Frank Ryan, the author of The Mystery of Metamorphosis, responds to Josh Trapani’s review of his book, which appeared online in the Independent on May 30. Josh Trapani’s response to Frank Ryan will appear on Thursday.

Thank you for the opportunity to respond to Josh Trapani’s review of my book which paints a somewhat blinkered vision. Can I begin by explaining that the mystery of metamorphosis, and not any exclusive focus on Donald Williamson and his theory, is my thesis, in a book that is aimed at an intelligent lay reader? Metamorphosis is truly fascinating and has baffled the greatest minds since Aristotle. Even today, though classical Darwinian Theory has made important advances towards understanding the most wonderful, and often quite startling examples in insects and marine forms, such as the spiny-skinned echinoderms, metamorphosis remains refractory to any universally accepted explanation. This is all the more intriguing since it seems likely that the solution could help explain an even greater mystery: the seemingly explosive origins of the animal kingdom during the so-called “Cambrian Explosion”.

Bizarre and challenging as the mystery undoubtedly is, we can only look to the known mechanisms of evolutionary biology to solve it. And if mutation, in the classical sense of errors in copying DNA responsible for protein translation, has only offered a partial explanation, it is surely reasonable to broaden the search to include the modern additions of mutation affecting developmental pathways and hybridogenesis – evolution through sexual crossing between different life forms. Thus, when, in the first of four parts of my book, I focus on Williamson, and his espousal of the potential role of hybridisation in the evolution of metamorphosis, I am following this inherent logic. But Josh Trapani ignores the fact that I devote the whole of the second part of the book to the hormonal discoveries of entomologists Vincent B. Wigglesworth and Carroll Williams, whose pioneering contribution to insect metamorphosis has never (to my knowledge) been introduced to an intelligent lay reader before. It is true that I look further into hybridisation theory in the third part, including Williamson’s collaboration with Sebastian Holmes, as well as the criticism of Williamson by Michael W. Hart and Richard Strathmann, meanwhile also looking at the splendid work of Rudolph A. Raff, at the University of Indiana, and the ongoing work of some Japanese biologists.

In the fourth and final part, I discuss the work of the modern American entomologists, James Truman and Lynn Riddiford, who have extrapolated the early work of Williamson and Williams to evolutionary development in insects, leading to a modern theory for the origin of the larva and pupa of moths and butterflies. For me, one of the most exciting lines of extrapolation, again not mentioned by Trapani, is the amazingly subtle continuity of discoveries that link the insect hormone, ecdysone, to thyroxine in vertebrates, providing a delightful trajectory that begins with the humble tadpole larva of the sea squirt, leading us through the metamorphoses of fish and reptiles, to the cusp-of-the wave exploration of the development of the human brain.

While acknowledging that Williamson’s theory has provoked controversy, and even outright rejection, among many biologists, I feel obliged to adopt a deliberately neutral perspective, which includes a detailed discussion of how and why Williamson arrived at such a seemingly contrary theory. A scientist should be judged on his work, and so I have devoted some time to the lengthy series of hybridization experiments he conducted in an attempt to confirm his ideas. Two experiments are critical to any such evaluation. In 1990, while working alone, Williamson fertilised eggs of the sea squirt, Ascidia mentula with sperm from the sea urchin, Echinus esculentus. (I can provide colour pics of the two animals if you want them). And further, in 2002/ 2003, now working in collaboration with British marine biologist Sebastian Holmes and biology student Nic Boerboom from the Netherlands Institute at Texel, Williamson crossed the eggs of the urchin, Psammechinus miliaris with the sperm of the sea squirt, Echinus esculentus.

Both experiments resulted in large numbers of putative hybrid offspring, the majority of eggs developing into the easel-shaped “pluteus” larvae typical of sea urchins. But these larvae did not develop as one would expect of plutei. The majority went on to retract their arms and to metamorphose into a much simpler, rounded form, which Williamson called a “spheroid”. The spheroids from the 1990 experiment had long since died off before Hart approached Williamson to offer genetic testing of the putative hybrid offspring, and so they could not be genetically evaluated. In the 2002/2003 experiment, the large crop of resulting spheroids reproduced asexually through budding – I have included microscopic photographs of this below.

It is important to grasp that these asexually reproducing forms represent a hitherto unknown organismic form. Asexually reproducing spheroids do not feature in either parental species – though asexual reproduction is known to occur in certain other species of the paternally related sea squirts. If genetically confirmed as true cross-phyletic hybrid offspring, these spheroids, and Williamson’s experiments, would be of general biological interest. Unfortunately to date, no such genetic testing has been performed. In my opinion, what is now needed is a repeat of the cross-phyletic hybridization experiments in a lab with high quality methodology and the potential for scrupulous genetic and molecular dissection. An excellent example of such methodology has been set by Rudolph Raff and his colleagues at Indiana University.

I do possess a pdf of a detailed poster presentation by Seb Holmes based on the results of the 2002/2003 experiment.

Figure 1: “hybrid” pluteus.
Figure 2: retraction of arms of pluteus
Figures 3 & 4: pluteus metamorphoses to “spheroid”
Figure 5: spheroid reproduces by budding.

Dr. Frank Ryan is a physician with an interest in evolutionary biology, based in Sheffield, UK. His website is at www.fprbooks.com

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