Coevolution, Mutualism, Parasitism

Reading; Smith and Smith, Chapter 17 (read), ecological application 5 (after pp. 355, read.)

• 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. – The mosquito, an insect

ectoparasite, is the host for certain stages in the life cycle for many different parasites, including malaria

Presumably, these complex life cycles allow parasites toexploit ecological niches thatwould not be available to themof they simply went from onehost to a similar host.

Example• The guinea worm,

Dracunculus madeninsis, exploits two hosts during the course of its life cycle.

• Larvae swim in freshwater and infect a freshwater copepod. They feed within the still-living host, until it is swallowed by a human.

• In the stomach, acid dissolves the copepod, and the larvae emerge.

Within about a year, it (the female) has grownto a worm that can be threefeet long, living within an artery.It will bore a painful hole to the outside, it releases huge amounts of eggsinto the water whenever the hostbathes 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 the biofilm from our respiratory tract and allows our immune system to attack the function

But the bacteria that producedthat biofilm were selected to do so,and one reason was that sneezesare excellent disease vectors.

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

– intraspecific. • Interspecific brood parasitism • can be obligate or facultative.

• It is particularly common

• in birds, wasps, and bees.

Cuckoo wasp

– 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.

Kirtland’s warbler (50birds.com)

Brown-headed cowbird

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 fusca

Formica sanginea, (which isalso an aphid farmer).

• 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 paper wasp, which will take over colonies of the closely related Polistes dominulus. This is called social parasitism.

Polistes dominulus

How important is parasitism?• Parasites have a pervasive effect on the populations of

plants and animals. • A growing body of evidence supports the notion that

many parasites effective agents of population regulation, acting in many of the same ways as top predators, but more ubiquitous.– Parasite induced mortality may interact with limitation of

resources to produce population control. Populations that are under nutritional stress are frequently more susceptible to parasite-induced mortality.

– Microparasites which are transmitted from one individual to the next typically require dense populations of hosts to sustain themselves.

– Hosts develop immunity to most microparasites. This causes the disease to burn through its supply of susceptible individuals in a local area (it will grow exponentially, at first), and unless it can infect a new area, it will go extinct.

– Long-term persistence is facilitated by long-lived infective stages, by the physiological ability to avoid inducing immunity in the host, or by evolving so quickly that subsequent generations of the parasite can attack previously immune individuals.

– The more virulent diseases spread more quickly, but generally require denser populations of hosts to persist-over time.

• If a disease kills the host too quickly, that population of the parasite dies before begetting new populations of parasites. Thus, there is interdemic selection in favor of reduced virulence, but within hosts themselves, there is selection for increased virulence, because virulent individuals make more copies of themselves. Thus, diseases frequently evolve a balance, such that transmission rate is balanced with virulence.

– There is an old saying, “a good parasite does not kill the host.” In fact, this is only true if the parasite relies on living hosts for transmission.

– A good parasite does not kill the host before it has infected more hosts.

Example-the Black Death• Historically, human populations have been kept in check both

by resource limitation and by disease.– In medieval Europe, circa 1346, populations were close to an

all-time high, and widespread famines periodically killed thousands.

– Filthy, crowded conditions combined with poor nutrition to create perfect conditions for an outbreak of Yersinia pestis, a disease that is not primarily a parasite of humans.

– In many places, the human population was reduced from 40-65%, and did not recover for centuries because of repeated outbreaks.

– The disease required dense populations of humans, rats, and fleas, because it was both epidemic and epizootic.

• Yersinia pestis is a bacterium, normally a parasite of rodents, especially rats and marmots.– It is transmitted among rodent hosts by

fleas, mostly the rat flea, Xenopsylla cheopis.

– In Central Asia, in 1345, a particularly nasty strain jumped from Marmots to Mongols.

• The Mongols transmitted it to the Genoese, who spread it to Europe.– In Europe, it went back in forth from rat-

human transmission, to a human-human form (pneumonic plague).

– Eventually, it ran out of human hosts, but persisted in rodent populations, facilitating later outbreaks

http://www.insecta-inspecta.com/fleas/bdeath/Path.html

– In general, macroparasites, which generally rely on animal transmission vectors, and usually have more complex life cycles, can parasitize populations that are much less dense, and persist exist at much lower population levels.

– They require an efficient means of infecting each host in their life cycle. – Because they rely on indirect transmission, they do not “burn” through

all the available hosts. Instead, a few individuals harboring huge parasite loads cause the parasite to persist in a local are for large amounts of time.

– Parasites survival depends upon keeping a few of these “disease bags” around-which means prolific reproduction.

Effect of Parasite on Distribution of the Host

• It is thought that parasites have enormous potential effects on the distribution of hosts, though our reasons for thinking this are indirect; most of what persists today are parasite-host systems that are coevolved and permit the existence of both species.– Cases of introduced parasites suggest that parasites can

easily eliminate a host from major areas of the range, or drive it entirely extinct.

• Examples; Dutch elm disease, chestnut blight

Mutualism– Mutualisms are ecological interactions in which both partners

benefit (at least some of the time at least), either in terms of an increased potential for population growth, or because they are able to live in environments that are unavailable to them otherwise.

