Coevolution, Mutualism, Parasitism Reading: Smith and Smith Chp 15 • Coevolution Occurs When One Taxon Exerts an Important Selective Pressure on Another

Coevolution, Mutualism, Parasitism Reading: Smith and Smith chp 15 • Coevolution occurs when one taxon exerts an important selective pressure on another. – This causes an evolutionary response by the second taxon, which in turn, exerts a selectiv pressure on the first. – Gene per gene coevolution is coevolution in the strictest sense, when it is possible to identify particular alleles in one organism that exert a selective pressure on particular alleles in another. – Diffuse coevolution is the other extreme, when a general group of organisms is thought to exert selective pressure on another general group of organisms; example, bees and flowering plants – Example or Gene Per Gene Coevolution-Wheat (Triticum aestivum) and Hessian Fly (Mayetiola destructor). – This system has been studied extensively because it is of tremendous economic importance. • The Hessian fly is an introduced species (which was probably introduced by Hessian mercenery soldiers during the American Revolutionary War). • It is actually a gall midge (Cecidomyidae)-a group of dipterans that induce tumors in their plant hosts. • Life cycle – There are two generations of Hessian flies per year, one attacks seedling winter wheat, or volunteer (weed) wheat. It overwinters in wheat stubble, emerges in spring, mates, and attacks wheat by depositing its eggs on the underside of wheat leaves. The larvae grow and inhibit the growth of the wheat by sucking sap and releasing substances which suppress its growth. • Resistance – Certain alleles in wheat confer a property called antibiosis-toxic compounds within the wheat specifically act to kill the feeding larvae. – This resistance occurs because of alleles at any one of 28 loci (named H1…H28). – New alleles confer very good protection against the fly, but as they become more common in wheat populations (via what strains farmers choose to plant, this is not strictly natural selection), they induce strong selective pressures on flies to be able to detoxify whatever it is that the wheat is producing to kill them. – Thus, Hessian flies ultimately evolve immunity to whatever resistance alleles wheat evolves, causing selective pressure on wheat to evolve new alleles. • Parasite-host systems show an amazing amount of coevolution – parasites can inflect significant losses of fitness on the host-this can cause strong selection for resistance – a successful parasite produces a large number of offspring despite the host’s attempts to stop it-this causes strong selection for virulence – additionally, there is selection for parasites to find new hosts • parasites are selected to modify behavior of the host in such a way as to cause it to spread the parasite • for some parasites, selection for increased virulence is tempered by the fact that dead hosts do not spread the disease Transmission Vectors • Microscopic parasites and pathogens have an amazing diversity in their modes of transmission, and a corresponding complexity in their life histories- each stage has special adaptations to defeat the defenses of a particular host. Presumably, these complex – The mosquito, an insect life cycles allow parasites to ectoparasite, is the host for exploit ecological niches that certain stages in the life cycle would not be available to them of they simply went from one for many different parasites, host to a similar host. including malaria Example • The guinea worm, Dracunculus madeninsis, exploits two hosts during the course of its life cycle. • Larvae swim in freshwater and infect a freshwater Within about a year, it (the copepod. They feed within female) has grown the still-living host, until it is to a worm that can be three swallowed by a human. feet long, living within an artery. • In the stomach, acid It will bore a painful hole to the outside, dissolves the copepod, and it releases huge amounts of eggs the larvae emerge. into the water whenever the host bathes or gets wet. Modification of Host Behavior • We don’t usually think of ourselves as slaves to our illnesses, but in many ways, parasites modify our behavior to increase the likelihood of transmission. • A good sneeze removes But the bacteria that produced the biofilm from our that biofilm were selected to do so, respiratory tract and and one reason was that sneezes allows our immune are excellent disease vectors. system to attack the function Host Resistance • Hosts are not helpless when confronted with parasites. Nearly every organism has evolved sophisticated mechanisms to protect themselves. – Passive defenses are always operational, frequently these guard against generalists – Induced defenses can be triggered by particular parasites • For example, the human immune system has both passive and induced defenses • Every