans is one of the
major milestones in life's history. The genetic and developmental events involved in this evolutionary transition are unknown but may
have involved the evolution of genes required for signaling and gene
regulation in metazoans. The genome of animal ancestors may be
reconstructed by identification of animal genes that are shared with
related eukaryotes, particularly those that share a more recent
ancestry and cell biology with animals. The choanoflagellates have long
been suspected to be closer relatives of animals than are fungi, the
closest outgroup of animals for which comparative genomic information
is available. Phylogenetic analyses of choanoflagellate and animal
relationships based on small subunit rDNA sequence, however,
have yielded ambiguous and conflicting results. We find that analyses
of four conserved proteins from a unicellular choanoflagellate, Monosiga brevicollis, provide robust support for a close
relationship between choanoflagellates and Metazoa, suggesting that
comparison of the complement of expressed genes from choanoflagellates
and animals may be informative concerning the early evolution of
metazoan genomes. We have discovered in M. brevicollis
the first receptor tyrosine kinase (RTK), to our knowledge,
identified outside of the Metazoa, MBRTK1. The architecture of MBRTK1,
which includes multiple extracellular ligand-binding domains, resembles
that of RTKs in sponges and humans and suggests the ability to receive and transduce signals. Thus, choanoflagellates express genes involved in animal development that are not found in other eukaryotes and that
may be linked to the origin of the Metazoa.
Evolution
A receptor tyrosine kinase from choanoflagellates: Molecular
insights into early animal evolution
*
To whom reprint requests should be addressed. E-mail:
sbcarrol{at}facstaff.wisc.edu.
www.pnas.org/cgi/doi/10.1073/pnas.261477698
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