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.