Darwin and the vermiform appendix

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Last night, I asked for a copy of an article (I have plenty now, thanks!) that was getting a lot of press. The reason I was looking for it is two-fold: the PR looked awful, expressing some annoying cliches about evolution, but the data looked interesting, good stuff that I was glad to see done. Awful and interesting — I’m a sucker for those jarring combinations. My favorite pizza is jalapeno and pineapple, too.

I’m going to split my discussion of this article in two, just to simplify dealing with it. This is the awful part. I’ll do the interesting part a little later.

The paper is about the appendix, that tiny little organ in your gut that doesn’t have a whole lot of obvious function. The point of the work is to try and show that yes, it does something — which is fine and interesting, although I will quibble a bit with their interpretation. Where they go awry, though, is in trying to pick a fight with a dead man, and making that the focus of their public relations.

Now, some of those same researchers are back, reporting on the first-ever study of the appendix through the ages. Writing in the Journal of Evolutionary Biology, Duke scientists and collaborators from the University of Arizona and Arizona State University conclude that Charles Darwin was wrong: The appendix is a whole lot more than an evolutionary remnant. Not only does it appear in nature much more frequently than previously acknowledged, but it has been around much longer than anyone had suspected.

“Maybe it’s time to correct the textbooks,” says William Parker, Ph.D., assistant professor of surgical sciences at Duke and the senior author of the study. “Many biology texts today still refer to the appendix as a ‘vestigial organ.'”

Charles Darwin is dead. Your research can’t be very cogent if your approach to drum up interest is to dig up a 120-year-old corpse and kick it around; is there anyone alive who disagrees with you who can put up a more informative and entertaining struggle? What this does is pick this one fellow as a symbol of the whole edifice of evolutionary theory, which has the advantage of making one’s work seem very, very important (even if one is stacking the deck to do it), but has the disadvantage of giving every creationist on the planet something to masturbate over, and they’re icky enough without your help.

It’s also annoying. Charles Darwin was wrong about many things — I’ll even give an example at the end of this article — and it’s part of the nature of science that everyone’s work will be revised and refined over time, and some of us will even be shown to be completely wrong. It’s rather unseemly to collect a lot of data that Darwin did not have, run it through PAUP 4.0 on a fast computer, map the data onto a molecular consensus phylogeny, and cackle gleefully over discovering something Darwin did not know. Really, it doesn’t make you a better scientist than Darwin.

To make it even worse, people who do this can’t even make the corpse-fight a fair fight — they have to stuff the pathetic dead body with straw. In this case, they’re padding Darwin’s investment in the appendix a fair amount. They cite one work by Darwin, The Descent of Man, which mentions this issue. He wrote one whole paragraph on the topic, and here it is, in its entirety; it was presented briefly as part of a long list of human rudimentary structures, such as wisdom teeth, muscles of the ear, and the semilunar fold of the eye.

With respect to the alimentary canal, I have met with an account of only a
single rudiment, namely the vermiform appendage of the caecum. The caecum
is a branch or diverticulum of the intestine, ending in a cul-de-sac, and
is extremely long in many of the lower vegetable-feeding mammals. In the
marsupial koala it is actually more than thrice as long as the whole body.
(46. Owen, ‘Anatomy of Vertebrates,’ vol. iii. pp. 416, 434, 441.) It is
sometimes produced into a long gradually-tapering point, and is sometimes
constricted in parts. It appears as if, in consequence of changed diet or
habits, the caecum had become much shortened in various animals, the
vermiform appendage being left as a rudiment of the shortened part. That
this appendage is a rudiment, we may infer from its small size, and from
the evidence which Prof. Canestrini (47. ‘Annuario della Soc. d. Nat.’
Modena, 1867, p. 94.) has collected of its variability in man. It is
occasionally quite absent, or again is largely developed. The passage is
sometimes completely closed for half or two-thirds of its length, with the
terminal part consisting of a flattened solid expansion. In the orang this
appendage is long and convoluted: in man it arises from the end of the
short caecum, and is commonly from four to five inches in length, being
only about the third of an inch in diameter. Not only is it useless, but
it is sometimes the cause of death, of which fact I have lately heard two
instances: this is due to small hard bodies, such as seeds, entering the
passage, and causing inflammation. (48. M. C. Martins (“De l’Unite
Organique,” in ‘Revue des Deux Mondes,’ June 15, 1862, p. 16) and Haeckel
(‘Generelle Morphologie,’ B. ii. s. 278), have both remarked on the
singular fact of this rudiment sometimes causing death.)

