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Web Conference with Dr. Tyrone Hayes Web Conference with Dr. Tyrone Hayes - 00:00 My interest in amphibians and all the kind of stuff I do now starts way, way back. I grew up in South Carolina in an area where amphibians and reptiles and well, animals in general were just abundant. And in particular I've been interested in amphibians because of their dual lifestyle. That they depend on the water and then they have this terrestrial component to their life. But I think most importantly, the fact that they have no egg shells, and that the eggs are just there in the water and as a child I could watch them develop and turn into tadpoles and turn into frogs and that whole developmental process simply fascinated me since the time I was a child. Long story short, I went from South Carolina to Harvard with very little, oh how can I say, it wasn't a strategic decision so I only applied to one school, I was a first generation high school graduate on my father's side of the family and the whole process was foreign to me. And Harvard did some good recruiting and I'd heard of the place and so that's where I ended up. And I went into Harvard knowing that I was interested in science, knowing that I was interested in experiments and animals and amphibians, but had no idea what career path one took when you enjoyed doing those kinds of things. Luckily I ended up as a freshman with a professor, Bruce Waldmen? Actually in a summer REU, in an NSF REU program. And I got involved in research as a freshman and then continued in that laboratory until graduation. And in college my interests were also in amphibians and also in amphibian development and metamorphosis and growth, but I was particularly interested in how the environment influenced development and growth and reproduction. Towards the end of my career I became interested in how temperature potentially affected sex differentiation and reproductive development in frogs. And right in my senior year for my honors thesis became interested in whether or not temperature had this effect by influencing hormone levels which in turn influenced things like time to metamorphosis and size of metamorphosis and whether or not you're a male, whether or not you're a female. So with my graduate work I came to Berkeley to learn more endocrinology, so to learn about hormone metabolism and hormone production and hormone function and activity, mostly steroid hormones. And then towards the end of my graduate work, for my graduate work I studied how the role of hormones in reproductive development Web Conference with Dr. Tyrone Hayes metamorphosis, but with more of a physiological organismal approach, a lot less field work. And then towards the end of my graduate work, got more interested in molecular mechanisms of hormone action, and then did a short post-doc at the NIH to learn some molecular techniques and then as a professor, I was hired shortly after I graduated, shortly after I got the post-doc, I've really desired to put everything all together, to look at animals in the field, to look at ecology, evolution, populations and how they're affected by the environment. And in turn, the mechanisms by which the environment induces those effects by altering hormones and how the hormones work. Somewhere in there it became clear to me that some of the biggest environmental influences on development through affecting hormones were chemicals in the environment that could either mimic or alter hormone production or degradation or through any number of mechanisms. And that led me to atrazine which has occupied most of my career for the last decade. - 03:32 2006 paper on pesticide mixtures, you mentioned a lot of studies that used pesticide concentrations around 10,000 times. The low concentrations used in your studies. Now your low concentrations are relevant because they are what in the environment. So why do these other studies use such high concentrations? Because it seems like it's obvious that if you use a very potent dose of a chemical you will get some effect but that won't really help you understand what's going on in the environment. - Well I think some of it's a cultural shift if you will. So lots of people, so let's back up I guess. The high levels are the result of people who are doing what I would say, classic toxicology. Historically the interest was, what is the safety level of which we can use this chemical. And that safety level was defined by things like mortality, like LD50 the lethal dose at which 50% of the animals die. Or growth deformities or things like that. And so what people typically did is they'd start with a relatively high dose, and then you'd go lower and lower and lower until you saw no effect, until you got to the so-called no observable effect level. The reality is though, that Web Conference with Dr. Tyrone Hayes hormones, sorry that chemicals that interfere with hormones are active at much lower doses than they would be considered toxic. So for example, what I referred to as a culture shift is that the levels that I work at from my point of view, are actually not low levels at all. They're actually fairly high levels. Because hormones are active at levels that are even 1,000 times lower than I'm using. So from a toxicologist perspective, I'm looking at levels of a pesticide that aren't toxic, you're not gonna die, you're not gonna be grossly deformed or outwardly deformed, and so they generally have been considered safe, so for example atrazine's active at 0.1 micrograms per liter. So that's 100 nanograms per liter, or 100 picograms per mill, estrogen itself is active at 100 to 1,000 times lower than that. So it's a low level to a toxicologist, to me it's not a low level at all. - 05:38 Then why is an atrazine exposure dose kind of, where as like you said, for frogs a lower dosage of atrazine has a certain specific effect? - I don't have that paper in front of me, but I believe I know what you're talking about. So the difference is, I think many of these curves are sort of bell shaped or inverted U shape, if you will. So for example, birth control pill. Estrogen works exactly that way. You would be infertile without estrogen, right? You need low levels of estrogen for the effects on the uterus and for ovulation and egg development. And so there's a range as you get more and more estrogen. You would increase your fertility, correct? Whereas at high doses, because of negative feedback on the pituitary, estrogens actually will inhibit fertility. So with some range of doses, you get an increase in fertility and then as you get to the high doses you start to get a decline of fertility. So I think what you're looking at when you compare the fish to the frog paper it's just different parts of the curve. So we've shown some similar effects in frogs, where as you give more and more estrogen, you induce Vitellogenin, Vitellogenin is the egg yolk, the gene for egg yolk protein that's produced in the liver. As you give more and more estrogen, you increase that effect, so if you look at just the bottom part of the curve, you'd see a dose dependency, but if you go to higher levels you can actually start to see the effect Web Conference with Dr. Tyrone Hayes decline. And I think that's in general true of hormones as well as hormone mimics and other endocrine destructors. - 07:02 In individuals with delayed metamorphosis due to pesticide exposure, I was wondering if there was one stage during which development was arrested or prolonged, or if it was more like a uniform lengthening of development? And I was wondering if, depending on that answer why that would be the case? - Ah that's a good question. I can tell you with hormones that inhibit metamorphosis or certain compounds like thiourea that inhibit metamorphosis, they do inhibit specific stages or they are more potent at certain stages for example. And some of them have to do with which aspects of an amphibian's development are thyroid hormone dependent. So a compound that blocks thyroid hormone production might not inhibit the very early stages such as the hind limb development. But would have a really big negative impact or inhibitory impact on the later stages which are thyroid hormone dependent. So that would be the explanation for why. In the case of the pesticides, or I can give you another example, stress hormones, like corticosterone, early in development, they are inhibitory because they're inhibiting thyroid hormone production. Later in development, they actually accelerate metamorphosis because they enhance thyroid hormone activity, so once you're already making the thyroid hormone, then you get the synergism, but prior to that you get inhibition for hormones, for the pesticide mixtures, I can't answer the question. Because all we know is their time to metamorphosis was delayed, so we didn't measure developmental stages along the way, we just know overall it took them longer, so the specific stages that were inhibited, we don't know. - 08:33 What do you think causes the deviation between the SSD frogs and the hermaphrodism treated with the same dose of atrazine? So why do some develop ovarian tissues while others produce only testes even though there's multiple in both cases? Web Conference with Dr.
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