Lec3 APS301 Slideshow

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Published on October 25, 2007

Author: Janelle

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Slide1:  APS 301. Co-operation & Conflict Professor Francis L. W. Ratnieks Lecture 3 Brood Site Pollination Mutualisms: Common Interests and Conflicts Between Two Species Slide2:  Relationship Party 1 Party 2 Mutualism + + Relationship Actor Recipient Parasitism + – Altruism – + Spite – – What is Mutualism? Slide3:  What is Mutualism? Mutualism is an arrangement where all parties benefit. Many symbiotic (living together) relationships are mutualistic. For example, many insects have micro-organisms living in their gut (e.g., termites and gut protozoa) or intracellularly which help in nutrition, particularly in species which live on a non-balanced or difficult to digest diet such as wood, plant sap, blood etc.). In previous lectures we have also seen that genes can usually be considered as mutualists. However, many symbiotic relationships are parasitic with one party exploiting the other. Slide4:  Brood Site Pollination Mutualisms The lecture will focus on obligate mutualistic relationships involving insect pollinators whose larvae also develop in the plants they pollinate (“brood site pollination mutualism”). Even though both partners need each other to reproduce, and even though the relationship is basically mutualistic, there is often an incentive for one party to overexploit the other, and hence conflict. In the next lecture we will also consider parasitic symbioses and see that the exploitation of the host by the parasite may vary from benign to harmful. Symbiotic relationships are a continuum from mutualism to parasitism. Slide5:  Pollination as a Mutualism The pollen of many angiosperms is moved between plants by insects and other animals. The plant, if it has both male and female function, receives two benefits. First, its pollen is transmitted to other plants. Second, it receives pollen from other plants to fertilise its own eggs. (Many plants have stigmas that reject their own pollen.) The pollinator usually receives an incentive to visit the flower in the form of nectar. Rewards also include the pollen itself, oils, and scents (used by male orchid bees, Euglossinae, to attract female bees). Slide6:  Pollination as a Mutualism Sometimes this relationship is not mutualistic. Some bees visit plants without pollinating them. Instead they work the flower from the side or even chew a hole through the corolla tube in which to insert their tongue. Some plants exploit pollinators. Some orchids mimic female bees and wasps and attract males who try to copulate with them, spreading pollen via attempted matings. Kiwi fruit (Actinidia) has both male and female function plants. Both have nectar-less flowers with pollen as reward. But the pollen produced by female function plants is in fact useless to the bees as food (and also useless in transmitting genes). It is just the non-digestible outer coating. Slide7:  Insect Cheating Plant Bumble bee (Bombus terrestris) worker taking nectar through a hole she has bitten in a honeysuckle (Lonicera) flower. Honeysuckle is normally pollinated by long-tongued moths. Slide8:  Bumble bee (left) and honey bee (right) taking nectar from base of corolla tube without pollinating. Some bees (including some bumble bees but not honey bees) can chew a hole through the corolla tube. Insects Cheating Plant Slide9:  Insect Cheating Plant Left. Honey bee (Apis mellifera) working Brassica flower from above, a position that results in pollination. Right. Honey bee inserting tongue from side in natural gap in flower, a position that does not pollinate. Slide10:  Plants Cheating Insect Pollination of W. Australian orchid Drakaea glyptodon by thynnine wasp Zaspilothynnus trilobatus. Top Left. Pre-mating posture of wingless female wasp. Top Right. Orchid flower. Bottom. Male wasp tipped back by flower into position needed for pollination. Slide11:  Orchid flower resembles female wasp Male wasp copulating flower Plants Cheating Insect Slide12:  Plants Cheating Insect Insect visitors to Ophrys orchids. Left. Male solitary wasp Camposcolia ciliata on O. speculum; Centre. Male solitary wasp Argogorytes