Dr. Kevin Pang is interested in early animal evolution. His recently completed doctoral dissertation research focused on the biology and genomics of ctenophores as one of the important animal groups for studying early metazoan evolution.
"The ctenophores, also called comb jellies, are a group of marine animals that are really important for understanding animal evolution. Their unique features and position near the base of the metazoan tree of life make them an ideal group for studying what was going on during the early evolution of metazoans."
These gelatinous creatures are clearly distinct from cnidarian medusae (e.g., jellyfish). Some of the key features of the ctenophore body plan include biradial symmetry, an oral-aboral axis delimited by a mouth and an apical sensory organ, two tentacles, eight comb rows composed of interconnected cilia, and thick mesoglea. Other morphological features include definitive muscle cells, a nerve net, basal lamina, a sperm acrosome, and light-producing photocytes.
"Aspects of ctenophore development made them attractive to experimental embryologists as early as the 19th century. Ctenophores have also been of interest more recently because of their effects on local ecosystems as an invasive species, such as was seen in the fisheries collapse in the Black Sea," Dr. Pang notes. "And most recently, because of their phylogenetic position, we have been working to establish the ctenophore, Mnemiopsis leidyi, as a model organism for studying developmental events in early metazoan evolution. So they're interesting animals to study for a variety of reasons."
Although the exact branching order is still quite muddy, it is clear that modern ctenophores are descended from one of the earliest branching, if not the earliest, group to evolve such complex features as neurons and a nervous system, muscle cells and 'mesoderm', gut cavity, basement membrane, germ cells, as well as such developmental properties of a stereotyped cleavage program and early specification of cell fates and body axes.
"These complex features are very similar to that of the Bilateria, so understanding the complexity in ctenophores is very important," notes Dr. Pang. "One possibility is that these are homologous features, which would suggest the Urmetazoan or Ureumetazoan - the hypothetical last common ancestor of all animals - was much more complex than people previously thought."
Depending on where ctenophores branched off, it is possible that muscle cells were lost in cnidarians and Trichoplax. Contrary to the long-held idea that the direction of evolution is from simple to complex, this would indicate that a loss of complexity may have played an important role in metazoan evolution. As an alternative hypothesis, these features could have evolved independently in ctenophores and bilaterians, an example of convergent evolution.
"This is equally plausible, as ctenophores have been evolving along their own lineage for the same amount of time as bilaterians," Dr. Pang points out.
In collaboration with researchers at the National Institutes of Health (NIH), Dr. Pang has been able to sequence the genome of M. leidyi using 'second-generation' sequencing technologies. The seven 454 sequencing runs resulted in 8.1 million "reads" totaling 2.7 gigabases. These "reads" were then assembled to create a draft of the ctenophore genome. They estimate that they have sequenced an approximately 50X physical coverage of the genome.
"With second-generation sequencing, it requires much more genome coverage in order to obtain a decent assembly," Dr. Pang explains. "For the first draft assembly, we have 5,100 scaffolds, with the largest being 1.2 Mb and an N50 of 187 kb. The genome size is less than previously estimated, at 156 Mb."
Dr. Pang has used the sequence information gained from the genome project to better understand the genomic content of ctenophores. One of the key steps in early animal evolution was to evolve multi-cellularity. And with this increased complexity came the need for cell-cell signaling.
"There are 6-7 major signaling pathways present in animals that are thought to be key to early animal evolution. We have just begun to search for the presence/absence of components of this pathway and, in comparison with that of other animals, better understand animal evolution".
Searches of the ctenophore genome have found near-complete Wnt and TGF-β signaling pathways. Notably absent are Axin and APC from the Wnt pathway and SARA from the TGF-β pathway. Also significant is the lack or scarcity of diffusible antagonists, including Dickkopf, Cerberus, Chordin, and Noggin.
"This is similar to what appears to be the case in sponges," says Dr. Pang. "This suggests that while the core components of the pathway were present early in metazoan evolution, later additions included these antagonists, which may have been one way to add complexity to the pathway and its regulation."
You can read more about Dr. Pang's research on ctenophores in his latest publication in EvoDevo.
Early cleavage and development of the lobate ctenophore, Mnemiopsis leidyi. (Video by Kevin Pang).