Making whoopee, coral style


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.