Gender Part Four: Plants Gone Wild

Note: This show originally aired on July 14, 2012.

We have been spending the past few weeks looking at the biological basis for and variation in gender. This week we continue our look at sexuality in plants.

The vast majority of flowering plants or angiosperms are hermaphroditic, having both male and female reproductive structures on the same plant and usually in the same flower, we discussed those in detail a couple of weeks ago. A small minority of plant species have bucked the trend and evolved to have fully separate sexes, with males and females occurring in entirely different individuals. These polymorphic plants are called dioceious. Gender in dioecious plants is determined genetically and there is, to use the word of one researcher, a “bewildering” level of variability in the sexual structures of dioecious angiosperms.

Plant sexual polymorphism (poly meaning many, morph meaning shape) is a relatively new invention in angiosperms, and this newness is the reason for the variability. Amazingly this form of sex determination is estimated to have evolved over 100 separate times. This means that many many lines of angiosperm plants have, in evolutionary terms, solved the same problem in the same or similar ways. These plants have evolved an XX/XY sex chromosome system, much like ours, but it is relatively new in evolutionary terms, so it is much more complicated and less well resolved than the relatively simple mammalian system. Dioecy (or the state of being dioecious) is not at all the straight forward boy meets girl scenario we humans are used to; in addition to the male meets female system, there are male meets female biased hermaphrodite and female meets male biased hermaphrodite, and mixes of all three as well.  These plants are experimenting wildly with sexual reproduction, beyond the dreams of even the most creative humans. This chaos (and it is chaos, if you don’t believe me, dive into the literature) is a result of the relative youth of the dioecious reproductive system. We’ve already said that angiosperms have the most diverse reproductive structures of any group of organisms. Think of these dioecious outliers as those on the forefront of a new evolutionary wave. Sometimes they succeed, other times they fail. While there is some evidence that this system yields a lower overall reproductive rate, I think it is too soon to judge the results of this evolutionary experiment.

As if all this sexual experimentation weren’t enough, it gets really interesting with the fact that many unisexual plants are diphasic, meaning they change sex, starting as one, ending as another. One reason for this life strategy is environmental stress. A close to home example is the unusual wood and wetland species Jack in the Pulpit, aka Arisaema triphyllum. This is a multi year perennial plant, and when it first emerges, it is non flowering. Then it develops into a male plant. Traditional thinking has it that pollen is a less energy intensive gamete to produce than the larger ovule, hence smaller plants with less resources to devote to gamete production will be male. If the environment is stressful enough, in terms of nutrient levels or space competition, the plant remains male. If it is able to gather enough resources to grow larger, it will switch sexes and present as female in later years. There is evidence that the sex ratio of the plants around it also effects its sex determination. These plants are able to do this because the have a “plastic genotype” that allows them to develop one of a few options depending on the environmental conditions they are presented with in any give growing season. This plasticity, I believe, is a result of the relative youth of this sexual system. This is the wild west of biology, the rules aren’t fully written yet, and these plants are testing the limits of the system, which, as noted above, can lead as easily to failure as to reward.

The other thing we should note, is that if these individuals can change sex, that means that they must have the genetic code for both sexes in their genome, even if they are using a proto version of an X/Y sexual determination system similar to ours. Many vertebrates have no sexual determination genetics, and rely totally on environmental factors for determining gender. As mammals evolved on a branch of the tree of life away from other vertebrates, is it possible that our gender development was as plastic as these plants we now observe? We will consider that in the coming weeks as we continue our exploration of the origins of gender. 

References:

From the Indian Academy of Sciences, a pair of articles in their science education journal Resonance. From 1998, Volume 3 No. 4 R.M. Borge’s Gender in Plants: Why do plants change sex? and Vol. 3 No. 11 R. M. Borges Gender in Plants: More about Why and How Plants Change Sex. Dated, but useful.

http://www.ias.ac.in/resonance/Volume3.htm

Barret, Spencer “The Evolution of Plant Sexual Diversity” Nature Reviews: Genetics April 2002, Vol. 3

Charlesworth, D. “Plant Sex Determination and Sex Chromosomes” Heredity 2002 (88) 94-101 Very technical, good luck with this one.

http://www.sciencedaily.com/releases/2008/08/080807144242.htm “Gene for sexual switching in melons provides clues to evolution of sex”

http://www.pitt.edu/~kalisz/Research.html Website for the University of Pittsburgh Kalisz Lab, a plant research lab.

