I probably would have never said this a few years ago, but penises are absolutely fascinating. The phalluses of terrestrial vertebrates exhibit an incredible diversity of shapes and sizes with some possessing elaborate coils, barbs, bony spines, and multiple lobes. Many of us learn about the rapid evolution of sexual characters in our undergraduate classrooms, but until recently I, for one, did not fully appreciate the striking diversity of this organ until immersing myself in the subject area.
Many biologists study the penis under the umbrellas of different research disciplines, but relatively little work has been performed to explain its anatomical diversity. For example, how many times has a penis/phallus evolved among terrestrial vertebrates? This may seem like a trivial question, but the diversity in form, function, and physiology in the adult phallus actually makes this question difficult to address. Historically there has been much conjecture, but little data to support whether the mammalian penis, squamate hemipenes, and phallus of turtles, crocodilians, and basal birds share a single evolutionary origin or are independently derived. But where comparative anatomy has struggled, comparative developmental biology has recently forged ahead. Within the last several months two independent groups have published a total of seven new research articles that help us resolve the question of phallus homology.
I previously wrote about a series of five papers published from the Cohn lab (University of Florida) describing the embryology and gene expression patterns for the developing phallus. Since then this group has published a sixth paper synthesizing this wealth of information, using it to lay out a number of outstanding questions regarding phallus development and evolution. More recently, the Tabin lab (Harvard University) published a paper comparing the cellular-level origins of the genitalia in the laboratory mouse, green anole, house snake, chick, and python. I have had the distinct pleasure of working with both groups as their “anole guy.” Although these studies vary widely in their experimental and comparative breadth, together they have shed much needed light on the evolution of vertebrate genitalia. Here my goal is to discuss how this new wave of research changes what we now know, what we don’t know, and what we think we know regarding the evolution of external genitalia among vertebrates. Take a look at the original research papers for details of the developmental analyses, which represent many technical steps forward in our use of anoles as a laboratory model system and intellectual advancements in our understanding of genital development.
During the gradual transition of life onto land, vertebrates evolved the amniotic egg to facilitate their departure from moist environments. Because the shell and extra-embryonic membranes are laid down within the female, this transition must also be associated with the origin of internal fertilization. Copulation with a phallus is likely the most efficient way to pass sperm from the male into the female, but it is certainly not only way. The tuatara and nearly all birds (>90%) lack a phallus and successfully mate through cloacal apposition. Ultimately, when searching for the details of genital evolution, it is important to not conflate the origin of internal fertilization at the base of amniotes with the origin of the phallus as the former cannot be unequivocally used as evidence for the later. Furthermore, many amphibians also rely on internal fertilization, but to my knowledge there is no evidence that support the idea that the common ancestor of all tetrapods (amphibians plus amniotes) used internal fertilization or that the caecilian phallus, the phallodeum, is related to the phalluses of amniotes.
Despite the variation in adult anatomy, the phallus of all amniotes transition through a similar series of early embryonic events. Through the detailed embryonic descriptions of the Cohn lab we have learned that the phallus of all amniotes develop from a series of paired swellings flanking the cloaca. In lineages with a single, midline phallus these swellings expand and fuse into the genital tubercle, which includes all three embryonic germ layers (ectoderm, mesoderm, and endoderm) and express many of the same molecules. In squamates these swellings expand but do not fuse at the midline, leading to the formation of the paired hemipenes on the lateral margins of the cloaca. Due to the lack of fusion hemipenes only possess ectoderm and mesoderm, but lack endoderm and the associated molecules. Taken together the similarities in this well choreographed embryological play with multiple swellings acting as the characters suggests that the amniote phallus evolved only once, but became secondarily modified in squamates to form the hemipenes. Even birds that lack a phallus as adults undergo a similar series of embryonic events before the genital tubercle regresses due to localized cell death. The phallus-less tuatara remains the curious enigma to this evolutionary scenario. Whether this lineage lost its external genitalia or retains an ancestral mode of reproduction remains unknown.
Unfortunately, these embryological similarities do not tell the whole story. Tschopp et al. observed that the cells that contribute to the phallus come from different embryonic regions, either from the proximal hindlimbs or tail bud, in different amniote lineages. At first glance, this might undermine the notion of developmental homology, but it can easily be explained by focusing on the cloaca. The cloaca expresses a diffusible molecule (Sonic hedgehog) that recruits the cells around it to form the paired genital swellings, essentially acting like a control center that says, “Form a phallus here!” However, vertebrates have changed their body proportions many times during evolution and the presence of an internally stored (squamates, turtles, and archosaurs) and externally visible (mammals) phallus has also been modified in different lineages. It is possible that these modifications have changed the positional relationship between the cloaca, hindlimbs, and tail bud during evolution, which, in turn, may have led to consequent changes in where the phallus’s embryonic cells are recruited. Thus, this observation does not necessarily undermine the likelihood that the phallus evolved only once at the origin of amniotes. It simply allows room for the evolution of development beneath homologous embryological structures.
Following publication of the Tschopp et al. article, the popular media had a field day with the notion that phallus evolved from limbs. It’s a catchy idea, right? It feeds right into the types of jokes adolescent boys tell on the playground (if you don’t get the reference, go ask a teenage boy). Unfortunately, with such limited sampling there just aren’t enough data to know yet if this is true. Perhaps the cloaca shifted anteriorly during the evolution of squamates? Conversely, perhaps the cloaca shifted posteriorly during the evolution of mammals? Caution should be taken drawing too many evolutionary conclusions from these initial observations until broader sampling of lizards, crocodilians, turtles, and basal mammals are conducted.
These studies take a very macroscopic approach to the study of genital diversity, yet set the stage for more refined studies of genital diversity among more closely related species. Understanding genital evolution at a finer level has the potential bring new light to our understanding of the mechanisms that lead to reproductive isolation of incipient species. When paired with the study of females, more detailed analyses of genital diversity will also add to our understanding of genetic antagonism between the sexes, or how the sexes evolve in concert because of their shared genetic architecture. With this broad foundation layed out by the Cohn and Tabin labs any number of research avenues have now become more accessible. I hope you agree with my earlier statement, penises are truly fascinating little organs and well worthy of further study.