Fish have Placentas! Yes, Fish!

In the midst of finishing my PhD, I neglected to write on In Defense of Duck Genitalia. The last two years consisted of data analysis, writing, more data analysis, and a lot more writing than I anticipated. Good news, I finally completed my PhD! While navigating the next step of my career, I strive to be more active on In Defense of Duck Genitalia – there is a whole lot of cool science to talk about! So, let’s restart with fish placentas.


When I started my PhD, I was interested in whether sexual selection* is an important factor in how and why species evolve (i.e., speciation). I spent six months reading, thinking, and designing potential experiments to answer this question, and was pretty determined to do so. Fortunately, during this time, my PhD advisors encouraged me to read broadly, just in case there were other topics that peaked my interest. This, of course, led me to a paper on the evolution of fish placentas (in the genus Poeciliopsis) by David Reznick (from the University of California at Riverside) and his colleagues that changed my entire research plan (bye-bye 6 months of work!). Let me explain why placental evolution in fish is so incredibly cool…

First (and likely most obvious) is the fact that fish could even have placentas! Poeciliids, includes Poeciliopsis, are livebearers, which mean females have internal fertilization, carry their eggs, and give birth to live young. Other more commonly known livebearers (that are loved as pets/by aquarium hobbyists) are guppies, swordtails, and mollies. Although uncommon among fish, this reproductive mode allows for the exchange of nutrients between the mother and embryo, which, for some fish, has led to the evolution of placental-like structures. In Poeciliopsis, the placenta is composed of two main structures: the maternal follicle (imagine a tissue that encloses each and every embryo) and embryonic pericardial sac (imagine a sac that extends from the abdomen to the top of head of the embryo). Interestingly, these structures were described and drawn in an old-school natural history paper by C.L. Turner in 1940, but have been mostly overlooked for 60 years!

Second, in this paper, Reznick and his colleagues showed that poeciliids could vary both the timing and amount of nutrients (provisioning) provided to the embryo, from none to moderate to extensive. In some species, mothers provide all the nutrients prior to fertilization (in the form of a yolk). While in other species, mothers provide moderate to extensive amounts of nutrients after fertilization and during (embryonic) development. What is even more fascinating is that these differences in provisioning can be found among closely related (sister) organisms.

Third, these placental-like structures in Poeciliopsis have evolved in 750,000 years or less (and Reznick believes this may be an overestimate!). This time scale is actually comparable to the evolution of the eye, which is an incredibly complex structure. In addition, there have been THREE independent origins of this complex structure. So, we are able to study the evolution of placenta – the transition from none to moderate to extensive amount of provisioning – in three different groups of closely related organisms. This is something that cannot be done in mammals, as their single common ancestor lived over 100 millions years ago.

It has been 15 years since Reznick and his colleagues published this groundbreaking paper, and we are still trying to determine the evolutionary processes that drive the evolution of placenta in poeciliids. So far, we have narrowed it to natural selection (e.g., locomotion, more difficult for pregnant females to escape predators), sexual selection (e.g., different provisioning supply/demand by the mother/father(s) in a promiscuous mating system), and parent-offspring conflict (e.g., different provisioning supply/demand by the mother and offspring; which I previously wrote about, but with respect to genomic imprinting). The latter two processes have found increasing support from observational and experimental/molecular studies (e.g., research led by Bart Pollux^). However, recent work in another well-studied poeciliid that provides extensive provisioning (Heterandria formosa) has found contradictory results for parent-offspring conflict (e.g., research led by Matthew Schrader).

Although I had my heart set out on working on why and how species evolve for my PhD, I spent the next six years studying how sexual selection and parent-offspring conflict can explain placental evolution in fish. In many ways, I am grateful that I stumbled on this paper. First and foremost, I had never been so incredibly pumped and excited about research, which reaffirmed my decision to start a PhD. Second, my PhD advisors granted me plenty of freedom to think and explore this new area of research, which gave me a taste of what it would be like to develop your own research program (which is important during your post-doc, when you are trying to carve your own identity). Third, I went through a lot of ups and downs with this project (mostly because of the fear of exploring an understudied area of research [and the possibility of failing]); learning how to cope and overcome setbacks was one of the most valuable lessons from my PhD.