Note why Darwin classed this appendage as vestigial: because it is greatly reduced compared to the homologous organs in non-human relatives, and because it currently exhibits a great range of variation, which is apparently non-functional. These are criteria which the paper in question does not refute at all. Darwin does say that the appendix is “useless”, and the paper will show some evidence that that is wrong. It’s also irrelevant.

The reason why it is irrelevant is that the presence of some function is not part of the definition of a vestigial or rudimentary organ — Darwin obligingly concedes that evolution will salvage some utility out of organs with little retention of their original function, but which are present as a consequence of contingency. He discusses this at greater length in On the Origin of Species, and here is a significant chunk of the relevant writing.

Organs or parts in this strange condition, bearing the plain stamp
of inutility, are extremely common, or even general, throughout
nature. It would be impossible to name one of the higher animals in
which some part or other is not in a rudimentary condition. In the
mammalia, for instance, the males possess rudimentary mammae; in
snakes one lobe of the lungs is rudimentary; in birds the
“bastardwing” may safely be considered as a rudimentary digit, and
in some species the whole wing is so far rudimentary that it cannot be
used for flight. What can be more curious than the presence of teeth
in foetal whales, which when grown up have not a tooth in their heads;
or the teeth, which never cut through the gums, in the upper jaws of
unborn calves?

Rudimentary organs plainly declare their origin and meaning in
various ways. There are beetles belonging to closely allied species,
or even to the same identical species, which have either full-sized
and perfect wings, or mere rudiments of membrane, which not rarely lie
under wing-covers firmly soldered together; and in these cases it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality: this occasionally occurs
with the mammae of male mammals, which have been known to become
well developed and to secrete milk. So again in the udders in the
genus Bos, there are normally four developed and two rudimentary
teats; but the latter in our domestic cows sometimes become well
developed and yield milk. In regard to plants the petals are sometimes
rudimentary, and sometimes well-developed in the individuals of the
same species. In certain plants having separated sexes Kolreuter found
that by crossing a species, in which the male flowers included a
rudiment of a pistil, with an hermaphrodite species, having of
course a well-developed pistil, the rudiment in the hybrid offspring
was much increased in size; and this clearly shows that the
rudimentary and perfect pistils are essentially alike in nature. An
animal may possess various parts in a perfect state, and yet they
may in one sense be rudimentary, for they are useless: thus the
tadpole of the common salamander or water-newt, as Mr. G. H. Lewes
remarks, “has gills, and passes its existence in the water; but the
Salamandra atra, which lives high up among the mountains, brings forth
its young full-formed. This animal never lives in the water. Yet if we
open a gravid female, we find tadpoles inside her with exquisitely
feathered gills; and when placed in water they swim about like the
tadpoles of the water-newt. Obviously this aquatic organisation has no
reference to the future life of the animal, nor has it any
adaptation to its embryonic condition; it has solely reference to
ancestral adaptations, it repeats a phase in the development of its
progenitors.”

An organ, serving for two purposes, may become rudimentary or
utterly aborted for one, even the more important purpose, and remain
perfectly efficient for the other. Thus in plants, the office of the
pistil is to allow the pollen-tubes to reach the ovules within the
ovarium. The pistil consists of a stigma supported on a style; but
in some Compositae, the male florets, which of course cannot be
fecundated, have a rudimentary pistil, for it is not crowned with a
stigma; but the style remains well developed and is clothed in the
usual manner with hairs, which serve to brush the pollen out of the
surrounding and conjoined anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a distinct one: in
certain fishes the swimbladder seems to be rudimentary for its
proper function of giving buoyancy, but has become converted into a
nascent breathing organ or lung. Many similar instances could be
given.

I’ve highlighted the part most important for this discussion. Darwin did not discuss the appendix or caecum at all in the Origin, but this description does apply. If a portion of the gut, a digestive organ, is diminished in size such that it no longer contributes to the primary function of the organ, but does retain a secondary function, such as assisting in immunity, or as the authors of the recent paper will argue, in acting as a reservoir of bacteria for recolonizing the gut, then it is still a vestigial organ. It has lost much of its ancestral function.