mystaceus on fly orchid, O. insectifera; Right. Male solitary bee Andrena maculipes on O. lutea. Slide13:  Bee Orchid Slide14:  Bee Orchid Slide15:  Brood Site Pollination Mutualism Most plants are visited by many pollinator species, and pollinators visit many plant species. Some pollination mutualisms are more specific. In some the plant provides a home for the larvae of the pollinating insects instead of or in addition to the more traditional rewards of pollen and nectar. We will look at two of these. We will see how the relationship between figs and fig wasps has less conflict in it that that between yucca plants and yucca moths. Fig/Fig wasp mutualism Fig plants (Ficus spp.) and Fig wasps (Agaonidae) Where: worldwide in tropical and subtropical countries Yucca/Yucca moth mutualism Yucca plants (Yucca spp.) and Yucca moths (Tegeticula spp) Where: N. America Slide16:  Fig - Fig Wasp Mutualism Slide17:  Strangler fig from SE Asia. The fig climbs an existing tree which it eventually kills. Figs are perennial plants ranging in size from a small bush to a massive tree. Photo courtesy Steve Compton Slide18:  Photo courtesy Allen Herre Female fig wasp entering fig via ostiole. Panama Slide19:  Cross section of fig at time of entry of mother wasps. Panama. Photo courtesy Allen Herre Slide20:  Top Left. Female fig wasp, Blastophaga quadraticeps. Bottom Left. Male of same species. Right. Female fig wasp, Ceratosolen arabicus, laying an egg inside fig. Forelegs are raised to extract pollen from thoracic pockets, where it is stored by the wasp. Fig Wasps & Figs Slide21:  Fig Wasp Life Cycle 1. Mated female wasp enters fig & lays eggs in fig flowers inside fig 2. Wasp larvae develop 3. Male (wingless) and female offspring wasps emerge & mate 4. Mated female wasps disperse to fig flowers on other fig plants. Slide22:  Photo courtesy Allen Herre Cross section of fig at time of emergence of progeny wasps. Panama. Slide23:  Male fig wasps showing absence of wings and large mandibles for male-male fighting. Panama. Photo courtesy Allen Herre. Slide24:  Fig Wasps & Figs The life cycle is complex, but you must understand it. A fig is an inflorescence arranged so that the flowers are all on the inside of a cavity (“synconium”). The flowers inside are connected to the outside world by a small hole in the fig (“ostiole”). One or more mated and pollen carrying female fig wasps, usually of a particular species per fig species, enter the ostiole. They pollinate the flowers, which are all female at this point, and lay eggs in some of them. The wasp larvae develop into adults as parasites of some of the fig flowers. Slide25:  Fig Wasps & Figs Offspring male and female wasps emerge inside the fig and mate. Male fig wasps often fight with each other. The female wasps collect pollen from male flowers that mature inside the fig at this time. The mated, pollen-carying, females now leave their natal fig through a hole in the side cut by the male wasps. They disperse to other fig plants which have young figs to enter. The fig matures after the wasps leave. Figs are eaten by vertebrates who disperse the seeds in their faeces. The mother fig wasps did not lay eggs in all female flowers, so the fig does produce seeds. Slide26:  Fig Wasps & Figs In addition to pollinating fig wasps there are species of fig wasp (Torymidae) which do not enter the fig to lay their eggs. Instead they use a long ovipositor to lay eggs through the outer coat of the fig without having to enter. These do not pollinate the fig and so are parasites. They are parasites of both the fig plant and the fig wasp. They need the pollinating (mutualistic) fig wasp to pollinate the fig that they lay eggs into. They need the fig for their developing larvae. Slide27:  Mutualistic & Parasitic Fig Wasps Ostiole Parastic fig wasp Pollinating fig wasp Outer coat of fig Flowers Flowers Slide28:  Parasitic female fig wasp (above), with long ovipositor and pollinating fig wasp (below), with shorter ovipositor. Panama. Photo courtesy Allen Herre Mutualistic & Parasitic Fig Wasps Slide29:  Cost to Fig of Parasitism by fig wasps Even though a large proportion of the fig’s ovules are parasitized by wasp larvae, the cost to the plant is small. Why? Because most