Development of electroreceptors: a “sixth sense”

Hi there! My name is Melinda, and I’m a postdoctoral researcher at Cambridge University in the UK in the lab of Dr. Clare Baker (http://www.pdn.cam.ac.uk/staff/baker/). I’ve just wrapped up my research trip to work on paddlefish embryos in the southeastern state of Georgia in the United States, generously funding by the Development Travelling Fellowship award!

We are used to experiencing the world with five senses: sight, smell, taste, touch, and hearing. Many of these sensory systems are generated by placodes, which are regions of thickened ectoderm found in the embryonic head that generate a variety of peripheral sense organs, such as the otic and olfactory placodes, which form the inner ear and nasal epithelium, important for hearing and smelling, respectively. Hearing and balance are mediated by the mechanical displacement of tiny ¨hairs¨ on specialized sensory ¨hair cells¨ in our inner ears (also simply called mechanoreceptors).  In fish and aquatic amphibians, a series of lateral line placodes generates the lateral line system, which also contains modified mechanoreceptor hair cells, much like those found in the inner ear. These are used to detect changes in the local water environment important for prey or predator detection and schooling behaviors. In addition to the mechanoreceptors, another type of modified hair cell can be found in all major aquatic vertebrate groups: these are the electroreceptors, distributed in fields of “ampullary organs” on either side of the lateral lines of mechanosensory hair cells.

As the name suggests, electroreceptors allow animals that possess them to detect weak electric fields in water. Similar to mechanoreceptors, this is also used to find prey and for orientation. However land vertebrates (including reptiles, birds and mammals), as well as frogs and most modern bony fish (such as teleosts), have lost this ancient ¨sixth sense¨. They are still found in many aquatic vertebrates including jawless fish (lampreys), cartilaginous fish (sharks, rays), primitive bony fish (e.g. sturgeon, paddlefish), and even some amphibians (salamanders). Interestingly, in a few groups of modern bony fish, such as catfish and “electric fish”, electroreceptors have been independently “re-invented”. Although an evolutionarily ancient sense, electroreceptors were only discovered in the 1950s, and very little is known about their development or formation, i.e., how they develop in the embryo, what genes control their development, and what makes the difference between the sensory hair cells that detect changes in electric fields and those that detect water movement.

That’s where the North American paddlefish (Polyodon spathula) can help! This is truly an incredible animal. It has the most electroreceptors of any living vertebrate: between 50,000 and 70,000 “ampullary organs” per adult, many of them located on their rostrum or “paddle”, which is an extension of their cranium that accounts for nearly a third of their total body length (typically 1-2 meters).  Although a vulnerable or “threatened” species, conservation and farming efforts have made this primitive fish commercially viable as a source of caviar (No, I’ve never tried it…maybe it’s just me, but I’m not crazy about the idea of eating what I study), thus allowing us to obtain embryos for studying hair cell, and more specifically, electroreceptor development.

Now, contrary to what people initially think about my “field” trips, I don’t even see the adult fish! I go to the lab of collaborator Marcus Davis at Kennesaw State University, which is located on the outskirts of Atlanta, Georgia. The actual process of fertilizing the embryos is done in Missouri at Osage Catfishieries (osagecatfisheries.com) by the Kahrs family, a terrific family owned business that we’ve worked with over the years. Fertilization is external, so mature adults are injected with hormones that, along with weather conditions, dictate whether they are ready to be squeezed  (or in the case of males “milked” for sperm). So like buying from Amazon, I get an approximate delivery date and anxiously wait until I get an email saying the box of embryos is on the way. Once there, our game faces come on and, it’s at least 14 days of working all kinds of hours to ensure we maximize the one or two clutches we get per year. That means all experimental manipulations (e.g. injections, electroporations, drug treatments) need to be done in a short amount of time, in addition to the husbandry and collection of fixed specimens for future work. To say it´s intense at times is an understatement.

For me, one of the biggest challenges working at this university is that they are still in “transition” from their previous role as a small two-year college to a large four-year undergraduate college trying to advance scientific research. To give you an idea of what this means, there are no graduate students or postdocs, and I am the first postdoctoral researcher to ever visit this department. While I do get to interact with other faculty and undergraduate students, for the most part, I work alone. While I appreciate the chance to get caught up on all the podcasts I let pile up, it’s a very different environment to what I’m familiar with. Also as a former “commuter” school, it is located just off the major interstate, convenient for drivers, but not close to town. So any excursions require driving, thus making it more difficult to explore the area when you are limited for time.

However, two of my favorite things about going to Georgia (besides working on paddlefish, of course) are southern food and spring thunderstorms. Coming from England, I know drizzling rain. But in Georgia, with little warning, thunder and lightening just roll in. It can be quite a show and then 15 minutes later it’s completely gone and if it’s still daylight, the sun comes back out. Usually, it’s no big deal and quite normal around here. This year was different though. Across much of the southern United States, many states experienced the worst storms and tornados in nearly four decades! Luckily, the area I was visiting was spared much of the destruction: we only had a couple of power outages, but it did make for a few sleepless nights. All in all, not a bad season.

To see a juvenile paddlefish eating, check out this video I took:

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