Making whoopee, coral style

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Pocil­lo­pora dam­i­cornis is one of the most com­monly studied coral species on the planet. Photo by David Combosch.

When it comes to making babies, most species pick a strategy and stick with it. Humans, for example, are per­fectly happy with our sexual mode of repro­duc­tion: Half the DNA from dad mixes with half the DNA from mom to form a brand new, genet­i­cally dis­tinct indi­vidual. We’re quite fond of this “genet­i­cally dis­tinct” bit because it gives us indi­vid­u­ality and our species plenty of oppor­tu­nity to come up with new com­bi­na­tions of great traits from both our par­ents, say dad’s blue eyes and mum’s bright smile.

The asexual crowd, how­ever, has a good strategy too. Instead of passing on half of the genetic info from two unique indi­vid­uals, these folks make babies simply by cloning all of the info in one of their own cells. No dad required. In this mode of repro­duc­tion, all of the good traits get pre­served from one gen­er­a­tion to the next. Because 100 per­cent of the DNA gets passed onto the off­spring, there’s no con­cern of sac­ri­ficing a very promising trait (like, say blue eyes and bright smiles, which we could say we’re lucky to have held onto given our sexual mode of reproduction).

It’s very rare that you do both sexual and asexual repro­duc­tion at the same time,” said doc­toral can­di­date David Com­bosch. “Because it’s very com­pli­cated.” Still, there are few species out there in the world that aren’t sat­is­fied to chose one strategy over another. Cock­roaches are a good example. They’re nor­mally sexual like us, but when there are no boys in the ‘hood’, or when cir­cum­stances are just gen­er­ally trying, female cock­roaches will clone them­selves instead, making babies the old fash­ioned asexual way.

In the marine world a long-​​studied coral species called Pocil­lo­pora dam­i­cornis, or “cau­li­flower coral” if you like, has been some­thing of repro­duc­tive mys­tery, said Com­bosch, who works in Steve Vollmer’s lab at the Marine Sci­ence Center. When researchers looked at the genetics of entire pop­u­la­tions of P. damicornis, he told me, they observed lots of dif­ferent geno­types, pointing rather matter-​​of-​​factly toward sexual repro­duc­tion. After all, there’s not much oppor­tu­nity for genetic vari­ability if you’re just cloning your­self all the time. But when­ever those same researchers looked at the larvae pro­duced by indi­vidual corals, the babies’ geno­types were always iden­tical to their parents’.

Com­bosch has spent the last few years trying to figure out what in the world is going on with P. dam­i­cornis reproduction…and why. By looking at sec­tions of DNA called “microsatel­lites,” which are short, hyper-​​variable sequence repeats, Com­bosch found that while the majority of larvae (94 per­cent) were indeed genet­i­cally iden­tical to their parent, a few seemed to have been pro­duced sex­u­ally. It turns out that while they’re making their genet­i­cally iden­tical larvae, indi­vidual coral are also pro­ducing a whole bunch of sperm, which are just itty bitty cells that carry a half set of genetic infor­ma­tion. He hypoth­e­sized  that this sperm is actu­ally needed regard­less of the repro­duc­tive strategy — sexual or asexual — to stim­u­late an egg to tran­si­tion into a larvae. “Most of the time they take that trigger but they don’t take the genetic infor­ma­tion that come along with the sperm,” said Com­bosch. “Just occa­sion­ally some of that sperm DNA sneaks in.”

He said this strategy actu­ally makes a whole lot of sense, because it allows for suc­cess­fully grown up coral colonies to pass along all their good genes intact while still leaving room for even better gene com­bi­na­tions to show up every now and then. “You put most of your eggs on the safe side with asexual repro­duc­tion but you still gamble with a little bit of your money,” said Com­bosch, whose results were recently pub­lished in the journal Ecology and Evo­lu­tion. “You allow for some of your eggs to go into that sexual lot­tery and hope that a new com­bi­na­tion with some of the parental genes that comes along with the sperm leads to an even better geno­type.” This strategy is very rare, but it has been observed before and in a wide range of organisms.

The really inter­esting thing is the pat­tern of sexual repro­duc­tion he observed. He found more asex­u­ally pro­duced larvae in bigger colonies. “Smaller colonies,” he said, “seem to put a higher pro­por­tion of their money into the lot­tery, but then the bigger they get the more their geno­type proves itself to be suc­cessful, the more they pro­duce asex­u­ally to pass it on in one piece.” As far as he knows, no other organism on the planet behaves like this, but then again, no other organ­isms are quite like corals, which con­tinue to grow indef­i­nitely making their size a good indi­cator of success.