Carl Zimmer reviews the new Darwin exhibit at the AMNH. He has a few complaints, but otherwise it sounds wonderful.
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
Carl Zimmer reviews the new Darwin exhibit at the AMNH. He has a few complaints, but otherwise it sounds wonderful.
I saw on Muton, and several readers have mentioned it to me, this article about the world’s smallest vertebrate, fish of the genus Paedocypris. It’s a gorgeous translucent cyprinid, so is somewhat related to my favorite fish, Danio rerio. They live in cool, slow moving water in peat swamp forests of Southeast Asia. One female, only 7.9mm long, contained about 50 eggs, so they know it was sexually mature.
That size isn’t at all shocking—my zebrafish larvae at about that size are active hunters with functioning visual systems, capable of coordinated bouts of swimming, and they’re also very impressive animals…but they don’t have sex. It takes about 6 months for zebrafish to reach sexual maturity, and they are several centimeters long at that point. I would love to know how old these fertile Paedocypris were, but they were captured in the wild and virtually nothing is reported about their behavior or lifecycle. Ah, to have a fish colony that could be raised in a set of beakers, and could produce a couple of generations of crosses in a single semester…
One other clue that these are fully functioning, sexually mature adults are the presence of some pelvic specializations. Males have a hook and flange widget on their pelvic fins, and an odd prepelvic knob. Again, though, without knowing anything about their behavior, we don’t know how these are used in mating and courtship. Wouldn’t it be cool to put a pair under my Wild M3 scope and watch courtship and mating?
Of course, in addition to not knowing their generation time yet, these fish have another drawback relative to zebrafish: tiny eggs. They extracted a range of sizes from the ovaries, but assuming the smallest are immature, they max out at around 0.3mm diameter. That’s respectable, but Danio eggs are about 1mm in diameter.
Can you tell I’d love to get my hands on a bunch of these little fish? Unfortunately, I’ve heard from fish importers that it is agonizingly expensive and time consuming to bring wild tropical fish into the country, and for good reason: to block invasive species, to prevent the spread of new fish diseases, and also to discourage the plundering of native populations. I may not ever see one of these animals, short of making a trip to Malaysia, and even then I won’t be bringing any home.
Cory Doctorow visited the Nature offices, and gave what sounds like a very thought-provoking talk. It’s all about the past and future of intellectual property, the net as a tool for creating relationships between authors and readers, and some odd legal maneuverings on the horizon.
Majikthise reports that an Australian couple has found a $295,000 lump of ambergris on a beach. Ambergris is cool stuff, so let me add to it’s splendor by bringing up two scientific views of it.
First, let’s hear from the chemists:
Since ancient times, ambergris has been one of the most highly valued perfumery materials. It is secreted in the stomach or intestinal tract of the sperm whale and released into the sea in the form of a grey to black stone-like mass. When exposed to sunlight, air and sea water, the material gradually fades to a light grey or creamy yellow colour and, at the same time, the main component, the odourless triterpene alcohol ambrein, is oxidatively degraded. Some of the products resulting from this chemical process are responsible for the organoleptic properties of ambergris.
We know the chemical structure of many of the active components of ambergris—it’s a beautifully complex collection of compounds.
And what about us biologists?
The effect of ambrein, a major constituent of ambergris, was studied on the sexual behavior of male rats. The rats were administered ambrein in doses of 100 and 300 mg/kg body weight. Male sexual activities were assessed by recording the erectile responses (penile erection) and homosexual mountings in the absence of female. The copulatory studies were carried out by caging males with receptive females brought into estrus with subcutaneous injections of estradiol benzoate and progesterone. The copulatory pattern of treated male rats (mountings, intromissions, ejaculations and refractory period), the pendiculations (yawns/stretches) and orientation activities towards females, the environment and themselves, were recorded. Ambrein produced recurrent episodes of penile erection, a dose-dependent, vigorous and repetitive increase in intromissions and an increased anogenital investigatory behavior, identifying the drug used in the present study as a sexual stimulant. It is conceivable from the present results that the ambrein-modified masculine sexual behavior in male rats supports the folk use of this drug as an aphrodisiac.
Mmm-mmmm. Good stuff, that ambergris.
Gorbachov M.Yu., Rossiter K.J. (1999) A New Electronic-Topological Investigation of the Relationship between Chemical Structure and Ambergris Odour. Chem. Senses 24:171-178.
Taha SA, Islam MW, Ageel AM (1995) Effect of ambrein, a major constituent of ambergris, on masculine sexual behavior in rats. Arch Int Pharmacodyn Ther 329(2):283-94.
