The Genome of the Ctenophore
Mnemiopsis leidyi and Its Implications
for Cell Type Evolution
Joseph F. Ryan, Kevin Pang, Christine E. Schnitzler, Anh-Dao Nguyen, R. Travis Moreland,
David K. Simmons, Bernard J. Koch, Warren R. Francis, Paul Havlak,
NISC Comparative Sequencing Program, Stephen A. Smith, Nicholas H. Putnam,
Steven H. D. Haddock, Casey W. Dunn, Tyra G. Wolfsberg, James C. Mullikin,
Mark Q. Martindale, Andreas D. Baxevanis*
Introduction: An understanding of ctenophore biology is critical for reconstructing events that
occurred early in animal evolution. The phylogenetic relationship of ctenophores (comb jellies) to
other animals has been a source of long-standing debate. Until recently, it was thought that Porifera (sponges) was the earliest diverging animal lineage, but recent reports have instead suggested
Ctenophora as the earliest diverging animal lineage. Because ctenophores share some of the same
complex cell types with bilaterians (such as neural and mesodermal cells), the phylogenetic position
of ctenophores affects how we think about the early evolution of these cell types.
Methods: We have sequenced, annotated, and analyzed the 150-megabase genome of the ctenophore Mnemiopsis leidyi. We have performed detailed phylogenetic analyses on these new data
using both sequence matrices and information on gene content. We conducted extensive genomic
inventories on signaling pathway components and genes known to be critical to neural and mesodermal cell types, among others.
Results: Our phylogenetic analyses suggest that ctenophores are the sister group to the rest of the
extant animals. We find that the sets of neural components present in the genomes of Mnemiopsis
and the sponge Amphimedon queenslandica are quite similar, suggesting that sponges have the
necessary genetic machinery for a functioning nervous system but may have lost these cell types.
We also find that, although Mnemiopsis has most of the genes coding for structural components of
mesodermal cells, they lack many of the genes involved in bilaterian mesodermal specification and,
therefore, may have independently evolved these cell types.
Discussion: These results present a newly supported view of early animal evolution that accounts
for major losses and/or gains of sophisticated cell types, including nerve and muscle cells. This
evolutionary framework, along with the comprehensive genomic resources made available through
this study, will yield myriad discoveries about our most distant animal relatives, many of which will
shed light not only on the biology of these extant organisms but also on the evolutionary history of
all animal species, including our own.
FIGURES IN THE FULL ARTICLE
Fig. 1. M. leidyi life history and anatomy.
Fig. 2. Previously proposed relationships
of the five deep clades of animals.
Fig. 3. Tree produced by maximum-likelihood
analysis of the EST set.
Fig. 4. Tree produced by maximum-likelihood
analysis of gene content.
Fig. 5. The origin of postsynaptic genes.
Fig. 6. Inventory of myogenic components
in M. leidyi.
Materials and Methods
Figs. S1 to S10
Tables S1 to S31
The phylogenetic position of the ctenophore
Mnemiopsis leidyi and its implications regarding
the origin of mesodermal cell types. (A) Adult
M. leidyi. (B) Summary of the relationships of the five
main branches of animals and the outgroup Choanoflagellata. (C) Inventory of myogenic specification
genes in Mnemiopsis. Components present in the
Mnemiopsis genome are in blue, and names are
underlined. Absent components are in red. The lack of
many of these factors in Mnemiopsis indicates that
ctenophore mesodermal cell types are specified differently than in bilaterians, suggesting that they perhaps
evolved independently in these two lineages.
READ THE FULL ARTICLE ONLINE
Cite this article as J. F. Ryan et al.,
Science 342, 1242592 (2013).
The list of author affiliations is available in the full article online.
*Corresponding author. E-mail: firstname.lastname@example.org
RESEARCH ARTICLE SUMMARY