I do not understand why this is so hard for so many people to comprehend. Biology is plastic and opportunistic. Accidents of history will always still be incorporated into the whole of the organism; we make do, or we die. Just because something is does not mean that the entirety of its nature is the product of selection.

I mentioned that I’d point out errors in Darwin’s understanding. They’re there, but note that seeing them now 150 years after he wrote his big book does not make me smarter than Darwin, nor does it invalidate the overall picture of his theory. You can see one ‘error’ in the quote above: we are now pretty certain that the original function of the swimbladder in fish was respiratory. It evolved first as a supplement to the gills, providing access to the rich oxygen content of the atmosphere, and was secondarily adapted to function for bouyancy. Hah, silly Darwin, that he did not know a detail of paleontology and phylogeny that would be worked out a century after his death!

He also made a more substantial error. He wondered how organs became smaller over time, and his answer was, unfortunately, a bit Lamarckian and also a bit muddled.

It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more
and more complete reduction of a part, until at last it became
rudimentary,- as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced by beasts of prey to take flight, and have
ultimately lost the power of flying. Again, an organ, useful under
certain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this
case natural selection will have aided in reducing the organ, until it
was rendered harmless and rudimentary.

“Disuse” is the magic word there: if a cavefish lived in the dark and never used its eyes, the idea was that its progeny would then have smaller eyes. This is not correct, but it was a central part of Darwin’s invalid theory of heredity. This is a much more substantial failing of Darwin’s work, but again, I can’t claim credit for figuring this out; it took the work of Mendel to get the core of genetics puzzled out, and then it took a whole generation of scientists to work out how genetics and evolution fit together. We can say “DARWIN WAS WRONG!” about that, but we can’t really say that about his treatment of vestigial organs in general, which seems to hold up fairly well…perhaps because Darwin himself was not so fervently committed to the absolute adaptedness of every single feature of every single organism as some of his later followers.

That said, I’ll move along to the substance of the paper next, which really does have some good stuff in it. Most of my complaints here are with the abysmal presentation of the ideas in it by the popular press, aided and abetted by the scientists themselves. Just keep in mind that whenever these press releases that declare “Darwin was wrong” appear, it’s usually an example of grandstanding and the regrettable tendency of competitive scientists to think the way to impress people with the importance of their work is to get into a penis-measuring contest with poor dead Chuck.

Smith HF, Fisher RE, Everett ML, Thomas AD, Randal Bollinger R, Parker W (2009) Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. J Evol Biol. 2009 Aug 12. [Epub ahead of print]

Gene regulatory networks and conserved noncoding elements

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We miss something important when we just look at the genome as a string of nucleotides with scattered bits that will get translated into proteins — we miss the fact that the genome is a dynamically modified and expressed sequence, with patterns of activity in the living cell that are not readily discerned in a simple series of As, Ts, Gs, and Cs. What we can’t see very well are gene regulatory networks (GRNs), the interlinked sets of genes that are regulated in a coordinated fashion in cells and tissues.

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Oh, no, not the Aquatic Ape hypothesis!

I’m getting a lot of email asking me to talk about the aquatic ape theory, the idea that humans went through a semi-aquatic stage in their evolutionary history. It’s complete nonsense; its proponents spew out a lot of inconsistent and mutually contradictory noise to ‘support’ their claims, and there is no evidence anywhere for such a stage. I don’t need to say more, though, because Jim Moore’s Aquatic Ape page is the definitive web resource for dissecting this fringe theory.

The evolution of Hedgehog

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PLoS has recently published a highly speculative but very interesting paper on how a particular signaling pathway, the Hedgehog pathway, might have evolved. It’s at a fairly early stage in hypothesis testing, which is one of the things that makes it interesting — usually all you see published is the product of a great deal of data collection and experiment and testing, which means the scientific literature gives a somewhat skewed view of the process of science, letting the outsider mainly see work that has been hammered and polished, while hiding the rougher drafts that would better allow us to see how the story started and was built. It’s informative in particular for those who follow the creationist “literature”, which often crudely apes the products of actual working science, but lacks the sound methodological underpinnings. In particular, creationism completely misses the process of poking at the real world to develop ideas, since they begin with their conclusion.

