Monday, August 20, 2012
Note: This show first aired on August 18, 2012.
Here on the show, we’ve spent the last several weeks talking about gender. As humans, our tendency is to think of gender as a hard and fixed quality with few exceptions. That is because we are mammals, and mammals have the hardest and most fixed sexual differentiation system on the planet. I hope that I’ve been able to convince you that in most other groups of organisms, gender is a variable and plastic endeavor; that is the norm, we are the outliers (our transgendered brethren not withstanding).
We humans may be in for a nasty surprise in the coming decades. It turns out that our genders are getting a tiny bit more plastic as a result of well, plastic, and a multitude of other substances that we have created in the lab in the past 50 years or so. It turns out that many of these chemicals (you know all the names: phthalates, bisphenol A, PCB’s, atrazine, flame retardants, DDT etc) are what we call endocrine disruptors. They have estrogen like molecular structures, and thus bind with estrogen receptors on our cells and turn on estrogen sensitive genes. The few that aren’t direct estrogen mimics still have a net estrogen like effect on the body. We talked about hormones and their effect on the sexual development of the human body in our discussion of why men have nipples; it will be useful to revisit it here.
So, the human form has a default mode, and that is essentially female. In the absence of other directions, our genetics describe a female body whose development is governed primarily by estrogen. Males develop because genes on the Y chromosome tell certain cells to make androgens like testosterone. You can have a Y chromosome, but if there is damage on it, and it doesn’t do its job, you won’t develop as fully male, or have a host of other reproductive problems. In humans, as in most organisms, expressed gender is a result of the balance between estrogens and androgens, regardless of the genetic make up of the individual. Which brings us to our endocrine disruptors.
Lets start with a chemical so common scientists refer to it as “ubiquitous” in terms of human exposure, phthalates. There are two main catagories (and many many individual chemicals), high density and low density phthalates. High density phthalates are used as plasticizers in plastics and polyvinylchloride (PVC). They make plastics softer. Chemicals in this class were banned for use in children’s toys nationwide in 2008. Low density phthalates are used in cosmetics and other smelly things, they make fragrances last longer (this is why the first hint that a woman is pregnant is that she starts purchasing fragrance free personal care products!—more on why in a moment). They are quite volatile and readily leach out of what they are in, so, phthalates are everywhere.
Phthalates function as an antiandrogen endocrine disruptor, by at least in part, inhibiting the synthesis of testosterone. No testosterone? No male development. What we actually see is a group of alterations to male sexual organs that has a name; “phthalate syndrome”; it includes reduced penis size and impaired testicular descent among other things. Studies have also concluded that phthalates interfere with male brain development, and result in “reduced masculine play in boys”. Many other anthropogenic endocrine disruptors work in similar ways, disrupting the testosterone pathways in the developing male. Its important to note that for many of these effects, the window for problems to occur is very early in embryonic development when those bipotential gonads are first differentiating, and requires very low exposure levels. Hence those pregnant ladies and their fragrance free lotions.
Endocrine disruptors affect females as well, to be sure, but they generally do not masculate them, in a mirror image of the male effect. Early onset of puberty and multiple reproductive abnormalities are among the more common effects noted for females.
So its not just feminized frogs and hermaphroditic polar bears anymore. The bird of our toxic legacy has come home to roost. Because many of the chemicals are relatively new (as in the past 30 to 40 years), and their impact is often in utero, but we don’t become reproductively active for a couple of decades after that on average, the negative effects on our reproduction can be delayed and difficult to connect back to our inutero exposure to an endocrine disruptor. So what does this mean for gender in humans? We might not be able to fill our biological roles quite as well as we once did, which, at the risk of sounding melodramatic, may have significant long term impacts on the future of the human species on this planet.