successful defense causes strong selective pressure on the parasite to beat the defense. Experiment-evolution of resistance to a parasitoid by a sarcophagid fly • Sarcophaga bullata is a blowfly that is the preferred host of the parasitoid wasp Nasonia vitripennis. • In populations of hosts that have never been exposed to Nasonia, host mortality and the reproductive rate of Nasonia are very high. • In a series of experiments conducted during the 1960’s, David Pimentel showed that Sarcophaga could evolve increased resistance to Nasonia in response to selective pressure. – Treatment 1-remove all flies that survive attack by Nasonia, add constant # new flies. – Treatment 2-flies that survive attack are allowed to reproduce, the rest are new • Result-in the treatment where survivors were allowed to reproduce, the reproductive rate of Nasonia went from 135 offspring per female to 35, with correspondingly high increases in the survivorship of Nasonia. • How do insects defend against parasitoids? – many insect larvae have special cells that can encapsulate a parasitoid larva-they recognize the attacker, and surround it with a layer of tough, melanin-containing cells. • Some parasitoids beat this defense – Ichneumenoid parasitoids have special polyDNA viruses, harmless to the wasp, that concentrate in their venom, and can destroy the potential of the hosts to defend themselves. • A parasitoid is the ultimate transmission vector for a virus, once the infected host is killed, viruses enter the pupating parasitoid larvae, and have perfect transportation to a new host! Other Forms of Parasitism • Brood parasitism occurs when members of one species rob members of another species of their reproductive effort, rather than taking energy or nutrients directly from the host. – It can be interspecific or Cuckoo wasp – intraspecific. • Interspecific brood parasitism • can be obligate or facultative. • It is particularly common • in birds, wasps, and bees. – Species of birds that suffer frequent brood parasitism will eject strange eggs from their nests. • Species of birds that have, historically, lacked brood parasites, lack defenses. – Example-after millennia of isolation from cowbirds, Kirtland’s warbler (Dendroica kirtlandi) lacks behavioral defenses against the parasitic cowbird. Brown-headed cowbird Kirtland’s warbler (50birds.com) Examples • Intraspecific brood parasitism. – Certain females of the barn swallow (Hirundo rustica) lay eggs in the nests of their neighbors, rather than building their own nests. Adult barn swallows may build nests, parasitize, or do both. • Interspecific brood parasitism. – Females of the cuckoo wasp, Trichrisius tridens, enter the nests of bees and wasps, laying their eggs on the provisions that have been provided for the development of the nest-builder’s egg. The T. tridens larvae hatches first and destroys the larvae of the host, then devours the provisions for itself. Cuckoo wasps have evolved heavy armor to avoid being stung and killed by their hosts. Some hosts dig “false burrows” to deceive cuckoo wasps. Kleptoparasitism-is where animals steal some important resource from each other. – Example of intraspecific kleptoparasitism-Certain females of the great golden digger wasp (Sphex ichneumenoides) will “enter” the nests of conspecific females, effectively taking them over and saving the trouble of digging their own. – If they encounter the original female, the two will fight ferociously. – Example of interspecific kleptoparasitism-Black headed gulls (Larus ridibundus) parasitize flocks of golden plovers (Pluvialis apricaria) and lapwings (Vanellus vanellus) in the English countryside. They harass unwary birds until they drop the worms they have just extracted from the soil. Slave-Making in Ants • Slave making is an interesting form of parasitic behavior among social insects, where it is the lifetime effort of worker ants that is, in fact, stolen. – If abducted and transported as larvae, worker ants of most species will emerge as adults and serve the colony into which they emerge, even if it is a colony of a different species of ant. – Thus, an ant colony can increase its fitness (in terms of the number of reproductives it produces) by abducting workers of another species and “enslaving” them. – Example: Colonies of Formica sanguinea will raid colonies of Formica fusca, and abduct their workers for use as slaves. Colonies of victimized species have evolved defenses against slave makers, including abandoning nests and moving the colony to avoid being victimized. Formica sanginea, (which is also an aphid farmer). Formica fusca • Similar behaviors are sometimes seen in bees and wasps, but in those taxa, queens of a related species will take over the entire colony of the host species and “enslave” it, for instance, Polistes nimphus is a parasitic

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