So take this description as a work in progress — we’re seeing the dynamic of building up a good working model. As usual, it starts on a sound foundation of confirmed, known evidence, makes a reasonably hypothesis on the basis of the facts, and then proposes a series of research avenues with predicted results that would confirm the idea.

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What caused the Cambrian explosion? MicroRNA!

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No, not really — my title is a bit of a sensationalistic exploitation of the thesis of a paper by Peterson, Dietrich, and McPeek, but I can buy into their idea that microRNAs (miRNAs) may have contributed to the pattern of metazoan phylogenies we see now. It’s actually a thought-provoking concept, especially to someone who favors the evo-devo view of animal evolution. And actually, the question it answers is why we haven’t had thousands of Cambrian explosions.

In case you haven’t been keeping up, miRNAs are a hot topic in molecular genetics: they are short (21-23 nucleotides) pieces of single stranded RNA that are not translated into protein, but have their effect by binding to other strands of messenger RNA (mRNA) to which they complement, effectively down-regulating expression of that messenger. They play an important role in regulating the levels of expression of other genes.

One role for miRNAs seems to be to act as a kind of biological buffer, working to limit the range of effective message that can be operating in the cell at any one time. Some experiments that have knocked out specific miRNAs have had a very interesting effect: the range of expressed phenotypes for the targeted message gene increases. The presence or absence of miRNA doesn’t actually generate a novel phenotype, it simply fine-tunes what other genes do — and without miRNA, some genes become sloppy in their expression.

This talk of buffering expression immediately swivels a developmental biologist’s mind to another term: canalization. Canalization is a process that leads organisms to produce similar phenotypes despite variations in genotype or the environment (within limits, of course). Development is a fairly robust process that overcomes genetic variations and external events to yield a moderately consistent outcome — I can raise fish embryos at 20°C or at 30°C, and despite differences in the overall rate of growth, the resultant adult fish are indistinguishable. This is also true of populations in evolution: stasis is the norm, morphologies don’t swing too widely generation after generation, but still, we can get some rapid (geologically speaking) shifts, as if forms are switching between a couple of stable nodes of attraction.

Where the Cambrian comes into this is that it is the greatest example of a flowering of new forms, which then all began diverging down different evolutionary tracks. The curious thing isn’t their appearance — there is evidence of a diversity of forms before the Cambrian, bacteria had been flourishing for a few billion years, etc., and what happened 500 million years ago is that the forms became visible in the fossil record with the evolution of hard body parts — but that these phyla established body plans that they were then locked into, to varying degrees, right up to the modern day. What the authors are proposing is that miRNAs might be part of the explanation for why these lineages were subsequently channeled into discrete morphological pathways, each distinct from the other as chordates and arthropods and echinoderms and molluscs.

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Werner Arber: Molecular Darwinism

This talk has me a little concerned: it’s proposing something rather radical, for which Arber is going to have to show me some unambiguous evidence to convince me, and I’m coming into it with a very skeptical mindset. Here’s the relevant portion of his abstract:

The theory of molecular evolution that we also call “Molecular Darwinism” is based on the acquired knowledge on genetic variation. In genetic variation, products of evolution genes are involved as variation generators and/or as modulators of the rates of genetic variation. These evolution gene products act together with several non-genetic elements that can be assigned to intrinsic properties of matter, to environmental mutagens and to random encounter. We conclude that natural reality takes actively care of biological evolution. The evolution genes must have been fine-tuned for their functions by second-order selection, so that spontaneous genetic variation with different evolutionary qualities occurs at quite low rates. This ensures a relatively high genetic stability to individuals, as well as an evolutionary progress at the level of populations.

The presence of evolution genes points to a duality of the genome: while many genes act to the benefit of the individuals for the fulfillment of their lives, the evolution genes act to the benefit of an evolutionary development, for a slow, but steady expansion of life and biodiversity.

You see the problem, I hope. These hypothetical genes that do not necessarily directly affect the fitness of the individual are assumed to be promoted in lineages by a higher level of selection. This is not easily supported by evolutionary theory: there isn’t a mechanism given for individuals to maintain a gene that will only help its many-times-great-grandchildren. It is inferring a kind of foresight to evolution that is doesn’t have a mechanism…unless, perhaps, Arber is going to give use one. We’ll see. This talk will start in about 15 minutes, and I’ll update this post as he fills us in.