Swan, S. H. et al, “Prenatal phthalate exposure and reduced masculine play in boys” http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2874619/?tool=pubmed
Swan, Shana S. “Environmental phthalate exposure in relation to reproductive outcomes and other health endpoints in humans” http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2775531/?tool=pubmed
McLachlan, John et al “Endocrine disruptors and female reproductive health” http://www.ncbi.nlm.nih.gov/pubmed/16522520
Info about the national phthalate ban http://www.cpsc.gov/info/toysafety/phthalates.html
Nicolopoulou-Stamati, P. and M. A. Pitsos “The impact of endocrine disrupters on the female reproductive system” http://humupd.oxfordjournals.org/content/7/3/323.short
Post Script: So, how do I avoid phthalates anyway? First, the good news is that phthalates have a very short residence time in the body, as short as a day or two. So if you are successful in lowering your exposure, your body burden will decrease rapidly. Due to the ubiquitous nature of phthalate use in consumer products, it is difficult, (but not impossible) to lower your phthalate exposure. Below are a few links with some good ways to start you on your way.
Saturday, August 11, 2012
Note: This program first aired on August 11, 2012.
Once upon a time there was an entertainment company, and they set out to make a film for children that was accurate about life in the ocean, but entertaining at the same time. They chose a clown fish for their main character, and featured several other real fish and animal species as supporting characters. They went scuba diving on real reefs, to see how real fish move, in an attempt to get their animation as accurate as possible. The film went on to be one of the top grossing kids movies of all time, and win four Academy Awards. You can guess by now I am talking about Disney’s Finding Nemo.
They got a lot right in finding Nemo, but there’s one thing they got wrong, really wrong. Here’s the thing about clownfish. Most of us know that they live in a symbiotic relationship with a host anemone, a creature that would sting and possibly kill most reef animals. What most of us don’t know is that clown fish live in strict social groups, of usually 5-6 animals. At the top of the hierarchy is the breeding female. She is the largest of the group, and calls the shots. The next in command is the breeding male. After that, the three to four smaller, non breeding, male fish. This is how it works, the female is in charge, but if anything happens to her, everyone below her gets a promotion and bumps up in the hierarchy. The male becomes the female, and the biggest of the asexual fish becomes the male. So, when Nemo’s mom got eaten by that barracuda at the beginning of the movie, Nemo’s dad should have switched it up and become Nemo’s new mom, because clownfish are sequential hermaphrodites.
In clownfish, it appears that stress hormones control this process. Clownfish are born male, but switch to female when they fully mature. In a social group the breeding female keeps all the other fish in her group stressed by harassing them and limiting their access to recourses. This keeps them from developing fully and with their resources going towards stress hormones instead of growth, they remain male. If something happens to the female, the stress pressure is temporarily released, and everyone in the group has a bit of respite. They actually grow, and the largest male, the one that was the breeder actually gets to mature into a female, at which point, he starts harassing all the other fish again, and the cycle starts over. This is the sex change suppression model, the normal course of development is impeded by strict social control. Its important to note here that the normal course of development is a switch from male to female, the system in question just controls when that happens. The other main model of sequential hermaphrodism in fishes at least, is the sex induction model. This requires an awareness on the part of the individual fish of sex ratios or size ratios in a large group. When the ratio becomes too skewed, a sex change is initiated to rectify the situation. For you salt water aquarists, Sea Goldies and various species of wrasse are subject to this kind of sex change.
Sequential hermaphrodism is common in many gastropods as well as a few flat worms, the random crustacean and even an echinoderm. None of these species are as cute, and screen friendly as Nemo though. Though I understand why Disney ducked this issue when it made the film, I think the world would be a more interesting and open minded place if parents had to explain sequential hermaphrodism to their children, and kids grew up understanding that when it comes to gender (and most everything else for that matter) the world doesn’t necessarily work the same way we humans do.
Interested in the challenges of animating a fish? http://news.nationalgeographic.com/news/2003/05/0530_030530_findingnemo.html
Personal communication, Dr. Ann Cleveland, Maine Maritime Academy, Castine Maine (once again, having a clownfish researcher for a boss has its perks!)