Gender 3: Male Nipples and Human Evolution

Note: This program first aired July 7, 2012.

I was asked recently, why men have nipples. As we are currently working our way through the biological issues of gender, I thought this might be a good side track because it takes us from the somewhat silly idea of men’s nipples, to the much deeper and fundamental concepts of evolution and human development.

My initial guess was that men have nipples because nipples are an evolutionary orphan—left over from a time when perhaps both sexes of mammals nursed young.   Another idea was that maybe nipples had some other function in early mammals; regardless, I thought men had them because they once needed them and now don’t, and, like our appendix, they haven’t fully evolved away.

It turns out that, to the best of our knowledge, a small part of my guess was right (which means that large part of it was wrong as well). The part I was right about was the easy part. If an individual has a trait that has a negative impact on its ability to reproduce, that individual’s genes (and the traits they encode for) will disappear from the population. This is the mechanism of natural selection. However, if an individual displays a trait that is neutral to its ability to reproduce, there is no selective pressure that will make that trait disappear.  The bottom line is that once a trait appears in a population, if it isn’t doing any harm, it isn’t likely to disappear. This is the argument for why men still have nipples. While it is possible for men to get breast cancer (not only do they have nipples but mammary tissue as well), it is rare and not affecting the overall reproductive rate of the human population. So, while they are arguably useless, they are likely here to stay.

But this doesn’t really address the primary question, why do they have them in the first place? My thought, that they have them because they once needed them, seems to be all wrong. Almost all male mammals have nipples, and there is no evidence in the geologic record or in current species, that males ever used them for nursing. Mammary tissue originated pre mammal—in a group called synapsids ( first seen about 310 mya—small lizard like animals). They didn’t have nipples per say, but simply nourished their eggs and eventually young with secretions that came out of their skin, through structures related to hair follicles (platypus and other monotremes still lactate this way, picture milk oozing out of the skin of your chest and your infant licking it off you). There is some debate about origin of this tissue, did it arise from sweat glands? sebaceous glands?  or some other related cutaneous gland? Did the secretions nourish live young or as some hypothesize, get absorbed directly through the thin shells of the eggs of these early animals? It is not clear, as this tissue does not fossilize well. But it is clear that lactation predates nipples.

So it seems that the real reason male mammals have nipples is due to the realities of our very early development as embryos. It is commonly said that all human embryos start out “female”; this is an oversimplification that has the ring of urban legend to me, but there is a bit of truth to it. Typically, and in most mammals, individual who carry the XX genotype for their sex chromosomes develop into females, individuals who carry the XY genotype develop into males. All the traits and structures that we think of as human are on the 22 other pairs of chromosomes, including the coding for nipples and mammary tissue. The only difference between males and females is the presence of that Y sex chromosome, and amazingingly, the only thing it does is tell certain cells to make testosterone several weeks into the development of an embryo. That testosterone then tells the sex organ cells to become testes and male genetalia instead of ovaries and female genetalia. Without that testosterone cue, an embryo with an XY genotype will develop in the default mode, as a human who’s development is governed more by estrogen than testosterone, someone we would identify as female. 

Another thing that testosterone pulse does is suppress the development of the mammary tissue (though in humans, not completely). In a few species of mammal, (rats, horses) the testosterone completely suppresses the development of this tissue, and as a result, males of those species have no nipples at all. Again though, nipples on human males don’t seem to cause many problems, so we haven’t evolved such radical measures. In fact, males who experience non typical hormonal patterns or who experience the lower levels of testosterone common in older age can develop more mammary tissue, breasts and even in very rare cases, can lactate.