A simple history lesson: modern evolutionary biology is the convergence of work that began with Miescher (1874: nucleic acids) which led to molecular biology, Mendel (1876) which led to genetics, and Darwin (1859) that approached the problem at the level of organisms and species. The neo-Darwinian synthesis fused the genetic and Darwinian line, molecular genetics brought together genetics and biochemistry/molecular biology, and molecular evolution brings all three together—he seems to claim some kind of intellectual ownership of the last concept, which is what he calls molecular darwinism.

How do bacteria generate new variants? By transformation, conjugation, or transduction. All are mechanisms that transfer genes from an external source to the bacterium. Work in the 1940s demonstrated that DNA was the carrier of genetic information.

Arber gave a little summary of E. coli gene structure, which I suppose would be helpful to all the chemists here. He defines mutation as an alteration of the nucleotide sequence; in classical genetics, it’s defined differently, as an altered phenotype that is transmitted to progeny.

Mutations are rarely favorable; often unfavorable, and very often silent or neutral. There is no good evidence for directedness of spontaneous mutations. Mutations do not appear in response to a need.

He argues that there are three elements to evolution: evolution is driven by genetic variation (mutation), directed by natural selection, and modulated by isolation as a mechanism for speciation. There are multiple mechanisms generating genetic variation: spontaneous DNA sequence alteration, DNA rearrangement or recombination, and DNA acquisition (horizontal gene transfer).

So far, this is all very unchallenging and basic, at least for someone with any background in genetics and cell biology. After sitting through one talk that completely lost me with a failure to explain the basic terms of the work, I can’t complain, but I confess, I’m having trouble staying alert through all this.

Some genes can affect the rate of occurence of mutations — these are modulators of the frequency of genetic variation. He calls these evolution genes. He says neatral reality actively takes care of biological evolution, and that this is an expansion of the biological theory of evolution. This leads to an expansion of biological diversity, and, he argues, higher complexity.

I’m not very impressed. This is a combination of the commonplace and some odd interpretations. Of course there is variation in fidelity of replication that is influenced by genetic variation. Some of it is simply thermodynamically necessary: perfect fidelity is impossible to achieve, and greater fidelity has a metabolic cost, so some of that variation is utterly unsurprising. Some is; when we have organisms that have specializations to directly generate greater genetic variation — and sex is the first to come to my mind — we have a problem to explain. I don’t see that Arber has proposed anything to explain the real problems.

At the same time, what Arber said here does not make him a friend to intelligent design creationism, or creationism of any kind, despite the claims of some unreliable creationist sources, a claim that Arber has directly rejected.

I’d have to say it was a nice enough overview, but didn’t really propose anything novel, and definitely didn’t demonstrate anything that can’t be explained in the context of modern evolutionary theory.

Visiting village dogs

I am horribly envious. I am speaking of the Village Dog Project, some current research going on that looks very cool.

Understanding the evolution and domestication in dogs requires genetic analysis of a global and diverse panel of non-breed-affiliated village dogs. With a network of worldwide and Cornell-affiliated collaborators, we plan to gather dog samples from remote villages, establish a genetic archive containing DNA and phenotypic information from these dogs, carry out genetic analyses on these samples, and develop computational methods for analyzing this dataset. In particular, we are interested in understanding the location, timing, and demographic conditions underlying domestication; the genetic changes involved in the transition of wolf to dog; the relationship between these village dogs and the breed dogs; and the effect that historical forces have shaped village dog diversity.

That looks informative and useful, and I’ll be looking forward to the publication of the research. That’s not what’s got me envious, though: for that, you have to look at their field work. The researchers are spending the summer traveling to exotic, remote locations (admittedly, to the kinds of places rife with scavenging village dogs, but still…) to collect blood samples. They have a travel blog that will be recounting their adventures, and also explains the science a little more.