So, thank you PhD advisors for telling me over and over again to take my time to read and think about science. And, obviously, thank you David Reznick for continuously doing inspirational and cool science!

*Sexual selection occurs when there is differential mating success among individuals in a population. There are three mechanisms of sexual selection: male-male competition, female mate choice, and sexual conflict [between the sexes].

^If you are interested in reading more about this paper and, generally, placental evolution in fish, go check out Ed Yong’s brilliant article on this research.

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Sexual Dimorphism and Why The Sexes Differ

“Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males having had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; and have transmitted these advantages to their male offspring.”

-Charles Darwin, On the Origin of Species (1859)

We see striking sex differences in all sorts of organisms, including plants, animals, and even humans. This is something that even fascinated Darwin, who wrote an entire book, “The Descent of Man” (1871), on why the sexes differ [eventually, leading him to propose the theory of sexual selection*]. Sexual dimorphism occurs when males and females differ in a shared trait, which can occur on a genetic, phenotypic, physiological, and behavioural level. Some examples that come to mind are differences in body size (e.g., extraordinary large male elephant seals presiding over a group of females), ornamentation (e.g., elaborate and extravagant tail of peacocks used to court peafowls), armory (e.g., ginormous antlers on male red deer used for combat with other males for access to females), and behaviour (e.g., courtship calls by male frogs to attract females to the breeding sites). The evolution of sexual dimorphism is rooted in differences in gamete size (i.e., anisogamy). Males produce numerous, cheap sperm, while females produce few, expensive eggs. This leads to different reproductive strategies between the sexes, where males compete for females and females may resist matings, and/or females choose among advertising males.

Interestingly, the sexes share many of the same genes [traits], so such different reproductive strategies may result in selection acting in different ways on the same genes [traits] in each sex. And let’s also not forget natural selection, which can speed up or slow down such sex-specific selection. For shared traits, selection often acts on males to increase mating success (i.e., sexual selection) and on females to increase fecundity and viability (i.e., natural selection), both of which cause correlated responses in the opposing sex. So, when natural and sexual selection act in opposing directions in the sexes to optimize each of their fitness for some shared trait, this results in sexual dimorphism.

A good example in biology comes from zebra finch and their bill colour. Nancy Burley and her collaborators elegantly showed that males with redder bill are preferred by females and produce more offspring, creating sexual selection in male for bright bills. However, females with same red bills have lower survival and fewer offspring, thereby creating selection, that is natural selection, in the opposing direction for dull bills. So, in zebra finch, bill colour is a sexually dimorphic trait, where males with bright bills (at one end of the spectrum) and females with dull bills (at the completely other end of the spectrum) are the most fit in the population.

Today, evolutionary biologists are still keen on studying sexual dimorphism because it allows us to understand how shared genomes respond to natural and sexual selection, both of which can vary in strength (e.g., strong vs. weak selection) and/or direction (e.g., divergent vs. direct selection). Traditionally, sexual dimorphism has been discussed in light of natural and sexual selection, but recently, we have started consider its role in other important evolutionary processes, including speciation, diversity, and genomic imprinting. (Genomic imprinting occurs when maternal and paternal alleles are expressed differently in the offspring).

So, like many ideas in evolutionary biology, we owe it to Chuck D for being an incredible naturalist and creating interest for simple, but not always easy to understand, observations and patterns in nature, like why the sexes differ.

 *Sexual selection occurs when there is differential mating success among individuals in a population. There are three mechanisms of sexual selection: male-male competition, female mate choice, and sexual conflict [between the sexes].

The World of Animal Genitalia: Bizarre, Fascinating, and Complex

There are few things Charles Darwin got wrong. Most of us can quickly point to his theory on inheritance (sans genetics), but a lesser-known oversight was his idea on the evolution of genitalia. Yes, genitalia! It turns out Darwin believed that genitalia can only evolve under natural selection, but not sexual selection.  It was not until a century later that evolutionary biologists started to consider genital evolution in light of sex, starting with the significant contributions of Dr. William Eberhard in the 1970s. Today, genitalia are known to be among the most diverse and rapidly evolving traits, and sexual selection is implicated in their evolution.

