When you put a hat on a starfish, the place would you set it? On the middle of the starfish? Or on the purpose of an arm and, if that’s the case, which one? The query is foolish, however it will get at critical questions within the fields of zoology and developmental biology which have perplexed veteran scientists and schoolchildren in introductory biology courses alike: The place is the pinnacle on a starfish? And the way does their physique format relate to ours?
Now, a brand new Stanford research that used genetic and molecular instruments to map out the physique areas of starfish — by making a 3D atlas of their gene expression — helps reply this longstanding thriller. The “head” of a starfish, the researchers discovered, is just not in anybody place. As an alternative, the headlike areas are distributed with some within the middle of the ocean star in addition to within the middle of every limb of its physique.
“The reply is far more difficult than we anticipated,” mentioned Laurent Formery, lead writer and postdoc within the labs of Christopher Lowe on the Stanford Faculty of Humanities and Sciences and Daniel S. Rokhsar on the College of California, Berkeley. “It’s simply bizarre, and almost certainly the evolution of the group was much more difficult than this.”
Starfish (sea stars) belong to a gaggle of animals referred to as echinoderms. Echinoderms and people are intently associated, but the life cycle and anatomy of sea stars are very completely different from ours.
Sea stars start life as fertilized eggs that hatch right into a free-floating larva. The larvae bob within the ocean in a plankton type for weeks to months earlier than settling to the ocean ground to carry out a magic trick of kinds — reworking from a bilateral (symmetric throughout the midline) physique plan into an grownup with a five-point star form referred to as a pentaradial physique plan.
“This has been a zoological thriller for hundreds of years,” mentioned Lowe, who can also be a researcher at Hopkins Marine Station and senior writer of the paper that revealed Nov. 1 in Nature. “How are you going to go from a bilateral physique plan to a pentaradial plan, and how are you going to examine any a part of the starfish to our personal physique plan?”
Mapping stars
For puzzles comparable to this one, researchers typically conduct comparative research to establish related constructions in associated teams of animals to glean clues concerning the evolutionary occasions that prompted the trait of curiosity.
“The issue with starfish is there may be nothing on a starfish anatomically you can relate to a vertebrate,” mentioned Lowe. “There’s simply nothing there.”
Not less than, nothing on the skin of a starfish. And that’s the place genetic and molecular methods are available.
Throughout his graduate analysis, Formery studied early growth in sea urchins — echinoderms, like sea stars, that additionally begin their life as bilateral larvae earlier than reworking into adults with fivefold symmetry. When Formery joined Lowe’s lab, Formery’s data of echinoderm growth mixed with Lowe’s experience in molecular biology methods to assist sort out the thriller of sea stars’ baffling physique plan.
The workforce used a gaggle of well-studied molecular markers (Hox genes are an instance) that act as blueprints for an organism’s physique plan by “telling” every cell which physique area it belongs to.
“When you strip away the pores and skin of an animal and have a look at the genes concerned in defining a head from a tail, the identical genes code for these physique areas throughout all teams of animals,” mentioned Lowe. “So we ignored the anatomy and requested: Is there a molecular axis hidden underneath all this bizarre anatomy and what’s its position in a starfish forming a pentaradial physique plan?”
To research this query, the researchers used RNA tomography, a method that pinpoints the place genes are expressed in tissue, and in situ hybridization, a method that zeroes in on a selected RNA sequence in a cell.
“First we sectioned sea star arms into skinny slices from tip to middle, high to backside, and left to proper,” mentioned Formery, noting that sea stars regenerate lacking limbs. “We used RNA tomography to find out which genes have been expressed in every slice after which ‘reassembled’ the slices utilizing laptop fashions. This gave us a 3D map of gene expression.”
“Within the second technique, in situ hybridization chain response, we stained sea star tissue and visually inspected the samples to see the place a gene was expressed,” mentioned Formery. This enabled the researchers to look at anterior-posterior (head to tail) physique patterning within the outermost layer of cells referred to as the ectoderm.
“This was made doable by the latest, huge, technical enchancment in in situ hybridization, referred to as in situ hybridization chain response, Formery mentioned. “This new technique supplies higher decision of the place the gene is expressed.”
The analysis revealed that sea stars have a headlike territory within the middle of every “arm” and a tail-like area alongside the perimeter. In an surprising twist, no a part of the ocean star ectoderm expresses a “trunk” genetic patterning program, suggesting that sea stars are principally headlike.
Mining really various biodiversity
Analysis is usually centered on teams of animals that appear to be us, the researchers defined. But when we deal with the acquainted, we’re much less prone to be taught one thing new.
“There are 34 completely different animal phyla residing on this planet and in over roughly 600 million years they’ve all give you completely different options to the identical elementary organic issues,” Lowe mentioned. “Most animals do not have spectacular nervous programs and are out chasing prey — they’re modest animals that reside in burrows within the ocean. Persons are usually not drawn to those animals, and but they most likely symbolize how a lot of life obtained began.”
This research demonstrates how a comparative method that makes use of genetic and molecular methods can be utilized to mine biodiversity for insights into why completely different animals look the way in which they do and the way their physique plans advanced.
“Even in latest molecular papers there is a query mark close to echinoderms on the evolutionary tree as a result of we do not know a lot about them,” Formery mentioned. “It was good to point out that — no less than on the molecular stage — we now have a brand new piece of the puzzle that may now be placed on the tree.”
Formery, Lowe, and Rokhsar are additionally researchers on the Chan Zuckerberg BioHub. Rokhsar can also be a researcher on the Okinawa Institute of Science and Expertise. Further Stanford co-authors are Ian Kohnle, Judith Malnick, and Kevin Uhlinger of Hopkins Marine Station. Further authors are from Pacific Biosciences in Menlo Park, California, and Columbia Equine Hospital in Gresham, Oregon.
This analysis was funded by NASA, the Nationwide Science Basis, and the Chan Zuckerberg BioHub.