First, a tiny bit of quantitative morphological data you can find in just about any comparative anatomy text:
| mammal | number of vertebrae | ||||
|---|---|---|---|---|---|
| cervical | thoracic | lumbar | sacral | caudal | |
| horse | 7 | 18 | 6 | 5 | 15-21 |
| cow | 7 | 13 | 6 | 5 | 18-20 |
| sheep | 7 | 13 | 6-7 | 4 | 16-18 |
| pig | 7 | 14-15 | 6-7 | 4 | 20-23 |
| dog | 7 | 13 | 7 | 3 | 20-23 |
| human | 7 | 12 | 5 | 5 | 3-4 |
The number of thoracic vertebrae varies quite a bit, from 9 in a species
of whale to 25 in sloths. The numbers of lumbar, sacral, and more caudal vertebrae also show considerable variation. At the same time, there is a surprising amount of invariance in the number of cervical vertebrae in mammals — as every schoolkid knows, even giraffes have exactly the same number of vertebrae in their necks as we do. What makes this particularly striking is that other vertebrates have much more freedom in their number of cervical vertebrae; swans can have 22-25. I was idly wondering why mammals were so limited, and stumbled onto a couple of papers that addressed exactly that question (Galis & Metz, 2003; Galis, 1999). Galis’s explanation is that it is a developmental constraint that may have something to do with the incidence of cancer.
Development is an intricately choreographed process that treads a dangerous line. On one side is stability; but development is in many ways a destabilizing process, in which cells have to change their path and form new tissues, and stability is not compatible with it. On the other side is chaos, unregulated proliferation — cancer. During development, the organism has to foster proliferation and change to a greater degree than it can tolerate later, and that loosening of constraints represents a danger. Galis suggests that one reason we mammals may always have 7 cervical vertebrae is that the regulatory genes that specify the number of vertebrae are coupled to processes that otherwise regulate cell fates, and that modifications to those genes that would cause variation in vertebra number would also lead to unacceptable increases in the frequency of embryonal cancers.
This isn’t at all an improbable idea. Genes exhibit bewilderingly complex patterns of expression, and pleiotropy (the regulation of multiple phenotypic characters by a single gene) is the rule, not the exception. The Hox genes, the particular genes that control the identity of regions along the length of the animal, are known to switch on and off in proliferating mammalian cell lines in culture. Perhaps the Hox genes involved in defining cervical vertebrae are somehow also involved in controlling cell proliferation, making them dangerous targets for evolution to tinker with?
Galis provides several lines of evidence that this is the case. To see whether variation in cervical vertebra number leads to increased incidence of cancer, we need to look for instances of variation in mammalian vertebrae.
There isn’t much variation in cervical vertebra number, though. There is an exception: sometimes, the 7th cervical vertebra is found to undergo a partial homeotic transformation and forms a pair of ribs, which are normally found only on thoracic vertebrae. Humans develop cervical ribs with a frequency of about 0.2%; do they also develop cancers? The answer is yes, with a frequency 125 times greater than the general population.
Another place to look would be in phylogenetic variation — between groups rather than within a population. It turns out that there are two groups of mammals that do have a non-canonical number of cervical vertebrae: one manatee genus and two genera of sloths. No data is available on frequencies of embryonal cancers in either, and Galis reports that manatees at least seem to have a low incidence of cancer. One explanation is that both sloths and manatees have exceptionally slow metabolic rates, which in itself will reduce the frequency of cancer, since it will reduce the rate of oxidation damage; the idea is that this low cancer rate may have made these organisms more tolerant of variation in these genes.
An open question is how birds can have greater variability in the number of cervical vertebrae — they certainly don’t have low metabolic rates. One suggestion is that the coupling between these particular Hox genes and a predilection for cancer is unique to mammals. Another possibility is that birds possess other, unidentified mechanisms that reduce free radical production, reduces oxidative damage, and makes them relatively cancer-free. Galis cites several studies that show that birds do seem to be less severely afflicted with cancers than us mammals.
It’s an interesting idea, but the evidence so far is a collection of correlations. I’d be interested in seeing some direct analyses of the role of patterning genes on carcinogenesis. Still, it’s the first answer I’ve seen to explain why such a peculiar restriction in morphology should be nearly universal within a whole class of animals, when other classes allow so much more diversity.
Galis, F and JAJ Metz (2003) Anti-cancer selection as a source of developmental and evolutionary constraints. BioEssays 25:1035-1039.