Under the sexual selection framework, male genitalia can evolve (1) to stimulate females to take up sperm, (2) for sperm competition, and/or (3) to hold onto resistant females. Given the complex nature of male and female sexual interactions, these three hypotheses are likely operating simultaneously. For example, some of these interactions occur inside the female, making it difficult to determine the role males and/or females play in the outcome. So, let’s say sperm succeeds at reaching the egg, it is hard to tell if it is because males have superior sperm competitive abilities, females choose sperm from the best males, or a bit of both. Although determining the exact mechanism(s) is difficult, evolutionary biologists have shown a role for sexual selection in the evolution of genitalia time and time again and in different organisms, ranging from birds to insects to fish to mammals. So, to kick-start your interest into the world of animal genitalia, below are just two examples from an impressively long, and sometimes bizarre, list.

Over the last decade, Dr. Patricia Brennan, now at the University of Massachusetts, and her colleagues have been studying the reproductive biology of ducks, which has one of nature’s most bizarre systems. Here, males have a long, cork-shaped phallus that twists clockwise, while females have a reproductive tract (called the oviduct) that twists counterclockwise. So, why would genital parts that twist in exactly opposing directions evolve? Using different glass shapes, resembling the oviduct, Dr. Brennan revealed that the counterclockwise glass shape actually slows down the eversion of the clockwise turning phallus of the males. (See the high-speed videos here: http://www.youtube.com/watch?v=bybYCwjkm_s&feature=plcp. Oh, it is probably NSFW.) It turns out that during the mating season, males harass females, even the ones that have paired off, and such harassment can often lead to injury and even sometimes death. So, to fight back, females have evolved a reproductive tract that makes it harder for males to simply waddle up to them and mate. It’s definitely not all fun and games when it comes to duck sex!

Now, let’s move onto everyone’s favourite pest, bed bugs. To many of you, bed bugs are a living nightmare: destroyed furniture, itchy and bloody bites, and many sleepless nights. To evolutionary biologists, bed bugs are one of the most beautiful examples of male-female coevolution! At the University of Sheffield, Dr. Michael Siva-Jothy has dedicated his career to understanding the fascinating reproductive biology of bed bugs. Here, males use a needle-like intromittent organ (called the paramere) to traumatically inseminate the body cavity of females. Yes, to mate, males stab females and not even at their reproductive tract! Not surprisingly, these traumatic inseminations come at a high cost for females, often leading to infections, lower fecundity, and shorter lifespan. Unable to avoid the males, females have evolved a secondary reproductive tract (called the paragenitalia), which contains blood cells associated with immune responses, as a counteradaptation. Interestingly, males have played along and restricted their traumatic inseminations at the paragenitalia, thereby lowering the cost of mating on females. Why would males take part in this strategy, if they can pierce elsewhere? One hypothesis is this strategy ensures that females are just fecund enough to ensure male paternity and hence, male fitness. This is, hands down, way beyond tough love.

Alongside ducks and bed bugs, there are plenty of other cool examples of genital evolution, including the spikey penis of male seed beetles and claws at the tip of the intromittent organ of male guppies! At the University of Toronto, we are fortunate enough to have a number of experts studying reproductive biology in light of evolution and in very different systems, including, but not limited to, Dr. Maydianne Andrade (Department of Biological Sciences; UTSC) – spiders, Dr. Spencer Barrett (Department of EEB; St. George) – water hyacinth, Dr. Asher Cutter (Department of EEB; St. George) – nematodes, Dr. Darryl Gwynne (Department of Biology; UTM) – crickets, Dr. Helen Rodd (Department of EEB; St. George) – guppies, and Dr. Locke Rowe (Department of EEB, St. George) – water striders.

Although the rapid, divergent evolution of genitalia is a general evolutionary pattern, we have still have a poor understanding of the mechanisms (i.e., the three sexual selection hypotheses) underlying this pattern. Evolutionary biologists are now focused on studying the function of genital traits via various and rather creative experimental approaches, including the use of lasers. So, stay tuned to the truly bizarre, fascinating, and complex world of animal genitalia, there’s plenty more to come! Love it or hate it, the evolution of genitalia is, at the very least, a great conversation starter on a first (maybe hold off until the second) date.