After initial domestication, dogs probably lived “breed-less” lives as human commensals (hanging around humans, not really helping or harming them but living off their trash) for many thousands of years. During this time, dog populations quickly expanded and spread across the globe. In the last few hundreds of years, several hundred dog breeds were formed from local dogs in many parts of the world; these breed dogs have entirely replaced the non-breed “indigenous” dogs in some parts of the world, notably in Western Europe and the USA. However, most dogs throughout the world still live their lives as non-breed, indigenous, commensal dogs. We refer to these dogs as “pariah” or “village” dogs. They tend to be smallish (25-40 pounds), often tan, short-haired dogs, though the type varies a bit according to the region you’re in. The important point is that these dogs have not undergone the intense genetic bottleneck associated with breed formation. Thus, while breed dogs have only a small subset of the total genetic diversity of all dogs, it is likely that village dogs have a much greater range of the total diversity. Thus, they are very useful for looking at the original domestication event. They are informative of the original genetic bottleneck that led to the formation of domestic dogs many thousands of years ago.

Hmmm. We don’t seem to have many dogs running loose around exotic, remote Morris, Minnesota, but there are a few feral cats living off the dumpsters near the grocery store.

I probably wouldn’t try to read about visiting small midwestern towns to collect cats, though.

Big love among the ostracods

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How can anyone resist an article titled “Sexual Intercourse Involving Giant Sperm in Cretaceous Ostracode”? You can’t, I tell you. It’s like a giant brain magnet, you open the journal to the index, and there’s that title, and you must read it before you can even consider continuing on to anything else.

Some organisms have evolved immensely long sperm tails — Drosophila bifurca, for instance, has sperm cells that are about 60mm long, or 20 times longer than the length of the entire adult body. The excessively long sperm tail is obviously not a structure that has evolved for better swimming; instead, it is thought to act as a tangled barrier in the female reproductive tract to prevent other males from fertilizing the female, and there is also some very interesting evidence that sperm coevolves with the female reproductive tract, so some sexual selection at the level of the gametes is going on.

At the same time, sperm morphology is extremely diverse, and seems to evolve very rapidly. Perhaps these mega-sperm are a transient fad? Not all species of Drosophila exhibit the phenomenon, and those that do vary considerably from species to species. What we’d like to know is if there are any lineages that maintain these patterns of giant sperm over long periods of evolutionary time…so what do we need to do? We need to go spelunking for sperm in fossils!

That’s what this short letter in Science is about: the authors looked at ostracodes, a class of tiny crustacea that invests heavily in reproduction. About a third of their volume is their reproductive system, with males building giant (relative to their size) sperm pumps, and females having large seminal receptacles for sperm storage. The individual sperm are also large, often longer than the body length of the adult, and are also aflagellate — no flagellar tail at all, just a long, threadlike cell body. You can tell if a female ostracod is a virgin just by looking at those seminal receptacles, since they inflate hugely with all the giant sperm tucked inside.

So, if you look at the large orange blobs, the seminal receptacles, in this 3-D scan of a fossil female ostracod (bottom right of this image), you can tell that she was inseminated before she died, and that her mate had very large sperm. Her condition was also very similar to that of modern ostracodes (bottom left).

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(Click for larger image)
Partial reconstruction of E. virens (extant) and H. micropapillosa (fossil). Anterior is to the left. Orange structures indicate central tubes of Zenker organs in males or seminal receptacles in females; brown, esophagus; turquoise, mandible; purple, upper lip; pink, lower lip; green, valves; and gray scales, whole-body reconstruction. All scale bars indicate 100 µm. (A) Lateral view of male E. virens with several organs included for comparison. (B) Male H. micropapillosa in lateral view with several organs in context of whole-body reconstruction. (C and D) Ventral views of several organs including tubes of Zenker organs of male H. micropapillosa. (E) Lateral view of female E. virens with several organs included for comparison. (F) Female H. micropapillosa in lateral view with several organs in context of whole-body reconstruction, including seminal receptacles.

So, the conclusion is that boinking with giant sperm is an enduring property of at least some lineages: they’ve been going at it for a hundred million years. The authors also suggest that this kind of technique could be useful for measuring sexual selection by assessing pre-mating parental investment in fossil invertebrates.

Matzke-Karasz R, Smith RJ, Symonova R, Miller CG, Tafforeau P (2009) Sexual Intercourse Involving Giant Sperm in Cretaceous Ostracode. Science 324(5934):1535.

Miller GT, Pitnick S (2002) Sperm-Female Coevolution in Drosophila. Science 298(5596):1230-1233.