One of the primary lines of research in the
lab is to understand the evolutionary relationships of major animal
lineages. This work is drawing on information from several different
sources to build a comprehensive, consensus understanding of animal
relationships. In particular, we are focusing on the Lophotrochozoan
clade. This is a clade of bilaterian animals that is defined be
all the descendants of, and including, the last common ancestor
of the annelids, mollusks, and the three lophophorate taxa (Brachiopoda,
Phoronida, and Bryozoa). The term Lophotrochozoa refers to the fact
that this lineage includes the traditional Lophophorate groups and
lineages with a trochophore larvae (sensu lato).
A current understanding of animal phylogeny,
complied from several sources, is shown. We know believe that three
major lineages of bilateral animals existed prior to the Cambrian
radiation (~ 550-600 MYA): Deuterostomes, Lophotrochozoans, and
Ecdysozoans. This later group consists of molting animals and includes
the model systems of Drosophila and Caenorhabdidtis. Deuterostomes
include echinoderms (seastars, sea cucumbers, urchins), hemichordates
(acorn worms and pterobranchs), and chordates. The cnidarian (jellyfish,
corals, anemones), ctenophore (comb jellies), and porifieran (sponges)
lineages were all established lineages well before the Cambrian.
Much of the current understanding of animal phylogeny was initially
based on the 18S nuclear ribosomal subunit (rDNA) gene, but information
from morphological cladistic analyses, mitochondrial genomes, and
other nuclear genes have also been used.
In the Halanych lab, we are using the 28S
nuclear ribosomal subunit and Hox- genes to explore animal phylogeny,
specifically Lophotrochozoan relationships. The 28S rDNA is approximately
3000 nucleotides long and linked to the 18S rDNA. Hox-genes are
a set of transcription factors involved in embryogenesis. These
genes occur in a linked cluster that is expressed in a colinear
fashion. We are employing genomic approaches to clone and sequence
the entire cluster (~250-300 Kb) from representative organisms.
Because only a single Hox cluster exists in invertebrates, we will
use gene geneologies and the presence of orthologous genes to infer
evolutionary history.
One of the organisms that we are most focused
on is the annelid Ophrytrocha diadema (Dorvilleidae). This annelid
has a 21-28 day generation time (22OC) and is easy to maintain in
the lab. Surprisingly, no model genetic system exists for the Lophotrochozoans,
the bilaterian lineage that includes the greatest diversity of animal
body plans. One of the goals of the lab is to develop this annelid
as a model system so that genomic evolution within animals can be
better understood.