Interesting perspective on just how unique the clownfish transformation is: http://rhodeslab.beckman.illinois.edu/fish/Fish%20Lab.htm
Abstract only online: Environmental Biology of Fishes
Volume 29, Number 2 (1990), 81-93, DOI: 10.1007/BF00005025
http://www.springerlink.com/content/m806xlu045751u21/ outlines the models of different mechanisms for sex change in fishes
Note: This program first aired August 4, 2012.
The animal kingdom is full of mind boggling diversity, when it comes to making baby animals. Years ago an astute listener sent me a book, called How Animals Have Sex: A Guide to the Reproductive Habits of Creatures Great and Small, which documents in an extremely humorous way, this amazing diversity. The book highlights the reproductive strategies of many creatures from the animal world, including a few of today’s topic, the simultaneous hermaphrodites.
The word hermaphrodite comes from Greek mythology. Hermaphroditos was the son of Aphrodite and Hermes. One day he met a beautiful water nymph Salmacis, who kissed him and prayed that they be united forever. The Greek gods were extremely literal in this case, and Hermaphroditos was forever after recognized as part male and part female.
Animals that practice simultaneous hermaphrodism are like Hermaphroditos, by having gonads with both functional ovarian and testicular tissue, meaning they can make both eggs and sperm at the same time (scrabble players take note, the organs are called ovotestes, singular ovotestis). Animals that can do this include many gastropods, like our common garden slugs and snails, many other groups of molluscs, flat worms, many segmented worms like earth worms and leaches, some bivalves, the ectoprotcs like bryozoans, and even a few fish. There are estimates that as many as one third of all animals, excluding insects, are hermaphrodites.
Some of these species are self fertile, and can use their own sperm to fertilize their own eggs, others have evolved barriers against self fertilization, like many hermaphroditic plants. When hermaphroditic animals mate, a typical scenario is to swap sperm. Some species practice a variation on that theme, with one individual acting as male and the other as female during mating. Then they switch roles and mate again. Sometimes there is a difference between the gonadal gender and the functional or behavioral gender of an individual. They may have fully functional male and female gonads but choose to present as either one or the other for social reasons, often having to do with relative size or population density.
Phylogeny is evolutionary history. The phylogeny of hermaphrodism is unresolved at best. It appears that hermaphrodism is what we call a derived condition, as opposed to an ancestral condition. This means that hermaphrodism is not the original, or ancestral state of animal sexuality, it has been derived at some later time in animal evolution. Because it is seen in many relatively unrelated groups this also means it has evolved independently several times. This is called convergent evolution, and when we see that happen, we know the populations involved are on to something good, something that works.
The thing that works is that hermaphrodism allows organisms to maximize their chances of getting their DNA into viable offspring. It can literally double their chances. Beyond the opportunity to fertilize yourself in a pinch (which we know from previous discussions isn’t actually advisable), having double duty gonads functionally doubles the population size of a group by having each individual act as both a male and a female. So not only do I have a chance at having offspring if I take your sperm and fertilize my eggs with it, I can double my chances by giving you some of my sperm to fertilize your eggs with. If I live in an area with low population density, this is especially helpful, because it means I can mate with anyone I encounter, instead of being one gender and wandering about looking for an individual of the opposite one. For these groups of organisms, these benefits outweigh the metabolic costs of having to make two different kinds of gametes and the elaborate sexual organs needed to manipulate these gametes.
This may not be at all what the Greek gods had in mind when they transformed Hermaphroditos, but it has made life easier for many of our invertebrate friends, and our experience of the natural world just that much richer.
This is the book I wish that I had written: Strorm, David, How Animals Have Sex Gotham Books, NY, NY 2005 ISBN: 1592401910 I include the ISBN because you know you want to buy this book.
Leonard, Janet ed. The Evolution of Primary Sexual Character in Animals Oxford University Press (partial preview available on Google Books)
Editorial in Seminars in Cell and Developmental Biology “Mode and tempo in environmental sex determination in vertebrates” 2009
Avise, J. C., Mank J.E. “Evolutionary Perspectives on Hermaphrodism in Fishes” Sexual Development 2009, 3: 152-163