To recap: Mammals evolved nipples because nipples worked better than sweating milk for feeding young. Men have them because they are in the generic blue print for human being, and there has been no selective pressure to fully get rid of them. And the more we know about the delicate nature of the early development of the human embryo, the more clear it becomes that both native hormones and pollutants that mimic estrogen and testosterone in the body can have a profound effect not only on our health, but on our very identity.

References: 

Andrew Simons, 2003 http://www.scientificamerican.com/article.cfm?id=why-do-men-have-nipples

Interesting thoughts from Cecil Adams, the “worlds’ smartest humanbeing” http://www.straightdope.com/columns/read/85/why-do-men-have-nipples In fact, there are many people blogging about this issue. If you Google “Why do men have nipples?”, you will primarily get blogs as top hits.

“The Mammary Gland and Its Origin During Synapsid Evolution” Olav T. Oftedal,  Journal of Mammary Gland Biology and Neoplasia Vol. 7, No. 3 July 2002

Gender: Part 2

Note: This program first aired in June 2012.

Today we continue our ongoing discussion of gender and how it relates to biological reproduction and the amazing diversity we see in nature. To review, we know that individuals we call females make ova or eggs, and individuals we call males make sperm, and in humans, this comes with a whole lot of extra baggage (to be fair, many animals exhibit the differences in appearance and behavior called sexual dimorphism, but I believe we are the only species to have come up with misogyny and homophobia among other things).

When it comes to reproductive equipment, flowering plants, technically called angiosperms, have all other groups of organisms beat. As a group, angiosperms have the highest variety of reproductive structures on Earth. In this group, the most common sexual form is the hermaphrodite, meaning both female and male components are found in the same flower. These flowers can also be referred to as bisexual, and are botanically classified as “perfect”. It is thought that all angiosperms evolved from a hermaphroditic ancestor, and single sex, non hermaphroditic strategies evolved later. This is why 75 to 85% of angiosperms are hermaphrodites.

What are the benefits of being a hermaphroditic plant? The one that jumps to mind initially is that if there are no other potential mates around, you can fertilize your self. In theory this works, but most plants actually go to great lengths to avoid this phenomenon (called “selfing”).  Like interbreeding in royal families, selfing tends to bring out genetic weaknesses over several generations. Think of selfing as a last chance scenario, especially for annual plants, who live only one growing season and “winter over” in seed form. If the plant doesn’t get to mate with anyone else, at least it can sustain itself over the winter as a seed generated from a self fertilization; ideally in not too many generations in a row.  

Another way that hermaphrodism increases an individual’s chances of mating is in a low density potential mate situation; a less extreme version of what we just talked about. If there are very few potential mates around, what happens if they are all male? Or all female? They aren’t the isolated individuals of the former scenario but they still can’t mate successfully in a single sex group. Hermaphrodism in plants ensures both sexes are represented, even in a low density population.

Yet another benefit of hermaphrodism in plants is that it extends their mating season. Hermaphrodites can functionally bias their flowers towards one sex or the other for various reasons. While the flower will have both male and female sexual structures (the androecium and gynecium respectively), they may not be functional at the same time. For example the male may mature first, and produce pollen. Later the female parts will mature, ready to accept pollen from a separate individual. The plant has then essentially doubled the amount of time it is spreading and mixing its DNA, and has eliminated the chance of selfing as well.

The bet hedging, flowering species of the plant kingdom have clearly benefited from utilizing both genders simultaneously for millions of years, yielding extraordinary biodiversity and resilience. And with all of our ideas about gender and value, sexuality and stereotypes, most of us just see the flower.

References:

From the Indian Academy of Sciences, a pair of articles in their science education journal Resonance. From 1998, Volume 3 No. 4 R.M. Borge’s Gender in Plants: Why do plants change sex? and Vol. 3 No. 11 R. M. Borges Gender in Plants: More about Why and How Plants Change Sex. Dated, but useful.

http://www.ias.ac.in/resonance/Volume3.htm

University of Oxford’s Science Blog features a conversation from March 2009, with Dr. John Pannell of the Oxford Dept. of Plant Sciences

http://www.ox.ac.uk/media/science_blog/090312.html

Dr. Spencer Barrett’s “The evolution of plant sexual diversity” is lengthy and quite technical, but dedicated readers may find much to ponder. April 2002, Vol. 3  www.nature.com/reviews/genetics

Water: Part One

Note: This program first aired on June 23, 2012. 

