BEACON Researchers at Work: Hemichordate Global Biodiversity and Evolution

This week’s BEACON Researchers at Work blog post is by University of Washington postdoc Charlotte Konikoff.

Research in the Swalla lab broadly focuses on elucidating chordate origins and evolution. If you are reading this, you are a chordate. More specifically, you are a vertebrate, and a member of the species Homo sapiens. Like other chordates, when you were an embryo you had a dorsal (back) notochord, a supportive rod that is replaced by cartilage or bone in most adult vertebrates. You also had a dorsal nerve cord, which was later modified into your brain and spinal cord. As an embryo, you also had pharyngeal slits and a post-anal tail, the latter of which is now present as the coccyx. These hallmark characteristics (notochord, dorsal nerve cord, gill slits and post-anal tail) are present in all chordates at some point during their life history.

As a chordate, you also belong to a broader category of organisms – deuterostomes (from Greek, meaning “second mouth”). During early stages of your embryonic development, an opening called the blastopore formed – this is the first opening to the primitive digestive tract. In protostomes this opening becomes the mouth, while in deuterostomes it becomes the anus. Another opening develops later to complete the primitive digestive tract (becoming either the anus or mouth). Tunicates, sea urchins and lancelets are also deuterostomes, and we all shared a common ancestor millions of years ago.

My research focuses on hemichordates. This phylum occupies a very special place on the tree of life, and has been rediscovered in the last 15 years due to its immense potential to inform us about the origins and evolution of chordates and deuterostomes.

Hemichordates are marine invertebrates that span a broad range of depths and habitats. Hemichordata (Bateson, 1885) comes from the Greek prefix hemi (“half”) and the Latin chorda (“cord”). The oldest available description of a hemichordate dates back to 1825, when Eschscholtz first described the acorn worm Ptychodera flava. Hemichordates were originally classified as other organisms, including chordates (because their gill slits look like chordate gill slits), but were later placed in their own phylum. As the name suggests, hemichordates share some but not all of the hallmark characteristics of chordates. They don’t have a notochord, but they do have a structure called the stomochord – a flexible tube that somewhat resembles the notochord. They also have gill slits. They are likely the closest extant relatives to the deuterostome ancestor.

Two extant classes of hemichordates exist – enteropneusts and pterobranchs. Enteropneusts are free-living acorn worms, while pterobranchs are colonial organisms. From anterior (head) to posterior (tail), their adult body plan is composed of the proboscis, collar and trunk. Their mouth is located in between their proboscis and collar regions. They also have a heart-kidney complex that is located in the proboscis region. Pterobranchs also have a stalk region, which connects the individuals in the colony. Below is a diagram showing the hemichordate body plan, as well as pictures of hemichordate species found throughout the world.

Figure 2 image source: Rychel, A.L. and Swalla, B.J. (2009) Stem cells and regeneration in hemichordates. In: Marine Stem Cells, Valeria Matranga and Baruch Rinkevich, ed. Springer Publishing. pp. 245-265.

Phylum Hemichordata has been reported to contain only about 100 species, but recent studies of taxonomy and phylogeny suggest that the species number has been hugely underestimated. One problem is that species must be described by experts, and historically few taxonomists have studied this group of marine invertebrates. In our recent work, we provided an overview of our current knowledge of hemichordates, with a special focus on global biodiversity, geographic distribution and taxonomy. Of the 120 living species currently documented, we found the majority (80%) are enteropneusts, with more species descriptions forthcoming. Hemichordates are found throughout the world’s oceans, but the reported number is highest in the temperate north Pacific and Atlantic. Interestingly, these areas also have many marine labs. Many species have also been found in the western Indian Ocean. As new marine habitats are characterized and explored, we anticipate new hemichordate species will continue to be discovered and characterized.

Further, gene expression studies in hemichordates can provide clues on the developmental origins of the gill slits and chordate nervous system. Some hemichordates (such as Ptychodera flava) also have the remarkable ability to regenerate. If they are amputated in half, they can reproducibly regenerate their anterior and posterior regions.

I’m particularly interested in investigating genes involved in signal transduction, and want to determine which genes and signaling pathways are crucial for regeneration in Ptychodera flava. Where are they expressed? Which ones are crucial for regeneration? What are their functions? During development, signal transduction pathways allow for cells to communicate with each other and orchestrating proper patterning in the developing embryo. These developmental signaling pathways also appear to play roles in regeneration in other organisms.

For more information about Charlotte’s work, you can contact her at ckonikof at u dot washington dot edu.

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