Sex is one of the great mysteries of evolutionary biology - why do organisms have it? It has numerous costs associated with it, including the two big ones, which are that only half the population will produce offspring in the next generation (technically really a problem more of anisogamy than sex, per se) and that successful gene combinations can be broken up via recombination. There are other costs as well. For instance, finding and wooing mates can be costly to an organism.
Nematomorphs, sometimes called hairworms, are parasites that live inside arthropods as larvae, but then exist as free-living aquatic adults. They often induce suicide in their insect hosts, by causing them to jump into water, where the worms then escape (see this previous post for another example). The adults typically seek out the opposite sex and can form "Gordian knots" of mating worms. Today's species, however, is found in larger and faster-moving waters - and in these big, complicated habitats, finding a suitable mate can be really tricky. So, today's parasite, has solved this problem through the evolution of parthenogenesis. Meet Paragordius obamai, (named after President Obama, in honor of it being discovered in Kenya, where his father was raised), a species of nematomorph that has completely given up on males. When brought into the lab, P. obamai only released female worms and nowhere inside these stringy parasites could male reproductive organs be found. Because bacterial symbionts can sometimes produce severe sex-ratio biases or even male-killing in insects and other invertebrates, the authors used pyrosequencing to look for evidence of these micro-manipulators, yet found no sequences similar to the taxa that have been observed to cause these biases in other hosts.
The authors now plan to use this new species, in comparison with a sexual congener, to test hypotheses on the evolution of and genetic mechanisms responsible for this novel parthenogenetic situation.
Source: Hanelt B, Bolek MG, Schmidt-Rhaesa A (2012) Going Solo: Discovery of the First Parthenogenetic Gordiid (Nematomorpha: Gordiida). PLoS ONE 7(4): e34472. doi:10.1371/journal.pone.0034472
Image from the paper.
Contributed by Susan Perkins.
"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift
April 13, 2012
For many parasites, host castration is a very effective strategy. By specifically diverting energy from the host's reproductive functions, the parasite can horde as many resources as it can without compromising any organs or functions that are vital to everyday survival of the host. It is a strategy commonly used by digenean trematodes (parasitic flukes) and some parasitic crustaceans.
But, those parasites infect hosts that exist as discrete individuals (unitary organisms) - what about modular animals like corals and salps that live as colonies composed of many genetically identical individuals? Such organisms undergo alternating bouts of asexual and sexual reproduction throughout their life-cycle - so are there parasites that can castrate such hosts?
Today's parasite is Tetracapsuloides bryosalmonae - it is a myxozoan, parasites that were once thought to be protists, but are actually related to jellyfish and corals in the phylum Cnidaria. Throughout their evolution, they have simply been heavily modified for a parasitic way of life. Tetracapsuloides bryosalmonae has a very comprehensive scientific name in terms of describing its life-cycle - the species name encompass both of the parasite's hosts; bryozoans and salmon. In salmon, it causes a serious disease call Proliferative Kidney Disease (PKD), but less is known about its ecology in its bryozoan host
Bryozoans are also called "moss animals", and they are quite odd (even if rather common) little critters (you can read more about them on the Bogleech website here). They are colonial animals that live as a collective of individuals called "zooids", and are encased in a mineralized exoskeleton that can take on various shapes including fans, bushes, and flat sheets. Most of the time, the colony grows by budding genetically-identical zooids (clones), and colonise new habitats when fragments of the colony break off and settle elsewhere. But during lean times, the colony starts producing statoblasts, which are tough little capsules of cells that can be released into the environment to start the colony anew elsewhere, rather like seeds.
Most of the time, T. bryosalmonae exists as a quiet, secretive infection in the form of single-cells embedded in the body wall and proliferates as the colony grows. But this parasite can also switch into a more overt mode where it starts producing multicellular sacs of parasite cells, and because individual zooids are connected to each other in the colony via a common body cavity, during such flare-ups, the infection can spread throughout the colony. During the overt phase of the infection, T. bryosalmonae effectively castrates the bryozoan, and infected colonies cannot produce statoblasts.
However, on the flip side, because infected colonies aren't diverting resources towards producing statoblasts, they are better able to survive periods of starvation and suffer fewer overall zooid deaths comparing with uninfected colonies. In the natural setting, T. bryosalmonae usually enters its overt phase during late spring or autumn when the bryozoan colony undergoes its greatest period growth of asexual growth - which gives the parasite more opportunities to spread. As the parasite subsides back into its covert phase again, the bryozoan colony once again has an opportunity to produce statoblasts.
