Biology concepts – polyploidy, invasive species
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There are 60,000 different species of weevils, a type of beetle. And almost all of them are polyploid! Polyploidy benefits speciation, so maybe them being polyploid is why there are 60,000 species. On the left is Trachelophorus giraffa, named for obvious reasons, and on the right is Rhigus nigrosparsus. You’ll have to go to Brazil to see one in person. |
We pointed out last week that polyploidy in plants has done a lot to promote speciation events, and this seems to be the case in fish as well. While some families have a few polyploid members, like the loaches, or the carps and minnows, other families are completely polyploid, like the Salmonidae (salmon). Wat is more, the families with the greatest number of polyploid members also have the highest number of species overall. Of the 28,000 known species and >60 orders of fish, 63% fall into the 9 orders that include polyploidy – coincidence? I don’t think so.
Remember that polyploidy in plants is well behaved, not much genome restructuring goes on even though there can be subfunctionalizationand neofunctionalization leading to speciation at the molecular level. In contrast, fish polyploidy seems to induce a tolerance of change, and gene ordering and genome restructuring seem to run rampant. This seems to be at least one reason for high rates of new species development in fish that are polyploid.
The effects of polyploidy on fish are similar to those we have talked about previously. Polyploid fish tend to be larger, ie. the gigas effect, and they tend to live longer and grow faster, ie. heterosis. Inductions of triploidy or formation of auto- or allopolyploid species tend to have fewer diseases. For some reason, sexual maturation in fish is linked to higher infection rates – most likely due to stress. Finding a mate and having kids is stressful, ask any adult. Stress is directly related to infection rates, as one of the effects of the stress hormone cortisol is to turn down the immune system.
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The sunshine bass on top is a diploid female which is filled with eggs (gravid). In contrast, the female on the bottom is triploid. She is bigger, even compared to a gravid fish. The gigas effect in polyploids is real, and effects sport fishing. Everyone wants to catch a bigger fish. |
In plants, a recent study indicates that a disease resistance cluster of genes in soybeans indicates that production of new disease resistance genes could develop by polyploid development. In an autopolyploid soybean, the number of disease resistance genes doubled, but they didn’t produce twice as much protein. It seems that they have begun to evolve independently. This may in turn produce newly functional resistance genes, or on the other hand, may eliminate one of the clusters. It appears that specific immune function and polyploidy may be interpreted only on a case by case basis.
But there negative effects that are similar to plants as well. Triploid species are often less reproductively active, due either to difficulties in gamete production or to aberrant sex steroid levels as a result of dosage imbalance.
In some cases though, sterility has been used to the advantage of humans - triploid salmon are less likely to return to spawning grounds, which means they stay in the ocean longer, growing fat and happy. For wild salmon fisheries, this means a greater number of bigger fish. For salmon hatcheries and commercial growers, it means less stress on the animals and a greater harvest. Triploidy can be induced in the salmon (and other species) by cold shocking the eggs near the time of fertilization or using chemicals to prevent chromatid separation during meiosis.
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Triploid oysters on the west coast are raised so that they can be harvested year round. They are bigger and taste better than spawning diploids. They are also more disease resistant, and this might affect pearl formation, since most natural pearls are induced by parasites that bore through the shell. |
Pacific oyster species use up to 80% of their body weight for production of sperm and eggs – not good for food harvesting. This can last for most of the late spring and summer, so the triploids allow for harvesting when people are accustomed to avoiding oysters – typically, the rule is don’t eat oysters in any month without an “r.”
But if we harvest fewer diploids, and introduce more triploids – could we end up with a glut of oysters? The diploids that would have been caught are free to reproduce and we end up eating the triploids that wouldn’t have been reproducing anyway. What ecological niches might be disturbed by too many oysters? You can discuss amongst yourselves whether this is a good idea in the long run – I render no opinion one way or the other.
You can argue both sides of the polyploid introduction argument; human efforts to enhance (alter) the zoological face of the planet have met with some disastrous failures, but remember that majority of foreign introductions have been ecologically moot. This point is often overlooked, but we have talked about itbefore.
Polyploidy in wild salmon is extremely common, so would there really be that great a change? The use of triploid induction is more common in commercial fisheries and in the shellfish industry because they believe it provides a hedge against escapement and breeding with wild populations. Triploid fish and shellfish are sterile, so even if they did escape into the wild, they would be unlikely to breed with the wild type populations.
But Mother Nature always finds a way, doesn’t she? There have several cases of reversion to diploidy in triploid oysters. These shellfish are then free to breed with wild species. And what is more, induction of triploidy is not 100% efficient in fish, so some organisms will remain diploid. The incomplete induction of triploidy has been illustrated brilliantly by the invasion of the asian carp.
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What we call the asian carp is actually four different species. But they all get big. O.K. usually not this big! They are moving up the Mississippi River and threaten to enter the Great Lakes. If they do, they could destroy a multibillion dollar a year fishing industry. |
The bighead carp and silver carp were introduced into the US in sewage treatment plants and aquaculture ponds as a way to produce clearer water. These two species eat zooplankton and the waste of other animals, so naturally they were a good choice to improve water quality. But as with the grass carp, they ended up in the Mississippi and now have become a great problem. As far as I can tell, bighead carp and silver carp were not required to be tested as triploid before release until 2005 or later.
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Nobody wants to bite into their striped bass fillet and find a yellow grub. Black carp were introduced to destroy the snails that serve as one life cycle stages of the grub. My Gosh! Did this guy actually circle the grub with his wedding ring?! |
Black carp love snails, so they were introduced into fish farm ponds in the 1990’s to interrupt the yellow grub life cycle. The plan worked, and worked well; too bad the work to induce sterility did not work as well - the black carp has ended up in the Mississippi as have the other species of asian carp.
The result of these escapes is that rivers in 23 states are choked with asian carp, to the point that many native fish die off. True - the fish are big, very big, so they could provide a source of food. But they haven’t caught on as a food fish, and some places (like Canada) won’t even allow them to be sold for food. Fishing them for sport isn’t going to work as well, as their diets don’t help. How would you bait a hook with a piece of grass or a zooplankton?
The numbers have grown so large in recent years that other problems have developed. The silver carp has a strange habit of leaping out of the water when a boat motor approaches; there have been hundreds of instances where people have been struck by the fish. Noses have been broken, boats have been damaged, and this is all on top of losing the native species in the rivers. Check out this videoof the silver carp problem and the birth of a new sport, aerial bowfishing.
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Jeff Goldblum said it in Jurassic Park – life finds a way. There is no way of predicting which turn life will take, or the lengths evolution will go to help life persist in the face of huge obstacles. Too often, we are that obstacle. |
But we haven’t even talked about one of the most interesting exceptions in nature that is related to polyploid development; the link between extra sets of chromosomes and the abandonment of sexual reproduction. To illuminate this exception, we will focus on the insect, lizard and amphibian polyploids next time.
For more information and classroom activities, see:
Polyploidy in aquaculture –
Asian carp –
http://www.ecy.wa.gov/programs/wq/plants/management/aqua024.html