ovules result in female wasps that transmit pollen from the same plant. Optimal sex ratio is normally 50:50. So if half the flowers result in female wasps (male function for the plant) and half in seeds this is fine for the plant. However, some wasps reared are males. Males do not transmit pollen. But they help by cutting exit holes in the fig. (In many fig wasps there is LMC, so most of their offspring are females anyway.) Slide30:  Cost to Fig of Parasitism by fig wasps The fig benefits by having some but not all of its ovules parasitized Janzen (1979) – suggests ways in which plants can control wasp entry over evolutionary time (also Herre 1996) e.g. tighter ostiole, shorter ostiole opening period, more rapid fig development “How many babies do figs pay for babies?” 41-77% of seeds per fig killed (mean of means = 55%) (but pollinating and non-pollinating wasps were not distinguished) Slide31:  Defence Against Non-Pollinating Wasps These wasps have a long ovipositor. Fig may keep them out by evolving a thicker skin. Could be expensive to do this Will probably cause an arms race. With wasps evolving longer ovipositors. Who would win? Fig or parasitic wasp? Slide32:  Gynodioecious figs. Figs from 2 plants. Left: female function plant has only long stigmas and nowhere for wasp to lay her eggs; Right: male function plant has short stigmas allowing oviposition by wasps. SE Asia. Photo by Steve Compton. Slide33:  Gynodioecious figs Some Old World figs are “gynodioecious” Some plants have only female flowers Some plants have female and male flowers Female only plants – all flowers have long styles Female wasp enters, pollinates but cannot lay eggs Fig produces seeds but no wasps – functionally female GOOD FOR PLANT, BAD FOR WASPS Why do female wasps enter female-function figs? Slide34:  Yucca - Yucca Moth Mutualism References Pellmyr, O., Huth, C. J. 1994. Evolutionary stability of mutualism between yuccas and yucca moths. Nature 372: 257-260. Pellmyr, O., Leebens-Mack, J., Huth, C. J. 1996. Non-mutualistic yucca moths and their evolutionary consequences. Nature 380: 155-156. Slide37:  Female yucca moth taking pollen from stamens of a yucca plant. USA. Photo courtesy National Geographic magazine Slide38:  Yucca moth with bundle of pollen in her maxillary tentacles, ready to pollinate yucca flowers. Slide39:  Female yucca moth pollinating the stigma of a yucca plant. USA. Photo courtesy National Geographic magazine Slide40:  Female yucca moth using ovipositor to lay eggs below stigma of yucca plant. USA. Photo courtesy National Geographic magazine Slide41:  Yucca Plant & Yucca Moth Left. Yucca flower. The stigma is centrally located and surrounded by stamens that produce pollen. Middle. Yucca moth deliberately obtaining pollen from stamens. Right. Moth mouthparts are specially modified for handling pollen. Slide42:  Yucca moths & Yucca plants The life cycle is less complex that in the fig wasps. Yucca plants live in N. America, particularly in the arid western states. Each spring a plant produces an inflorescence of many flowers. Yucca moths hatch from their pupae in the soil and mate. The females deliberately collect pollen from the stamens of plants and deliberately use this to pollinate the stigmas of the flowers of other plants.The moths have specialised mouthparts for handling the pollen. The pollinating moth also saws a slit in the ovary of the flower it is pollinating and lays eggs. The eggs develop into larvae which feed on some of the developing seeds. Slide43:  Yucca moths & Yucca plants Plants can drop (abort) flowers into which too many eggs have been laid or which have not been pollinated adequately. Moth larvae pupate away from the plant. Thus, a female moth will probably not pollinate the plant on which it develops. In contrast, a fig wasp does transmit the pollen of the plant in which it developed. Thus, the interests of plant and insect are more closely aligned in the fig system than in the yucca system. As with the fig wasps, there are species of yucca moths which do not pollinate. Slide44:  Abortion (Dropping) of Yucca Flowers The graph (Pellmyr & Huth 1994) shows that when flowers are aborted, they are normally dropped before the moth eggs hatch. In this way the plant can