The Choanoflagellates – Evolution, Biology and Ecology
Written by: Barry Leadbeater
My interest in choanoflagellates came about by accident.
In 1970 I spent a month in Norway attempting to collect coccolithophorids (calcified phytoplankton) which are usually present in coastal waters during summer. However, August 1970 proved to be an exception in that there were no coccolithophorids but instead I encountered mysterious looking single-celled flagellates each within a silicified basket-like cage. These I soon discovered were loricate choanoflagellates.
In 1970, the choanoflagellate literature was sparse and widely scattered in zoological, marine biological and taxonomic journals. My own knowledge of choanoflagellates was limited to a brief mention in an undergraduate lecture of their resemblance to sponge choanocytes.
However, this apparent lack of interest was misleading because during the nineteenth century choanoflagellates had been recorded extensively by pioneering protistologists starting with Henry James-Clark in 1866. Revisiting the early literature was a revelation.
Since 1970, major advances in all forms of microscopy, together with the digital capture of still and video images, have enormously extended our knowledge of the morphology and behaviour of choanoflagellates. This has been complemented by the exponential development of molecular genetic procedures that have given us unique insights into the phylogeny (evolution) and molecular biology of this most interesting group of protists.
Far from being an esoteric group of protists, choanoflagellates are of major significance in at least three distinctive respects.
Firstly, in an evolutionary context, they are the sister group to the Metazoa, which means they are the closest, unicellular, living relatives of the animal lineage.
Secondly, in an ecological context, they are an important and universal component of aquatic microbial foodwebs (the microbial loop).
Thirdly, in a cell biological context, they demonstrate a remarkable ability to be able to produce silicified components that are assembled outside the cell to produce an elaborate basket-like lorica.
My recently published book entitled The Choanoflagellates: Evolution, Biology and Ecology brings together, in an easily readable format, all the information currently available on choanoflagellates. It is comprehensive in range, and provides up-to-date summaries on ecological and evolutionary aspects, including the unicellular origins of metazoan multicellularity. The cell biology chapters, which include the utilization of silicon and the mechanisms involved in lorica production and assembly, develop a novel example of ‘regulatory’ evolution, whereby evolution occurred by the reorganization of existing structures and mechanisms.