快猫短视频s trying to work out where a starfish鈥檚 head is have come to a startling conclusion: it is effectively the whole animal. As well as solving this longstanding mystery, the finding will help us understand how evolution generates the dramatic diversity of animal forms on Earth.
Starfish, also known as sea stars, belong to a group of animals called echinoderms, which includes sea urchins and sea cucumbers. Their strange body plans have long puzzled biologists. Most animals, including humans, have a distinct head end and tail end, with a line of symmetry running down the middle of their body dividing it into two mirror-image halves. Animals with this two-sided symmetry are called bilaterians.
Echinoderms, on the other hand, have five lines of symmetry radiating from a central point and no physically obvious head or tail. Yet they are closely related to animals like us and evolved from a bilaterian ancestor. Even their larvae are bilaterally symmetrical, later radically re-organising their bodies as they metamorphose into adults.
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These profound differences make it hard for scientists to find and compare equivalent body parts in bilaterians to work out how echinoderms evolved. 鈥淭he morphology cannot tell you anything, almost,鈥� says at Stanford University in California. 鈥淚t is just too weird.鈥�
Formery and his colleagues decided to look at a set of genes known to direct the head-to-tail organisation of all bilaterians. In these animals, these genes are turned on, or expressed, in stripes in the outer layer of the developing embryo. The genes that are expressed in each stripe define which point on the head-to-tail axis it will become.
The idea was to see if the gene expression patterns would reveal a 鈥渕olecular anatomy鈥� hidden in echinoderms. 鈥淭his particular suite of genes is just good for exploring animal diversity in its most extreme forms,鈥� says team leader , also at Stanford University. 鈥淚 think echinoderms represent a really extreme experiment in how to use that bilateral network to produce a very, very different body plan.鈥�听
To the team鈥檚 surprise, the genes that determine the head end in bilaterians were expressed in a line running down the middle of each arm on the underside of the starfish. The next head-most genes were expressed on either side of this line, and so on.
Even more strangely, the genes normally expressed in the trunk of bilaterians were missing in the outer layer of the animal. This suggests that starfish have jettisoned their trunk regions and freed up the outer layer to evolve in new directions, says Formery.
The findings show that 鈥渢he body of an echinoderm, at least in terms of the external body surface, is essentially a head walking about the seafloor on its lips鈥�, says at the University of Victoria in Canada, who wasn鈥檛 involved in the study. Animals like us may have kept their trunks to escape predation by swimming away. 鈥淓chinoderms hunkered down and armoured themselves, so they didn鈥檛 need a trunk,鈥� says Lacalli.
The idea that echinoderms are 鈥渉ead-like鈥� animals is 鈥渋nteresting and powerful鈥�, says at the University of Guelph in Ontario, Canada. It raises some very important and fundamental questions about how ecological factors shape the evolution of anatomy, he says. 鈥淔inding underlying conserved patterns really provides critical insights into how development evolves.鈥�
Nature