prevent itself from being overexploited by having too many eggs laid, which would result in most developing seeds being eaten. Slide45:  Abortion (Dropping) of Yucca Flowers The graph below (Pellmyr & Huth 1994) shows: Left, that flowers with more eggs are preferentially dropped. Right, that flowers with very few oviposition scars are also more likely to be dropped. This suggests that the plant is sensitive to being overexploited (too many eggs) but also drops flowers that may not have been sufficiently pollinated (few scars meaning not much visitation by moths). Plants do not have the resources to mature all flowers so selectively abort those of least benefit. Slide46:  Non-Mutualistic Yucca Moths As in figs, some pollinators take advantage “Cheater” non-pollinating species of Tegeticula have evolved Consume seeds but do not pollinate 76% seeds eaten in population of plants with pollinator and non-pollinator speces; 26% with pollinator only Tegeticula yuccasella is probably a complex of numerous species, both pollinators and cheaters Slide47:  Non-Mutualistic Yucca Moths There are two types of cheater Early cheaters Adults active during latter part of flowering Oviposit into fruit wall a few days after pollination Late cheaters Use elongated ovipositor to cut into full-sized fruit This is ca. 2 weeks after pollination AND after the abortion phase (cunning!) If both types occur, up to 85% of seeds may be destroyed Slide48:  Phylogeny of Yucca Moth Strategies The phylogeny of yucca moths on the following slide (Pellmyr et al. 1996) was constructed using 2100 base pairs of mitochondrial DNA. Each end point represents a moth. Above this are the names of its host plant and the USA State in which they live. The phylogeny enables the following inferences to be made. 1. There is a single origin of active pollination of yucca plants by moths. 2. There is a single origin of non-pollinating via “early cheating”. (Early cheaters lay their eggs in yucca flowers a few days after they have been pollinated, and after the flower abortion period.) Slide49:  Phylogeny of Yucca Moth Strategies 3. There is either one or two origins of non-pollinating via “late cheating”. (Late cheaters have an elongated ovipositor and lay their eggs in the swollen seed pod of the plant c. 2 weeks after pollination.) 4. Within a single plant population with two moth species, one pollinating and one non-pollinating, the two moths are not closely related phylogenetically. This suggests that non-pollinating arose when one moth species colonised an area in which plants were pollinated by another species, allowing one species to evolve to be a non-pollinator, rather than a single species both pollinating and non-pollinating. 5. Non-pollination evolved c.61-66% of the time since active pollination. Slide50:  Phylogeny of Yucca Moth Strategies Y. whipplei sCA Y. whipplei nCA Y. brevifolia NV Y. schidigera CA Y. torreyi TX Y. utahensis UT Y. elata AZ Y. baccata NM Y. intermedia NM Late Cheaters (non-pollinators) Evolution of active pollination of yucca plants by yucca moths Outgroup Early cheaters (non-pollinators) Evolution of Tegeticula yucasella complex of yucca moths Slide51:  Effect of Cheating on Plants The figure below (Pellmyr et al. 1996) shows the proportion of seeds of Yucca filamentosa destroyed by yucca moths in populations with and without cheaters (= non-pollinating moth species) in Ohio and Tennessee. A sample of 25 fruits were collected in each population. Two populations had cheaters and six did not. The proportion of seeds destroyed was only 26% without cheaters but role to 76% with cheaters. Slide52:  Things to Ponder Why don’t fig wasps avoid female function figs? Why don’t fig wasps over-exploit the plant by laying eggs into all the flowers? How might a fig plant be able to able to prevent over-exploitation? What are the similarities and differences between the fig/wasp and yucca/moth mutualisms? How can a plant make decisions, such as when a yucca plant aborts flowers with excess eggs? Parasites have evolved in both mutualisms. Why is this? Is it inevitable? This is an ongoing area of research – see references, search Web of Science etc. if interested

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