A single-celled creature originally found in shallow sea sediments around Hawaii develops into multicellular structures with remarkable similarities to animal embryos. The finding could help scientists understand more about how and when embryonic development evolved.
One of the biggest questions in biology is how a single cell, the fertilised egg, coordinates its development into a complex multicellular body with many different cell types all doing the right things in the right places. Researchers have learned a great deal about how this happens, but how the process evolved from our single-celled ancestors remains something of a mystery.
Some clues have come from our modern single-celled relatives, many of which go through a brief multicellular stage as part of their life cycle. Our closest single-celled relatives, the choanoflagellates, for example, divide to form small colonies. Unlike animal embryos, however, they do this in response to an environmental change and don鈥檛 seem to have an internal program coordinating the development of different cell types.
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Recently, researchers have looked at more distant relatives, such as the ichthyosporeans, whose ancestors branched off from the lineage that gave rise to animals about a billion years ago. They have a multicellular stage of their life cycle that arises independently of any environmental cue.
To find out more, at the Swiss Federal Institute of Technology in Lausanne and his colleagues studied an ichthyopsporean species called Chromosphaera perkinsii. The team found that once single C. perkinsii cells reach a fixed diameter, they divide without growing in size, just as animal embryos do in the first stages of development.
The cells remain in this cluster, which can reach hundreds of cells, for around a third of their life cycle 鈥� far longer than other ichthyosporean species studied so far 鈥� and form at least two different cell types. 鈥淭hat was the biggest surprise,鈥� says Dudin.
What鈥檚 more, the way in which the cells divide and the three-dimensional structures the colonies adopt are remarkably similar to animal embryos in the earliest stages of development. Studies of gene activity in the colonies revealed intriguing parallels with that seen in early-stage animal embryos.
All of this suggests that genetic programs for guiding complex multicellular development were either present at least a billion years ago, long before the evolution of animals, or evolved independently in ichthyosporeans, says Dudin.
The findings could also help to resolve a long-standing debate about some 600-million-year-old embryo-like fossils that some scientists have used to argue for a more ancient evolutionary origin of animals. The new findings raise the possibility that they were instead colonies of ichthyosporean-like organisms.
The apparent degree of different cell type development in C.perkinsii colonies is 鈥渢ruly novel and striking鈥�, although it isn鈥檛 clear to what extent they cooperate with each other, says at the Pasteur Institute in Paris. His team has just described a choanoflagellate species that that were previously believed to be distinct.
This and the C. perkinsii findings are adding to a growing body of results challenging conventional notions of multicellularity and showing that there is more diversity and versatility than we thought existed even a few years ago, he says.聽 鈥淚t is just fascinating how freshly discovered or characterised species just keep defying earlier expectations or generalisations. There are many more ways to assemble the 鈥榖uilding blocks鈥� of multicellularity than we previously thought.鈥�
bioRxiv
