We are bilaterians: We have a front, back, left, right, up, down … you get the point. Our body plans contrast those of animals that are radially symmetric, such as cnidarians (jellyfish, Hydra). Yet bilateral symmetry is often superficial, as many of our organs are not bilateral - our single heart is biased to the left; the right lung has three lobes while the left has only two (to accommodate the heart); most of the liver resides on the right whereas the pancreas takes the left.… Our reproductive system, though, is as symmetric as a card-carrying bilaterian would expect. We have two symmetric gonads, and all the plumbing associated with the gonads is symmetric along to the midline. While these traits are true for most bilaterians, some have instead taken this to the extreme. Most adult birds have only one ovary and one oviduct, and these structures do not occupy the midline. The solo ovary resides on the left side of the bird, connected to a substantial oviduct that routes eggs to the midline cloaca - yeah, most birds have a cloaca, a single outlet for everything: urine, feces, and eggs. Fortunately (in my mind at least), the oviduct protrudes out of the cloaca during egg laying so the egg is actually not mixed with poop (Big Relief!). The average domesticated hen produces more than 300 eggs each year - one almost every 24 hours, from just one ovary. The vast majority of time in between layings is spent on construction of the shell - one of the most amazing I packaging devises ever, albeit a fragile one. Two ovaries actually begin development in a female chicken following gastrulation. The left ovary continues development, acquiring an expanded cortex containing large numbers of young oocytes surrounded by granulosa and steroidogenic cells, and a central medullary region containing largely stromal tissue (de Melo Bernardo et al., 2015), whereas the right gonad remains rudimentary. Both gonadal primordia do, however, acquire primordial germ cells that begin an asymmetric wave of meiosis. Sadly, the minimal cortical region in the right gonad does not support oocyte growth. Yet the right gonad appears to remain partly functional through adulthood, as evidenced by the expansion of the right ovary into a testis-like organ with steroidogenic activity following an ovariectomy of the left gonad (Groenendijk-Huijbers, 1965). Weird. In contrast, adult males possess two functional, similarly sized and symmetric testes. During development, though, the left testis is often somewhat larger than the right - but not to the extreme difference observed in the female. Clearly, the sex-determination mechanism initially patterns a slight asymmetry that is super-sized as female development progresses. Recall that birds use a ZW sex chromosome arrangement, where the female is heterogametic (ZW) and the males are homogametic (ZZ). In other words, the egg determines the progeny's sex since sperm possess only the Z chromosome. Perhaps the W carries the gene(s) responsible for ovarian asymmetry - keeping the secret within the female line.… But why - why do most birds have only one ovary? One argument has been baggage: stay light for flight, as one gonad weighs less that two. Indeed, flightless birds (e.g. individuals of the order Apterygiformes, or kiwis) have two functional ovaries. (So, maybe if kiwis could simply jettison an ovary, they could fly?) A deeper analysis of this argument though does not hold its own weight, given that the single ovary is of comparable size to both testes -even during the breeding season, when the testes increase considerably in mass. Others argue that the fragility of the eggshell is actually the key. The stability and durability of the shell is essential for the embryo to survive during incubation, considering that a hen is usually sitting on the eggs to keep them warm. If a hen had two oviducts, producing an egg in each side, would the final product be as robust - particularly if the eggs would be clanging against each other during flight? And why does the singular gonad stay on the left? Answering “why” questions are always the hardest because they flirt with the philosophical. Nevertheless, determining why the single avian ovary evolved may reveal much about the evolution of bilaterians, of the role of reproductive organs in this process, and their mechanisms of asymmetry. Gary M. Wessel
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Aug 1, 2024
Tao Wang + 7
Open Access