The late spring and early summer of 2012 has proven to be a damp one here in Maine. While not as biblical as the spring a few years ago, when it rained every day for weeks on end, I still hear complaints about the showers and lack of sun everywhere I go. I admit, I hear them in my own head and feel them in my own heart as well. And having just returned from a trip to the desert, where the sky is inevitably blue and sun always shines, the damp has felt especially insidious.

I’d like to take this time however, and remind us all, how blessed we are. Water is precious, a fundamental requirement for life on earth, and increasingly rare. Our physiology demands it at regular intervals on a daily basis. Modern industry uses (and abuses it) in vast quantities. Our food supply is critically dependent on fresh water, 70% of all appropriated freshwater currently goes to irrigation of crops, so where there is water shortage, there is food insecurity. Over 7 billion people live on this planet, and by 2025, over half of them will live in areas subject to severe water stress. Millions of people on the Indian sub continent and in South America rely on glacial melt water as their primary water supply. With global glaciers in fast retreat and several projected to disappear in the coming decades where will those people’s children get water from? Subtropical regions the world around are already typically desert zones. As climate instability increases in coming decades, these regions, home to millions of people, are expected to experience intensified drought. The leading cause of death globally is diarrhea, overwhelmingly due to lack of access to clean water.  When I said we are blessed, I meant it. Clean water, so necessary for life, literally falls from the sky here.

We primarily get our water (for industry, for drinking and municipal use), from one of two sources: deep underground aquifers (referred to as ground water) or surface water (water that is readily available in ponds, lakes, rivers). Ground water is the form of the vast majority of liquid fresh water on Earth. According to a UN World Water Development report “Ground water supplies nearly half of all drinking water in the world” and is crucial for the well being of people, especially rural poor people, all over the world.

We should (but don’t) think of ground water as a non renewable resource. Water moves readily through the hydrologic cycle: it evaporates from the land and ocean surface, and is transpired from vegetation into the atmosphere. From there it condenses from a gas back to a liquid in the form of clouds and eventually rain or snow. That precipitation returns the water to the surface of the Earth, where it infiltrates the soil, or runs off and returns to surface water reseviors. Much of the water that infiltrates the surface is taken up by vegetation or simply evaporates back into the atmosphere. Only a small fraction eventually makes it down into the ground water aquifer. The rate of recharge depends not only on how much water hits the land surface, but also on the depth of the saturated layer, and the porosity of the underlying geological layers the water has to travel through. Many ground water aquifers are being depleted, due to extraction rates that exceed inflow or replenishment rates. Basically, we take more water out than is going back in through natural processes, thus making ground water use unsustainable.

So the next time you are thinking about complaining about mosquitos or black flies-insects that require water in which to lay their eggs and provide habitat for their larva, pause and remember: their presence is an indicator of our good fortune and our richness; abundant clean water that falls from the sky.

References:

“Environment and Development Challenges: The Imperative to Act” http://www.unep.org/home.asp
www.unep.org/pdf/pressreleases/Blue_Planet_synthesis_paper.pdf  This is a summary statement from all Blue Planet Award Winners.

 “Managing Water under Uncertainty and Risk” United Nations World Water Development Report  4 http://www.unwater.org/ Cheery reading.

https://www2.ucar.edu/atmosnews/news/2904/climate-change-drought-may-threaten-much-globe-within-decades Report from the National Center for Atmospheric Research (NCAR).

http://www.ipsnews.net/2011/09/200-million-depend-on-melting-glaciers-for-water/ Excellent 2011 article that provides an overview of the glacial meltwater issue.

Gender: Part One

Note: this program first aired on June 16, 2012.

We humans tend to think of gender as a fixed and clear cut matter. Males are males, females are females, and they each have a fixed and distinct biological role in sexual reproduction. Unfortunately, many of our human cultures have not displayed much tolerance for individuals who have blurred the lines of our clear cut ideas about gender.