By cycling between a covert and overt phase, the parasite can persist without causing damage that can compromise the host's survival. Asexual colonial organisms can avoid permanent castration like that seen in snails infected with parasitic flukes. Theoretically, colonial modular animals can potentially live indefinitely due to their mode of reproduction. But another benefit associated with such a life-style could be an increased tolerance for infection - whereas discrete, unitary animal might be completely sterilised by host-castrating parasite, when infected with a parasite like T. bryosalmonae, modular animals can simply bide their time and reproduce when the parasite subsides between periods of overt infection.
Image from the Natural History Museum
Hartikainen, H. and Okamura, B. (2012) Castrating parasites and colonial hosts. Parasitology 139:547-556
April 2, 2012
Most animals are infected by multiple species of parasites or multiple strains of the same parasite species that are not close kin. Rarely does an individual parasite (and its close kin) gets to monopolise the resources of an entire host. So do parasites like to share? The answer to that depends on the host (and parasite) in question. Because different species often exploit a single host in different ways, a parasite can share a host with many other species without ever coming into conflict with them. But, when a parasite finds itself sharing a host with members of its own species, competition is more likely to occur as they are all going to be after the same (limited) resource (whatever that may be) from the host.
For some parasites, competition arising from coinfection can favour the most virulent strains, leading to greater overall harm to the host. But more recent studies have shown that the most competitive strains are not always the most virulent. In some cases, competition between co-occurring parasites can actually be beneficial for the host as the parasites end up mutually suppressing each other. How this plays out depends on how the parasite uses its host, and in parasites that have complex life-cycles, this can change from host to host.
The parasite we are looking at today is Diplostomum pseudospathaceum - more commonly known as the eyefluke. As with most parasitic flukes, D. pseudospathaceum uses a snail for the clonal stage of its life-cycle to make thousands of larval stages (called cercariae), which are then released into the water to infect any nearby fish. In the fish, the parasite migrates to its eye and uses it as a temporary vehicle to reach its next host - a fish-eating bird. Diplostomum pseudospathaceum essentially turns its snail host into a parasite factory, using the snail's bodily reserves as raw material to produce its army of clones. There is only so much to go around inside a snail, and when a particular D. pseudospathaceum strain has to share a snail with other strains, it ends up producing many fewer cercariae than if it had the whole snail to itself. But while it is detrimental for D. pseudospathaceum to have close company in the snail, it is a different story in the fish.
While D. pseudospathaceum undergoes a resource-hungry spree of rampant asexual multiplication inside the snail, it is relatively dormant inside its fish host. All it needs from the fish is a space to tuck into within its eye - and there is plenty of room in the fish's eye for these flukes. In fact the more the merrier given that at high numbers the eyeflukes can induce the formation of cataracts - and a half-blind fish is more likely to be eaten by a bird. But there is also another reason for D. pseudospathaceum to be more welcoming to strangers in the fish host.
The immune system of the fish is remarkably adept at responding to intruding parasites, and the immune system can be quickly "primed" towards recognising and destroying particular parasite strains. To reach the fish's eye, D. pseudospathaceum has to complete a treacherous journey through the fish's body, all while under close scrutiny and assault by the fish's immune system. In fish that have previously been exposed (and thus "primed") to D. pseudospathaceum cercariae, the chances of the parasite successfully reaching its destination are greatly diminished. But while a fish might be "primed" towards cercariae of a particular strain, a double- or even triple-prong attack is likely to overwhelm its finely-tuned targeting system. When exposed to simultaneous attacks from multiple, genetically-diverse strains of D. pseudospathaceum cercariae, the fish's immune system becomes overwhelmed, which in turn allows more eyeflukes to slip by.
In the life-cycle of D. pseudospathaceum, there is a time and place for everything - while there is a time to be on the look out for number one, there is also a time to love thy neighbour.
image by Tina Loy, modified from here
Karvonen A, Rellstab C, Louhi KR, Jokela J. (2012) Synchronous attack is advantageous: mixed genotype infections lead to higher infection success in trematode parasites. Proceeding of the Royal Society B 279: 171–176