An interesting theory in need of some additional research. The examples were fascinating ( )

I read this book because I got hooked by caterpillars over the summer, googled for information about metamorphosis into butterflies, and ... OMG, the process is incredible! This book is about the scientific investigation of metamorphosis, focused on two men: Vincent Wigglesworth in the 1930s and Don Williamson in the 1980s. It is neither a comprehensive history nor a comprehensive account of current knowledge, but it combines elements of both into a fascinating narrative story. (Note: I have no educational background in biology, I am merely a casually interested layperson, so please assume that any errors in this summary are my misinterpretation, not the author’s.)

Vincent Wigglesworth, an entomologist whose career began with investigating relationships between the physiology of insects and the spread of tropical diseases, wondered about the evolution from incomplete to complete metamorphosis. The fossil record shows that insect metamorphosis did not evolve from marine arthropod metamorphosis. The protective exoskeleton of insects is composed of chitin, which is derived from mucoproteins (such as the slime of slugs), and strengthened by the protein sclerotin. Growth occurs by molting. The more primitive insects change size but not form, and do not have wings. Among the most ancient insects with wings, dragonflies undergo incomplete metamorphosis, with buds on the outer surface of nymphs developing into wings on adults, and no larval or pupal stage. In the complete metamorphosis of beetles, bees, butterflies, etc, the wings develop internally. Complete metamorphosis had been associated with imaginal disks, discovered in mid 1700s and investigated in the mid 1800s, determined to be active in the pupal stage when the larva is liquified and reconstructed into an adult. Wigglesworth was aware of variations and exceptions, and not convinced of the unique role of imaginal disks. What if the key mechanisms of metamorphosis were an extension of molting? What if the pupa had evolved to fill the gap between increasingly divergent stages of larva and adult? His experimental subject was the kissing bug (Rhodnius prolixus), which undergoes incomplete metamorphosis in five nymphal stages, with the most significant changes occurring throughout the body in the final molt. Sudden universal changes implied a signal, and the most likely candidate was a hormone. In a series of gruesome experiments involving beheadings and surgical conjoinings, he determined that the corpus allatum gland produced an inhibiting hormone in the first four stages (this is now known as the juvenile hormone, and is found across insect families); the molting hormone came from... where? This question was answered by Carroll Williams, who was studying diapause in silkworm moths (Hyalophora cecropia). As soon as the caterpillar spins a cocoon, cell activity ceases. When spring arrives, metamorphosis begins. Another series of gruesome experiments, with chilled brains transplanted to parts of severed bodies, established three components to metamorphosis: a hormone secreted by the prothoractic gland, a master hormone secreted by the brain, the absence of a hormone secreted by the corpus allatum gland. The molting process is now known to be this: neurosecretory cells in the brain send signals along nerves to the neurohaemal organs / corpora cardiaca (located near the corpora allata), which release prothoracicotropic hormone, which is carried by the bloodstream to the prothoracic gland, and stimulates production of the ecdysone, which triggers molting. This is the simplified version. I’m including scientific jargon because I want search terms. The jargon is peripheral and not a concern; I ignored it while I was reading. What comes through are people keenly curious about nature.

Don Williamson, a marine biologist who specialized in plankton, was intrigued by anomalies. In classic evolutionary theory, each developmental stage of the life cycle (egg, embryo, larva, adult) is modified by natural selection. A starfish, for example, swims as a larva at the ocean surface for wide dispersion, but settles as an adult on the ocean floor for predation. The developmental stages, though, do not always flow nicely from one to another; for example, the symmetry of an animal may change from bilateral to radial. He constructed and compared evolutionary trees based on larval forms and adult forms, and they did not precisely coincide; larval forms of one lineage sometimes appeared in another, as if somehow transferred. Nonsense, he thought for years, and then he decided to take the idea seriously, as a hypothesis of “larval transfer”. This though was not an explanation; he needed a mechanism and experimental evidence. The only plausible mechanism was hybridization, producing offspring by mating different species. He chose animals even more distantly related, from different phyla: crustaceans and echinoderms, eggs from shrimp (Gammarus dubeni), which are bilaterally symmetrical, and sperm from sea urchins (Echinus esculentus), which are radially symmetrical. He could see development through a microscope, but most of the hybrid eggs did not survive beyond a few days, and none hatched. Recognizing difficulties, he reconsidered, and focused on sea squirts (Ascidia mentula), in the phylum of urochordates. Sea squirts are bilaterally symmetrical at all stages, but the larva does not gradually transform into the adult; instead, the adult grows inside a tadpole larva, with two nervous systems and two brains coexisting until one is destroyed. Sea urchins develop similarly; the adult grows inside a pluteus larva. Williamson fertilized eggs from sea squirts with sperm from sea urchins. Some larvae developed to the radially symmetrical pluteus stage, remarkable because sea squirt eggs would normally develop into bilaterally symmetrical tadpoles, but no further. In 1985, he wrote a paper proposing that anomalies in the evolutionary tree might be explained by the transfer of genetic material from one branch to another, and submitted it to several journals, but it was too much at odds with standard theory for publication. In 1987, it was accepted to a journal on oceanography, whose editor had also been puzzled by larval forms and the assignment of taxonomic relationships. In 1988, it found its way to Lynn Margulis, whose theory of endosymbiosis had initially been rejected by established forces, and whose sympathies as a consequence were with iconoclasts. Doors opened. Over the years, a smattering of biologists have been interested enough to give the idea a public airing and considered response, and to attempt repeat experiments. The theory of larval transfer is quite possibly (probably?) wrong. And yet, much about metamorphosis remains unexplained. The story is compellingly told, with the twists and turns of a struggle for understanding, and the appropriate caution.

(read 30 Sep 2012)

I got a copy of this book to review through the Amazon Vine program. It ended up being an interesting, and at times boring, read. If you are really interested in evolution theories that compliment or go against Darwin’s theory of evolution this might be a good book to read.

The first part of this book goes into great detail about the experiments of a guy named Williamson. Williamson has a theory that marine species did not necessarily evolve but in some cases cross-hybridized to form new species.

The second part of the book is about moths and insects and the metamorphosis they go through. This portion basically discuses the finding that insects have hormones which drive their molting and metamorphosis. The main scientist featured here is Wigglesworth.

The third part of the book details the genetic studies that were done to prove or disprove Williamson’s theory of hybridization in evolution. The results were some what inconclusive because of the limited samples that were studied.

The last part discusses some about human metamorphosis.

It is mid-level reading as far as technicality goes; I am a chemist and understood most of what the author was talking about. It does help to have some basic theory of biology and the theory of evolution in understanding the experiments and theories discussed. This is definitely not an easy read, but also not one that you need a PhD in Biology to understand.

It is a somewhat interesting book, but a bit wandering. It leaves one theory and then goes to another, the author does attempt to bring the theories together at the end. This book might be of some interest to those interested in the theory of evolution. As a non-biology major I could follow much of the book, but got a bit bored when things got too in depth.

The examples focus mainly on marine and insect life. There are some fascinating examples in here and some neat facts. The theory of of cross-hybridization and hormones as drivers of metamorphosis is kind of repeated over and over.

It seemed to me that the book is basically lauding Williamson’s work and how it is finally showing some merit as other scientists do more DNA mapping and genetic studies.

Overall an okay read for the layman. If you have an interest in the theory of evolution I would recommend this. It does a decent job of summarizing the work of those who are trying to add to or elaborate on Darwin’s theory of evolution. There are some interesting facts in here and the background behind the scientists doing the work is included. The book reads more like a non-fiction read than a text book so that is nice, but it is definitely not a quick or easy read. ( )