In my last blog – which was much longer ago than I intended – I provided a loose working definition for a parasite. Hopefully, it wasn’t too pedantic…Anyway, in that definition, I argued that a parasite is any organism that requires another to complete its life cycle (not including predators or herbivores). That’s kind of vague, I admit. But I did assert that parasites do not necessarily take nutrients from their hosts, and I gave a few examples to that effect. Continuing in this vein of thought, I decided to write a post about sexual parasites, another example of parasites that do not obtain nutrients from their hosts. I recently read about them in Trends in Ecology and Evolution. See citation at the end of the post.
The term sexual parasite is rather misleading. Many parasites reproduce sexually, but sexual parasite refers to sexually parasitic organisms. What? How does that clarify the things? Well, when sexual parasites mate, only one of the parent’s genetic contribution survives the process. For example, in gynogenesis a female sexual parasite mates with a male (the host in this case). Instead of fertilizing the female’s eggs, the male’s sperm merely activates the eggs and enables them to develop into embryos. We call this form of reproduction sexual parasitism because the male sperm does not contribute genes to the eggs, and so the male’s mating effort was wasted. The female sexual parasite needed a male to initiate her reproductive process; she parasitized the male. The male counterpart of this process – a male sexually parasitizing a female – also exists, and is known as androgenesis.
Other than being a really great example of non-traditional parasites, sexual parasites are interesting in that they exist. Or rather, they are interesting in that sexually parasitic species have survived for longer than we would have predicted possible. Here’s the thought process: Sexually parasitic species either mate with closely related species that have non-parasitic members, or have both parasitic and non-parasitic members. In either scenario, there’s a problem. Sexual parasites have much more fitness than their non-parasitic counterparts. The hosts waste energy in each mating with the parasites, and the parasites are more effective at spreading their genes in the population because there is no sex, no dilution of genes through multiple parents contributing genes to the offspring. This double-edged sword ensures that the parasitic life style should spread through the population, at which point the parasites will have no hosts with which to mate – and they will become extinct. Moreover, sexual parasites produce clone offspring; the offspring have the same genes as their parents. As such, mutations will tend to accumulate, a process known as Müller’s Ratchet. In other asexual organisms with very high reproductive rates, such as bacteria, so many offspring are produced that chances are high that a few have no mutations. Selection will favor these offspring and prevent the accumulation of mutations. In multicellular organisms reproducing sexually, reproductive rates are much lower, and sex reshuffles genes to produce offspring with limited mutations. In sexual parasites, which reproduce without sex, no such reshuffling occurs, and mutations will tend to accumulate since they do not have enough offspring to produce a few without new deleterious mutations.
So how do sexual parasites survive, some for as long as 100,000 years? The current hypotheses involve mate preference and patchy distribution of parasites and hosts. Mate preference could preserve sexually parasitic species because if the hosts can differentiate between parasitic and non-parasitic members, then they will be able to mate much more effectively and preserve the non-parasitic life style. However, mate preference would have to evolve before the parasite life style reached a critical level in the population. If hosts represent five percent of the population, for example, their chances of finding another host with which to mate could be very low, meaning that even if the host can discern with perfect accuracy the parasitic species members, the host may never find another non-parasitic member with which to mate, and so the non-parasitic life style would not be passed to the next generation. Patchy distribution of parasites and hosts could preserve the sexually parasitic life style through preventing the depletion of hosts. For instance, if hosts and parasites are separated in space, hosts could occasionally migrate into a parasite zone and at such a rate to sustain the parasites within their zone. Meanwhile, the hosts could maintain themselves in their location. Again, this situation has a caveat: I imagine that parasite migration would have to be minimal. If the parasites can migrate in large numbers, they could successively wipe out each patch of hosts.
Cool stuff, huh? I thought so. Anyway, I had never heard of sexually parasitic organisms before I read the below article, so I looked some up. I’ve posted links to a few pictures and articles.
Tassili Cypress tree: https://en.wikipedia.org/wiki/Cupressus_dupreziana
Amazon molly: http://www.fishbase.org/summary/Poecilia-formosa.html
Edible frog: http://en.wikipedia.org/wiki/Edible_Frog
Tremblay’s salamander: http://en.wikipedia.org/wiki/Tremblay%27s_salamander
For more information see: http://books.google.com/books?hl=en&lr=&id=z4NyJt21NO8C&oi=fnd&pg=PR6&dq=tremblay%27s+salamander&ots=iFSCLbV3WG&sig=PfbhcdwUjWKVLsLP5dHKzNBZ77Q#v=onepage&q=tremblay%27s%20salamander&f=false, and for some pictures see: http://www.a2gov.org/government/communityservices/ParksandRecreation/NAP/Herps/salamanders/Pages/HybridSalamanders.aspx
Lehtonen, J., et al., Evolutionary and ecological implications of sexual parasitism. Trends Ecol Evol, 2013. 28(5): p. 297-306.
DeGraaf, Richard M., Deborah D. Rudis, and Abigail Rorer. Amphibians and Reptiles of New England: Habitats and Natural History. Amherst, MA: University of Massachusetts, 1983. Print. Pg 19.