Evolution of direct development in echinoderms

In chapter 14 of the Origin of Species, Darwin wondered about the whole process of metamorphosis. Some species undergo radical transformations from embryo to adult, passing through larval stages that are very different from the adult, while others proceed directly to the adult form. This process of metamorphosis is of great interest to both developmental and evolutionary biologists, because what we see are major transitions in form not over long periods of time, but within a single generation.

We are so much accustomed to see a difference in structure between
the embryo and the adult, that we are tempted to look at this
difference as in some necessary manner contingent on growth. But there
is no reason why, for instance, the wing of a bat, or the fin of a
porpoise, should not have been sketched out with all their parts in
proper proportion, as soon as any part became visible. In some whole
groups of animals and in certain members of other groups this is the
case, and the embryo does not at any period differ widely from the
adult: thus Owen has remarked in regard to cuttlefish, “There is no
metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed.” Landshells and fresh-water
crustaceans are born having their proper forms, whilst the marine
members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified
habits, or are inactive from being placed in the midst of proper
nutriment or from being fed by their parents; but in some few cases,
as in that of Aphis, if we look to the admirable drawings of the
development of this insect, by Professor Huxley, we see hardly any
trace of the vermiform stage.

Why do some lineages undergo amazing processes of morphological change over their life histories, while others quickly settle on a single form and stick with it through their entire life? In some cases, we can even find closely related species where one goes through metamorphosis, and another doesn’t; this is clearly a relatively labile character in evolution. And one of the sharpest, clearest examples of this fascinating flexibility is found in the sea urchins.

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Beware the frogs

A good way to recover from the fra…fra…frammmm… that topic is to go watch the freaky frogs. If it’s late at night and dark where you are, though, don’t watch them. The first one will creep you out, and the second one will deliver the coup de grace; you won’t be able to get to sleep for fear of the amphibians outside your window.

Born of a virgin

Add hammerhead sharks to your list of animals that don’t need males. A captive bonnethead female in Nebraska gave birth in 2001, and genetic testing has revealed that it was produced by parthenogenesis. In a way, this isn’t a surprise: I could have told her that Nebraska is no place for a self-respecting shark to look for a boyfriend.

Parthenogenesis had been suspected, because the shark had been isolated from males for at least 3 years, and because she lacked the obvious bite marks that result from shark sex (which is another reason a lady shark might not want to have anything to do with sexual reproduction), and now the tests have shown it for sure. Nifty!

Ascidian evo-devo

Here are three animals. If you had to classify them on the basis of this superficial glimpse, which two would you guess were most closely related to each other, and which one would be most distant from the others?

i-3c5822c1c21ece64c8664c4ac32d9b63-ascidian.jpg

i-56ee51e328b10451feb168cd9bab0ea5-amphioxus.jpg

i-703ea1f1beca939b01785054c9529b6c-fish_larvae.jpg

On the left is a urochordate, an ascidian, a sessile, filter-feeding blob that is anchored to rocks or pilings and sucks in sea water to extract microorganismal meals. In the middle is a cephalochordate, Amphioxus, also a filter feeder, but capable of free swimming. On the right are some fish larvae. All are members of the chordata, the deuterostomes with notochords. If you’d asked me some years ago, I would have said it’s obvious: vertebrates must be more closely related to the cephalochordates—they have such similar post-cranial anatomies—while the urochordates are the weirdos, the most distant cousins of the group. Recent developments in molecular phylogenies, though, strongly suggest that appearances are deceiving and we vertebrates are more closely related to the urochordates than to the cephalochordates, implying that some interesting evolutionary phenomena must have been going on in the urochordates. We’d expect to see some conservation of developmental mechanisms because of their common ancestry, but the radical reorganization of their morphology suggests that there ought also be some significant divergence at a deep level. That makes the urochordates a particularly interesting group to examine.

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