Genetic Conflicts: A Family Affair

**This article was originally published in The Varsity, University of Toronto’s Student Newspaper**

The bond between parents and their children is often regarded as something loving, caring, and supportive (well, maybe except for during puberty). However, in evolutionary biology, we see this bond more as a battle, tug-of-war, and conflict. To explain this, we have to visit the groundbreaking work of two evolutionary biologists: (the late) William Hamilton and Robert Trivers. In the 1960’s, Hamilton introduced the notion of inclusive fitness (total fitness including kin, e.g. offspring, relatives, etc.) to studies of altruism. Specifically, he proposed that a gene for altruistic behaviour can spread if BrC > 0, where B is the benefit received by the recipient, C is the cost to the actor, and r is the coefficient of relatedness between the actor and recipient. This came to be known as Hamilton’s rule, and it singlehandedly altered the way many view social behaviours.

A decade later, Trivers (now at Rutgers) recognized that Hamilton’s rule could partially explain the social dynamics between parents and their offspring. Focusing on the coefficient of relatedness (r), Trivers proposed that the fitness interests of parents and their offspring should differ because the offspring are twice as related to themselves (r = 1) than to their parents (r = ½). This asymmetry in relatedness, now known as parent-offspring conflict, should lead to differences in parental investment, with the offspring demanding twice as many resources than the parent is willing to provide. Support for parent-offspring conflict has been numerous and comes from both theoretical models and empirical studies.

One of the coolest things that have come from these studies is the kinship theory of genomic imprinting. Genomic imprinting occurs when maternal and paternal alleles are expressed differently in the offspring. (Remember, with genes, we inherit one copy from our mother, and the other copy from our father.) The kinship theory explains the evolution of genomic imprinting by using parent-offspring conflict as a platform. It focuses on parental investment but on a genic level, specifically why and how maternal vs. paternal genes are expressed in the offspring. Differences in gene expression are expected when the fitness interests of the mother and father differ, such as in polygamy, where the mother is equally related to all the offspring, but the father is only related his offspring and not those sired by other males.

Since the 1990’s, David Haig, an evolutionary biologist at Harvard, and his colleagues have elegantly developed and shown that the tug-of-war between the mother and father over parental investment in the offspring does occur! One of the coolest examples comes from mice and their growth-stimulating gene (insulin-like growth factor 2; Igf2) and its corresponding growth-inhibiting gene (receptor of Igf2; Igf2r). In the fetus, only the paternal copy of Igf2 and maternal copy of Igf2r are active, and knocking out either gene results in drastic weight changes at birth (paternal Igf2: -40%; maternal Igf2r: +125%)! This sex-biased gene expression of Igf2Igf2r tells a story where a father is seeking additional resources for his offspring, but the mother plays D to ensure equal distribution of resources for all her current (and future) offspring.

Recently, we have started to take a more applicable approach to imprinting by exploring the link between genetic conflicts and human diseases. Given the intimate contact between the mother and her (unborn) child in the placenta, it is no surprise the kinship theory has been associated with complications in pregnancy (which, by the way, was an idea first proposed by Haig). Recent work has revealed a potential link between imprinted genes and pre-clampsia, a common condition where pregnant women suffer from dangerously high blood pressure. Aside from pregnancy, genomic imprinting has been linked to a number of behavioural, cognitive, and/or developmental problems after birth. More recently, there has even been a shift towards studying genetic conflicts in the brain. Led by Bernard Crespi, an evolutionary biologist at Simon Fraser University, we are starting to explore the role of genomic imprinting on mental disorders, including autism, schizophrenia, bipolar disorder, and depression.

Since the days of Hamilton, we have made great strides in our evolutionary understanding of how genes, albeit selfish ones, can cause rifts between the mother vs. offspring and mother vs. father. Studies of such genetic conflicts are not just fascinating and cool, but there are now hints of its practical applications. With potentially a few hundred imprinted genes in human, perhaps it is time to think of the bond between parents and their offspring more like a battle, tug-of-war, and conflict, like we do in nature.