• Mutualisms fall along a continuum from facultative to obligate.

– Mutualistic symbioses are mutualisms in which both partners live in, on, or in very close proximity to each other.

– Mutualisms run the gamut from chance ecological interactions, to interspecific interactions that have coevolved over millions of years, in which one partner cannot hope to survive without the other.

Pollination• One of the most ubiquitous and important

mutualistic interactions in terrestrial communities involves animal pollination in angiosperms.– Angiospersms are either wind pollinated (no

mutualism) or animal pollinated. – Animal pollination is generally mutualistic, though it

can be parasitic.

• Animal partners include: bees, butterflies and moths, flies, beetles, thrips, hummingbirds, bats, and occasionally other animals.– These mutualisms range from obligate to facultative.

– Advantages to wind pollination-straightforward, effective when plants grow in dense stands, range of species is not affected by availability of mutualist pollinators.

• Disadvantages-uses a lot of pollen (energetically expensive), can be unreliable.

– Advantages to animal pollination-depending upon pollinators, can be reliable over moderately long distances, reduces amount of pollen plant needs to produce and thus is easier on the energetic cost of sex.

• Disadvantages- requires the existence of a pollinator, incentive must be provided to pollinator for their services.

Pollination Syndromes– Pollinators fall into several

general categories. – The selective pressures exerted

by these pollinators causes flowers to evolve certain sets of characteristics to attract the type of pollinators most likely to increase the plant’s fitness.

www.loc.gov

This is an Indonesian “corpse flower”, the world’s largest inflorescence (Amorphophallustitanum). It smells like dead people to attractcarrion beetles and flies

Pentstemon barbatus This is a great hummingbird plant.

borderlandnews.com

Pentstemon digitalis, a bee pollinated plant. Melittid bees are commonly specialized to feed on Pentstemon, though the flowers will accept any beethis is a Megachile bee. www.clc.edu

– Fungus gnats-smells like mushroom, flowers not terribly showy

– Moths; strong smell, open at night, white– Hummingbirds; usually red, long corolla, exserted anthers,

dilute nectar– Short-Tongued Bees; short corolla, purple-yellow-bee

purple, concentrated nectar– Long-Tongued Bees; longer corolla, purple-yellow-bee

purple-white, landing platform, concentrated nectar– Butterflies; long corolla– Bats-Big flowers, musky scent, dull colored, open at night,

copious dilute nectar– Carrion Beetles-smell like rotting flesh, weird looking

Example-Melittid Bees– The melittid bees are a family that includes about 100 species worldwide.

– Some are specialist pollinators (ie., obligate mutualists, others are generalists-facultative mutualism)

– All melittids are solitary bees.

• Ecological specialized species have life cycles timed to synchronize with the plants they forage upon. The bees are inactive as resting pupae for the rest of the year

Macropis steironematis from the UICgreenhouse, foraging upon catnip Nepeta catara.

Example:Caullauthidium isa small solitary bee.Males defendthe flowers upon which females specialize (Pentstemon).

– Pentstemon flowers provide pollen, a rich source of protein that the female bees use to provision nests, and concentrated nectar, which is an important food source for both adults and larvae.

• Pentstemon also have eliaphores-which provide energy-rich oils along with pollen as provisions for developing larvae.

– Most flowers in the genus are either white or purple, a color which is attractive to bees. Melittids are fairly large, generally, and Pentstemon flowers have a “landing pad” to make it easier to get in and get out.

• In return for nectar and oil, the flower gets an efficient pollinator. Melittid bees are very active, and can fly great distances, and will visit the same type of flower in search of resources-thus ensuring efficient pollination.

– On t• I.

– The mutualism is assymetric. Although Callauthidium specializes in Pentstemon, Bee pollinated species of Pentstemon will accept many different floral visitors.

• Other bees in the same family, such as Macropis, will visit many types of flowers.

• It is possible that the ecological specialization of some species of bees results from interspecific competition,

• OR it is possible that the only way for a species to carve out a particular ecological niche is to specialize-specialists that are active only a short period of time are not limited to habitats where flowers are continuously available all summer long.

• Specialization is definitely a factor limiting their range, since the right type of floral resource must be present for the bee to occur.

• On the flower’s end, it is not likely that the flowers evolved specialization per se, it is more likely that the most effective pollinators exert sufficient selective pressure that the flower evolves certain traits that make it most suitable for a particular species of pollinator.

– Remember the bumblebee snapdragon study? (Freeman and Herron, pp. 75) Note that the yellow color was selected against and the white color-spotted color scheme was favored. Bees are especially attracted to flowers with patterns.

• OR it is also possible that there is indeed selection for specialization, to increase the efficiency of pollination and avoid “wasting” floral resources..

Seed Dispersal• A major class of plant/animal mutualisms

involves seed dispersal. Certain plants, with heavy seeds, rely upon animals to disperse them.

• There is a great section in your book on seed dispersal (pp 328-330)– Why is Clark’s nutcracker the only seed predator

that effectively acts as a mutualist to the whitebark pine?.