The hybrids are winning!

Although at this stage it's a close race. Two and a half weeks ago I spawned sea urchins in the lab, setting up several purple urchin crosses with the hope of re-doing the feeding experiment that I lost this past summer when I was on the DL (that's Disabled List, for those of you who don't speak baseball). I was also fortunate enough to set up a hybrid cross, fertilizing purple urchin (Strongylocentrotus purpuratus, or "Purp") eggs with red urchin (Mesocentrotus franciscanus, or "Red") sperm. I would have done the reciprocal hybrid cross (red eggs by purp sperm) as well if I'd gotten any of female red urchins to spawn. However it wasn't really spawning season for the reds, and I consider myself lucky to have persuaded that one male to release some sperm for me.

This is the first time that I've tried to raise the hybrid larvae, although I know it can be done because my colleagues Betsy and John did it many years ago, before I came to the marine lab. All of my larvae are the exact same age and are being raised side-by-side, so I can make direct comparisons between the Purp by Purp crosses and the Purp by Red hybrids. Incidentally, when speaking or writing about a hybrid cross the convention I've adopted is to reference the female parent first, so when I say Purp by Red I mean a Purple eggs fertilized by Red sperm. A Red by Purp hybrid would logically result from red urchin eggs fertilized by purple urchin sperm.

My experience raising sea urchin larvae is that things almost always go well for the first 48 hours or so; most (but not all) of the fertilized eggs develop into embryos and undergo the crucial processes of gastrulation and hatching. In some cultures the hatching rate is close to 100%. After that there's a window of 3-4 days when cultures can crash for no apparent reason, although food availability or quality may be a factor. If the larvae make it past their first week of post-hatching life they generally cruise along until the next danger period which occurs at about 24 days. I change the water in the culture jars and observe the larvae under the microscope twice a week.

Today the larvae are 18 days old. It's a little early for that second mortality period, but some of the Purp by Purp cultures never really took off. The larvae don't seem to be growing or developing as quickly as I'm used to. Perhaps this has to do with lower water temperatures, especially after the prolonged period of high temps in 2014-2015. In any case, two of the four Purp by Purp crosses are doing well and the other two are just hanging in there.

There are two things I can see with the naked eye that give me a heads-up when cultures are crashing: the first sign is an accumulation of debris at the bottom of the jar and the second is an absence of larvae in the water column. The debris can be due to excess food, a build-up of fecal matter (not usually the case, as I'm pretty good at doing the water changes on time), the disintegration of larval bodies, or some combination thereof. If the water column is clear then the culture has already crashed and everybody is dead.

Today one of my jars had crashed. The water column was very clear and there was a lot of fluff at the bottom of the jar. I'd been wondering if I could figure out what the fluff was made of, so I sucked up a bit in a pipet and examined it under the microscope. I thought I'd see dead algal cells or pieces that look like defecated algal cells. This is what I saw:

18 January 2017
© Allison J. Gong

Silly me. I had forgotten that the corpses of pluteus larvae would disintegrate pretty quickly, leaving behind only the skeletal rods. The rods get all tangled together and trap the organic stuff, which is probably a mixture of uneaten and defecated algal cells and the soft tissues of the larval bodies. This explains the clear water column in the jar.

While the Purp by Purp larvae have had mixed success so far, the Purp by Red hybrids have been doing well. From the outset they appeared to be more robust than the Purps, and even though the fertilization rate was only about 50% the post-hatching mortality seems low. The hybrid larvae are also larger than the Purps, and are developing more quickly. In the two photos below the scale bar indicates 100 µm.

Pluteus larva of Strongylocentrotus purpuratus, age 18 days.
17 January 2017
© Allison J. Gong
Pluteus larva of a hybrid cross between S. purpuratus and Mesocentrotus franciscanus, age 18 days.
17 January 2017
© Allison J. Gong

The hybrid larva is about 10% larger than the Purp larva. Other than that they look similar, but to me the hybrid larva seems farther along the developmental process: its arms are proportionally longer and have a more mature look (although I don't have any way to describe that to a naive observer). There's something about the gestalt of the animal that makes me think it's more robust than the Purp individual.

We'll see how the pure Purps and the hybrids do from here on. I actually have the Purp larvae divided up into different feeding treatments, which I may discuss in a future blog post. In the meantime I'm trying to baby the hybrid larvae as much as possible, to maximize their probability of successful metamorphosis in six weeks or so.

2 thoughts on “The hybrids are winning!

  1. Jamie Grover

    So if the hybrid larvae are doing so well, I wonder what disadvantages them in the wild? Do you know if anyone has successfully bred red/purp hybrids with one another?

    1. Allison Gong

      For one thing, some hybridization probably does occur in the wild. I know that I've seen urchins in the field that have that sorta-red, sorta-purp look. I don't know whether or not anyone has looked at the DNA of these individuals to determine whether or not they are actual hybrids, though.

      It may be that the different spawning seasons of the two species evolved as a pre-zygotic mechanism of reproductive isolation. There's enough overlap in habitat that I think there would be much more hybridization if they spawned at the same time. Purps are generally (but not entirely) intertidal while reds are mostly subtidal, seen intertidally only during the lowest low tides. The habitat overlap would be in the subtidal, where purps and reds often live side by side in large numbers.

      As with other hybrids, it could simply be that they are not all that well suited to either habitat, and would be outcompeted by the parent species that dominates in a particular habitat. It would be a very interesting thing to study.

      Betsy Steele and John Pearse did a big experiment where they crossed reds and purps both ways, then bred the F1s back to their parent species. This was all before my time and I don't know all the details of the results, only that there were a couple dozen urchins in the lab that Betsy referred to as "hybrids" but I think were actually F1 back-crosses. I wish I had been able to get even one red female to spawn, so I could do the Red by Purp cross and compare.


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