Have you ever wondered how animals became so diverse? Professor Bernard Degnan at the University of Queensland and many collaborators embarked on a genome project that would hopefully answer questions about our origin and the evolution of animal complexity. The answers resided within the sponge genome.
Core Animal Features
The first step in understanding animal complexity is to identify what are the principle features associated with true animals. First, animals are all eukaryotes meaning they have cells with complex membrane structures including a nucleus. The eukaryotes then break into the unicellular and multicellular organisms (Metazoa). The multicellularity of animals contains six key characteristics: regulated cell cycling and growth, programmed cell death, cellular adhesion, developmental signalling and gene regulation, allorecognition and innate immunity, and the specialization of cell types. Degnan looked at each of these characteristics, but for simplicity I am mainly going to address cell specialization, the signalling pathways (neurons) and cell communication, the presence of tissues and organs, and motility of animals.
Sponges (Phylum Porifera) are considered metazoa, but their general characteristics don’t seem to fit with what we consider a ‘true’ animal. These multicellular organisms do not have tissues and organs or a nervous system. A sponge adult is a sessile (non-motile) filter feeder with specialized cells to allow for water movement to go through pores and allow the organism to feed. Their unique features also allow it to completely change and adapt to its environment allowing these mostly marine species to persist for 800 million years! In general, this phylum may have cell specialization, but the remaining animal characteristics seem to be missing.
Amphimedon queenslandica Genome
Degnan sought out a sponge with ideal features for extracting DNA and genome sequencing. The final subject was a species of Demospongiae that they named Amphimedon queenslandica. Genome sequencing of this sponge revealed the same kind of gene size and structure that we have. Nearly 60% of the genome is similar to ours with about 1300 genes that are shared with all other animals. This means that the sponge gene architecture hasn’t really changed in millions of years and may be the common ancestor of all animals. Additionally, even though these genes have been preserved, the sponge is evidently not using these genes in the same way that we use them, indicating that the gene functions are not the same for all organisms.
Further analysis of the Amphimedon has also led to some changes to what we originally thought about sponges in general. The sponge has all the genes and components that we have for nerve cells suggesting this is the beginning of signalling pathways allowing cells to communicate with each other. This is not quite a nervous system, but a definite precursor to one.
Also, the larval form of Amphimedon has a perfectly round pigment ring. The dark ring has long cillia around it that is responsive to light. Although sponges don’t have organs, this primitive eye certainly functions as one. Furthermore, signalling molecules accumulate at this spot suggesting there may be signals being sent and contributing to the pigment ring.
Degnan’s research also found many other components such as genes, proteins, and structures that support the sponge innovation of the six main characteristics of multicellular animals. Thus, the true origin and diversity of all animals lies at the point in which unicellular organisms branched into multicellular organisms and split into metazoans and the eumetazoans (see bold metazoan stem in phylogeny chart above). Surprisingly we all have a sponge ancestry that led to the further evolution and diversity of all animals!
Top 5 Take-Home Messages
- Genome sequencing can reveal more about our origin and animal diversity.
- Genes may have more than one function.
- The increase in animal complexity involves an expansion of gene functionality.
- Sponges are more similar to other animals than originally thought.
- The unique features of all animals started with the metazoan stem.
- The Amphimedon queenslandica genome and the evolution of animal complexity; Srivastava et al., 2010
- Zoology: Vision with no nervous system; Nature, 2012
- Contribution of sponge genes to unravel the genome of the hypothetical ancestor of Metazoa (Urmetazoa); Muller et al., 2001