Galis, F (1999) Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. J Exp Zool (Mol Dev Evol) 285:19-26.
Since I mentioned yesterday that penis size mattered, upon stumbling on this article about the horrific effects of a trematode infestation, I thought everyone might enjoy a grim and vivid picture of what trematodes can do to a poor, innocent mollusc.
This is a photo of a trematode, or fluke. Trematodes are parasitic flatworms with very complex life cycles; this particular one is a cercaria, or tailed larva. They swim about and infest various hosts at various stages, proliferating and spreading through tissues, before moving on to infect the next host in their cycle.
I hate those commercials on cable TV for Enz*te, that fake “male enhancement” product that promises a “boost of confidence” for all the guys who take their little pill. I don’t believe it, of course—it’s probably a concoction of sawdust and rat droppings. But the phenomenon of male confidence as a function of the size of their physical attributes might just have some validity.
AL Basolo, who did some well-known work on mate preference in swordtails a few years ago (short answer: lady swordtails prefer males with longer swords), has a couple of new papers on the subject. She has looked at competition between males—the fishy equivalent of checking out the other guy’s equipment in the lockerroom—and found that the length of the sword makes a big difference in the struggle between males, even with no females involved.
Xiphophorus helleri is a common aquarium fish with a distinctive feature: that long sword on its tail. The males have competitive interactions with each other that are fairly easy to assess: dominant males chase away inferiors, and inferiors avoid the winners, so you just have to record who is chasing who to sort out who thinks they are in charge. Usually, it’s the fish that is bigger overall that wins. The investigators suspected that the size of the sword might also be a deciding factor. Observations of pairs of fish matched for body size, but with natural differences in sword length, did not bear this out, however, showing little correlation. That suggests, as one might expect, that there are multiple factors that influence competitions.
To simplify those factors, they carried out what sounds to me like a very cruel experiment. Pairs of fish matched for body size were anesthetized, their swords chopped off, and replaced with transparent plastic swords of identical size. The difference, though, was that different length swords were painted on the transparent plastic—one lucky fish got a new painted sword roughly the same length as the old one, while the other got a sword half the length.
After recovering from their implant surgery, they were put together in a tank…and the truncated male consistently lost all competitions. I guess size matters, after all.
Without the gross surgical modifications, however, size wasn’t such a clear indicator of victory, so other factors must also play a role. A companion paper looked at stripes on the sword, and how they affected female interests. This work modified the tails digitally; a video recording of a hunky male was made, and then edited to either remove the stripes from its entire length, to remove them from the proximal half or the distal half, or left intact. The video was played back to a female, and the length of time she paid attention to it measured (longer is better).
The lessons are clear. Having a long sword will help you intimidate and beat up your competition, and painting stripes along its length (or at least at the tip) will win you the admiration of females.
If you’re a fish, that is.
There is no necessary expectation that it will help at all if you’re a hairless ape, but if anyone tries it, let me know how it turns out.
Benson KE, Basolo AL (2006) Maleâmale competition and the sword in male swordtails, Xiphophorus helleri. Animal Behaviour 71(1):129-134.
Trainor BC, Basolo AL (2006) Location, location, location: stripe position effects on female sword preference. Animal Behaviour 71(1):135-140.
DarkSyde has another of his Science Friday interviews on dKos—this time with the gang at Real Climate.
Nematostella, the starlet anemone, is a nifty new model system for evo-devo work that I’ve mentioned a few times before—in articles on “Bilateral symmetry in a sea anemone” and “A complex regulatory network in a diploblast”—and now I see that there is a website dedicated to the starlet anemone and a genomics database, StellaBase. It’s taking off!
Variation is common, and often lingers in places where it is unexpected. The drawing to the left is from West-Eberhard’s Developmental Plasticity and Evolution(amzn/b&n/abe/pwll), and illustrates six common variations in the branching pattern of the aortic arch in humans. These are differences that have no known significance to our lives, and aren’t even visible except in the hopefully rare situations in which a surgeon opens our chests.
This is the kind of phenomenon in which I’ve become increasingly interested. I work with a model system, the zebrafish, and supposedly one of the things we model systems people pursue is the ideal of a consistent organism, in which the variables are reduced to a minimum. Variation is noise that interferes with our perception of common underlying mechanisms. I’ve been thinking more and more that variation is actually a significant phenomenon that tells us something about where the real constraints in the system are. It is also, of course, the raw material for evolution.
Unfortunately, variation is also relatively difficult to study.