What I would like you to know today is that in nature, gender is not as fixed or defined as we humans would like to think. Initially gender in the form of separate sexes arose as a result of sexual reproduction, which is simply the recombination of genetic information. In its simplest form, an individual is “encoded” by two sets of genetic information, one from one parent and one from the other. Without going into detail about the steps of meiosis (the process of making sperm and eggs) understand there is nearly infinite variation on the sets of genes that each parent can give, which is why each individual that results from sexual reproduction is completely and utterly unique.

When put this way, the obvious question is: Why do parents have to be two separate genders? Why couldn’t two individuals simply exchange gametes? Why is there gender at all? It turns out, initially, there wasn’t gender. This makes sense, the earliest organisms on earth were bacteria, and bacteria don’t have gender (nor do they generally reproduce sexually, but that is a different matter). Sexual reproduction in higher multicellular organisms was achieved through isogamy (a genderless exchange of gametes—many fungus and algae still reproduce this way).  Due to natural diversity, some individuals had larger gametes and some had smaller gametes. Over time, it seems that the most successful matches were between a larger gamete and a smaller one. This pushed natural selection towards favoring these two sizes of gametes, and separating the world into individuals who make big gametes and those who make small ones. Essentially, it is to your benefit to do either one of two things: invest a lot of time and energy into making a few large gametes (we now these now as eggs), or invest your energy into making lots and lots of small, short lived gametes, that you can continue to make anew your whole life (we call these sperm). The smaller and more mobile and less energy intensive the better, or the larger and more stable the better. In the evolution of sexual reproduction, the middle ground falls away.

This brings us back to the original question: gender. There is then indeed a biological basis for gender. Some individuals make large gametes, the eggs or ovum. We recognize those as female. Other individuals make smaller gametes, called sperm. We recognize those as males. But that is truly a simplification. As we will discover in coming weeks, gender is truly a human construct, and as usual, nature, in its infinite intelligence, elegance and practicality has something more amazing and beautiful to show us.

References:

Personal communication with Dr. Ann Cleveland. May 2012. Working in Academia has it advantages, including having a biologist for a boss. Over lunch, Ann enlightened me as to the origins of gender.

Sperm Biology: An Evolutionary Perspective By T. R. Birkhead, David J. Hosken, Scott Pitnick ( online exerpt, Google Books)

The origin and evolution of gamete dimorphism and the male-female phenomenon: (abstract only): http://www.sciencedirect.com/science/article/pii/0022519372900070

Phase Change

Note: This program first aired on June 9, 2012.

Phase change. It’s a term that refers to the change of a state of matter. When a substance changes from solid to liquid, or liquid to gas it is said to change phase. What makes phase change interesting is that there is a set energetic pattern associated with it, and both the natural world, and human industry have incorporated that energy in useful ways.

When a substance, any substance, changes from a solid to a liquid, or a liquid to a gas, it requires energy. Energy must be absorbed by the substance for this to happen. We know this. It takes energy for ice to melt right? Or liquid water to boil away to steam.  As the molecules of the substance absorb more energy, they move more. The more and further they move, the ‘looser ‘ substance’s structure becomes. At a certain point, the molecules absorb enough energy that they are behaving differently enough to appear as a different state of matter.

One wonderfully reassuring facet of nature is its symmetry. In this case, if energy is absorbed in one direction of phase change, the opposite happens when we reverse the phase change. When a substance goes from liquid to solid, or gas to liquid, energy is released. The molecules must release all that extra energy in order to slow down enough to get close enough to each other to go into a denser “lower energy’ state of matter. And of course, we understand this too; we have to cool water off to freeze it solid.

If you keep this concept in mind, you will recognize it in many places. Its why your refrigerator works, why the propane tank gets cold when you run it, why sweating works to cool us off, why panting helps dogs. Sweating, and panting are just operations that utilize the physics of evaporation. Our (and our dogs’) bodies are producing a liquid and taking advantage of the fact that turning it into a gas will remove energy from the system, in this case, our bodies. That is why sweating cools us off, evaporating that water off our skin absorbs energy, the extra heat we are trying to get rid of.

