With the advent of new molecular biology techniques, including bioinformatics, scientists have new ways to determine relatedness between organisms. Even if two organisms have no readily apparent comparable features at the macro level, scientists can analyze DNA sequences and protein family function to look for proteins and sequences that are widely conserved throughout the animal kingdom.
For the sake of this discussion, we will assume that one of the primary differences between plants and animals is the development of a nervous system. The most primitive organisms which display rudimentary nervous systems are cnidarians ( jellyfish and corals, exhibit radial symmetry ) and bilaterans ( flatworms and such, which display a anterior, posterior, left and right ). Even more primitive than these organisms are the sponges, which display cell type differentiation, and development of identifiable tissues such as epithelium. It is known that sponges do not transduce signals using nerve cells in the way that higher animals do, but they do have specialized cells called globular cells that act as primitive sensory cells.
To see if developmental signals ( proteins called transcription factors) related to nerve development, or neurogenesis, were present in the sponge genome, a group of scientists took a DNA sequence from a known neurogenic transcription factor, basic Helix Loop Helix ( bHLH ) and searched a sponge genomic database for matching sequences. Upon scoring hits, the group of French and Australian scientists began a series of experiments to see how bHLH transcription factors might be employed within the sponge. The question here is “why does an organism that does not display the traditional function of bHLH transcription factors, neurogenesis, have them”.
Notch is a cellular membrane receptor. A trans-membrane receptor has a portion of it’s domains on the surface of the cell and a domain on the cytoplasmic, or inside surface of the cell. Notch, like many widely conserved gene families was named for it’s first observations in fruit flies. Early genetic experiments involved irradiating fruit fly larvae and observing the resulting mutant phenotypes. It was noted that when the gene for “Notch” was disabled, the resulting fly would exhibit notches in it’s wings, as it’s most identifiable attribute.
It was later discovered that notch is a receptor on cell membranes that helps receive signals from certain classes of “ligands”, or signal molecules that react with cellular receptors. In the case of the present study, Delta is the ligand associated with notch.
The authors of the present study are not studying fruit flies, they are studying the sponge Amphimedon ( Amphimedon queenslandica), an inhabitant of the great barrier reef in Australia if I am correct. They have noted that during the development of amphimedon, certain cells exhibit Notch receptors and can react with Delta ligands. The scientists used a method called whole mount in-situ hybridization (ISH) to track the certain genes of interest. ISH involves using laboratory designed probes to label individual certain cells. In ISH, a probe is a piece of DNA or RNA that has either incorporated radio-labeled nucleotides, or has been bound ( conjugated) to a fluourescent molecule such as flouorescein. The probe is then applied to a mounted tissue specimen, and the nucleotides in the probe bind to complementary sequences and form double stranded, or hybridized, forms of the nucleotide sequence. The remaining unbound probe an be washed away, or enzymaticaly destroyed with an enzyme specific for single stranded nucleodide sequences.
The authors in this paper used ISH probes for three molecular targets, bHLH , notch and delta gene expression. By using these three probes, they visualized globular cell development and migration in the developing sponge embryo, and concluded that this process is a predecessor the the process that gave rise to neurogenisis. In other words, the molecular signals associated with nervous system differentiation had roles in life before they were “drafted” into their neurogenic role.
Sponges exist right at the junction of what we might consider an organism and what we would consider a colony of independent organisms. If we were to simply view the methods used by people who culture sponges, that is, merely cut up a sponge, tie the pieces to a scaffold, and let them grow back, we might lean toward calling a sponge a colony.
As a result of this study, we see that sponges not only form epithelium, a tissue type characteristic of animals, but they have sensory sells called globular cells that migrate and coordinate sensory input for the developing organism. This provides further ties between sponges and the animal kingdom as opposed to the plants and protozoa. I guess we could say welcome to the animal kingdom sponges. I for one am not expecting much from the new guys at the party.
References
Richards, G. S. et. al. Sponge genes provide new insight into the evolutionary origin of the neurogenic circuit Current Biology 18, 1156-1161, August 5, 2008
