Sponge species sheds light on animal evolution

Published in The Hindu on August 5, 2010


Amphimedon queenslandica living with an octocoral. — Photo: Maely Gauthier

What is so unique about sea sponges that scientists should care to sequence its genome? Simply put, sea sponges are considered the oldest surviving species of the multicellular metazoan lineage.

Hence studying their DNA can help in identifying the genetic innovations that enabled transition from single-celled to multicellular life (metazoan animals) some 600 million years ago. Metazoan animals make up a major portion of the animal kingdom.

Not ‘true animals’

Sea sponges do not have a gut and nervous system, and are hence not generally considered as ‘true animals.’ Yet, scientists are of the opinion that other multicellular metazoan animals evolved from sponge-like ancestors.

A paper published today (August 5) in Nature, shows that the sea sponge species (Amphimedon queenslandica), found in the Great Barrier Reef of Australia, has a “developmental toolkit [of genes] similar to that of modern complex bilaterians [those that exhibit bilateral symmetry].” The scientists were able to arrive at this conclusion by comparing the genome of A. queenslandica with that of other species.

This strengthens the argument that multicellular metazoan animals did evolve from sponge-like ancestors.

Remarkable similarity

They found that A. queenslandica possesses a wide range of metazoan-specific genes. In other words, the draft genome sequences of the sea sponge species studied show a remarkable similarity to other advanced animal genomes in terms of content, structure and organisation. This, despite the fact that sponges diverged from other metazoans over 600 million years ago.

The genome of the sea sponge studied allowed the researchers to look at the origin of the six characteristic features of multicellularity.

The most characteristic similarity between sponges and advanced eumetazoan animals (which have true tissue organised into germ layers), is seen in developmental signalling and transcription.

Right at the beginning

The presence of both developmental signalling and transcription in the Amphimedon species suggests that origin and development of an individual organism (ontogenetic development) took place right at the beginning of the metazoan lineage.

A. queenslandica sponge may not have a mesoderm (middle layer of a tissue), but they have the wherewithal for differentiation of muscles and nerves. This ability arises despite the sponge not having a neuromuscular system.

The study also showed that the metazoan ancestor ( A. queenslandica) has a complex sensory system, and has many of the molecular requirements for neural development and nerve cell function.

The scientists conclude that the sexually-reproducing metazoan ancestor has the “capacity to sense, respond to, and exploit the surrounding environment.”

They also found that sea sponge may lack certain specialised cell types such as neurons and muscles, but it shares with the advanced metazoans some of the genes that are essential for the form and function of multicellular organisms.