A refrigerator is an elegant example of how we use phase change in both directions to our advantage. We can construct a simple model of a refrigerator by imagining a tube in a closed loop. Half of it is inside a box, and half is outside. We put a substance in the closed loop that wants to be gas at room temperature (or even a little cooler). This means that left to its own devices, it at room temperature it will readily absorb energy and boil, changing from liquid to gas. Humans have figured out that if they compress that gas into a liquid, it will release energy. A refrigerator is simply a system that takes that substance, compresses it to a liquid form, pumps it into the space to be cooled, allows it to evaporate in the tube (hence cooling the surrounding area), pumps it out of the space to be cooled, compresses it back to a liquid (thus releasing the heat) and back in and back out, and around and around it goes. Evaporation happens inside the refrigerator absorbing energy, condensation happens outside the refrigerator, releasing energy. That’s why its warm behind the fridge—its not because of the motor, its because the heat that was in the stuff inside the fridge has been absorbed and moved out, via the substance in the tube (or coils—lots of tubes). And what is this substance? In most domestic refrigerators modern refrigerant gasses similar to the chloroflourocarbons of ozone layer fame are used, rendered less dangerous by having the chlorine removed. 

What is remarkable to me is the simplicity and consistency of this pattern. It doesn’t change, when things condense, they release heat. Period. When things evaporate, they absorb energy, no questions asked. With frenetic pace of life today and the unprecedented opportunities and distractions we face in every moment, I find these indisputable facts infinitely reassuring.

Snowflakes

Note: This piece first aired in January 2008

If you were paying attention to the immediate environment anywhere in Maine during December, you know it was covered with snow. That may be hard to remember during the current January thaw, but December 2007 was record setting for snowfall. Snow is frozen water, formed through a remarkably complex process in the clouds. We've talked about clouds before, they are formed when water vapor, the invisible gaseous form of water, condenses out of the air to form liquid water—that's what condensing is-going from gas to liquid. So clouds are masses of tiny drops of liquid water, suspended in the air. When the cloud gets cold enough, the droplets of water can freeze, forming the foundation for a snowflake. Then water vapor that is still in the air as gas freezes directly onto the ice nucleus—going directly from gas to solid—this process is called deposition-it's also the same process that causes freezer burn. So snow isn't frozen rain—that does occur but we call it sleet—solid ice pellets that form when rain falls through a cold air layer on its way down to the ground. Snow is the direct deposition of ice crystals onto an ice nucleus, and the shapes and forms it takes are highly dependent on both the temperature and the relative humidity of the air where the snowflake originates.

We've all heard the saying that no two snowflakes are alike, which is really neither here nor there, as it would be impossible to prove or disprove anyway. What is certain, and easily observable for most of us, is that snowflakes come in several distinct shapes. Almost all snowflakes have some sort of 6 sided or hexagonal structure. This comes from the way the water molecules arrange themselves when they freeze and form crystals, due in large part to the unique qualities of the water molecule. But that's another story.

When it is very cold, the air does not hold much water vapor, and the snow that forms tends to come in very small, simple shapes, plates-flat hexagons, or small columns (like a stack of plate snowflakes). As the air warms up more complex forms show up, stellars—which are star shaped, and dendrites—which are lacy or leafy—the classic snow flake we all imagine. Snowflakes also come in the form of needles! Any of these forms can show secondary effects as well, like when the snow flake falls or gets recirculated through a higher humidity cloud or fog. When it makes it down to the ground it looks like little bits of Styrofoam and is called graupel.

So you have an assignment. The next time it snows, go outside with a dark jacket on. Catch some falling snow flakes on your arm and look closely at them. See if you can determine their general form. Don't be discouraged if you can't tell at first, when its windy snowflakes get broken up on the way down, and it its warm they stick together. You will probably be able to identify parts, and if it's a nice calm snow event, you should be able to see snowflakes in their entirety.