Essays 4

Outline the wide variety of mechanisms of pollen and seed dispersal which can be found in plants of the Mediterranean ecosystem. Discuss possible explanations for this diversity.

In the Mediterranean ecosystem a principle coordinate analysis of reproductive characteristics showed that there were three major trends in shrubs.
1) Small flowered, fleshy fruited with one or few seeds, pollinated by small insects.
2) Large flowered, dry fruited, many seeded, hermaphrodites pollinated by large, efficient insects carrying large loads.
3) Small flowered, rewardless, mono- or dioecious, pollinated by the wind.
Dispersal is by bird and mammal (mainly the fleshy fruits), ants, wind and perhaps even reptile.

In Mediterranean ecosystems water and nutrient stress are the main environmental influences on shrubs. The different breeding systems may be different strategies for maximizing reproductive output.

1) The entomophilous dioecious species. In this type of plant it is common to find lower nectar production in the female plants as nectar production has been shown to reduce seed production. So there is a division of labour with the female devoting resources to fruit and seed formation, and the male to pollinator reward. These species are predominantly relics from tropical Tertiary forest environments, and are pollinated mainly by nectar-seeking insects.

2) The hermaphrodite entomophilous group. This is the largest group, they have small-seeded fruits some of which need fire to break dormancy. Fire is a common occurrence in the Mediterranean type ecosystems, so this group may have evolved under the existing Mediterranean conditions. Many do not produce nectar, but copious pollen, in one, Cerinthe major, many individuals produce little or no nectar while a few produce a lot. Some of the orchids are completely rewardless with neither nectar or insect-available pollen. This has been termed "pollination on the cheap".

So in all cases it would seem that nutrient and water stress are influencing the reproductive strategies of these plants. The three main trends show the differing ways of allocating resources towards reproduction and consequently this resource allocation affects the method of pollination and dispersal.

3) The wind pollinated group have been found to produce pollen of lower calorific value than animal pollinated species. This may be because wind pollination is a less efficient method, and so requires the production of greater quantities of pollen. It was also found that in Phillyrea latifolia (hermaphrodite and male plants) there was greater pollen production in male plants than in hermaphrodites, which may indicate an evolutionary trend towards dioecy enabling the hermaphrodites to concentrate on the female function.

Discuss the origin and evolution of sclerophyllous vegetation, with emphasis on plants of the Mediterranean.

Mediterranean climate. The history of a separate Mediterranean climate really started 6 million years ago. The collision of India into Asia caused the rise of Tibet, and Africa's movement north caused mountain building in Spain and North Africa. The Tethys sea dried up leaving a huge salt pan. Presumably most vegetation in the area that was subject to wind blown salt dies, also the temperature was very high and the area arid. Then the Mediterranean opened and a fairly lush vegetation grew with invasions from the surrounding areas. The onset of glaciation brought climatic fluctuations which gradually settled to resemble more or less the climate we see today. This caused a gradual disappearance od plant unable to withstand periods of summer drought - these would either die out or retreat north - the south was even drier. So there was a gradual change favouring plants that could withstand quite long droughts on a soil that had never been very nutrient rich and may still have contained salt which could rise to the surface by capillary action. Added to this were the frequent volcanic eruptions causing local extinctions, but more widespread depositions of pumice etc. During this time such plants as remained would have to evolve mechanisms to cope with the increasingly harsh and unpredictable climate.

Sclerophyllous leaves are found on plants from dry, hot and nutrient poor habitats. They are usually relatively small, with small thick-walled cells, and often both faces of the leaf are the same. They are usually covered by a thick cuticle and epidermis. Some exude wax or resin, and the stomata are often sunk into the leaf and may be protected by hairs. The leaf may also roll up to protect the stomata from excessive evaporation; this is common in plants from salty areas. Most of these modification appear to have arisen to conserve water, but they are also useful in nutrient poor habitats. For instance smaller leaves require less nutrients as do leaves that last for more than one year.

Sclerophyllous vegetation. This type of vegetation takes its name from the leaf which is the most visibly adapted part, but the vegetation share some other common adaptations to drought and poor soil. Many of the species have protected roots to maximise nutrient uptake in areas of local enrichment. Many are legumes or other species that have symbiotic relationships with nitrogen fixing bacteria. Many make use of mycorrhizal associations again enhancing nutrient as well as water uptake. A few are even carnivorous, so extending their sources of nutrients. And some are parasitic on other species, or their own species, or even their own roots. Sclerophyllous vegetation usually have small flowers which may be wind pollinated or pollinated by small insects, but they do not invest heavily in nectar and other pollination rewards. However there is fairly heavy investment in producing a single, or few seeded fleshy fruit. Most have the ability to spread vegetatively, which is useful as most are dioecious. So as far as reproduction goes there is a distinct pattern of investment in pollen as opposed to nectar, and reproduction without sex. This is taken to extremes by some plants, e.g. Osyris alba, where the female flowers are rewardless but mimic the male so pollination can occur. Also the female flowers occur less often than the male.

Origins. It is postulated that the sclerophyllous vegetation of the Mediterranean regions has evolved from the lusher tropical vegetation that may have existed before the onset of the climatic changes. And the various adaptations we see today came about in order to cope with the increasingly harsh conditions, and those that couldn't cope simply went extinct.

The future. For a plant so limited in nutrients and water the investment in the production of fleshy fruits seems excessive. But these fruits are dispersed by birds, and birds like fleshy fruits. This is another common link to the lush tropical flora that is believed to have existed previously. Recently it has been noticed that ants play a greater part in dispersal of seed than had previously supposed. If a plant relies on ants entirely it will have no need for fleshy fruits, as the ants are interested in the lipid-rich elaiosome around or in a groove of the seed. So there may be a general trend in the sclerophyllous Mediterranean vegetation towards dry fruit dispersed by ants. This is common in other Mediterranean climates, but there may be dangers ahead as in some places the Argentine ant, Iridomyrmex humilis, has displaced the native ants. The Argentine ant is not interested in elaiosomes, so will not disperse seed. These plants may also be able to reproduce vegetatively, but after fire, seed dispersal is important. And unless the seeds are cached by harvester ants they are unprotected. So regeneration after fire is severely limited. This may lead to soil erosion and desertification.

Outline pollination mechanisms

Mimicry. The hammer orchid in Australia mimics the wingless female wasp. The female wasp climbs up a flower and releases a perfume to attract a mate. The mate flies in (males have wings) and carries the female off to mate on the wing. After mating he deposits the female on a stalk or a flower. The orchid has a small flower on a long stalk. The flower is in two parts hinged together. One part resembles the female wasp, and the other is a pad with two pollen sacs. The male wasp lands on what he thinks is a female, then tries to take off on the mating flight, but he cannot. The hinge catapults him against the pollen pad, and this happens at every attempt he makes to take off with what he thinks is a female wasp. After a few hits the pollen sacs peel off the anthers and stick to the back of the male wasp. The pollen sacs are so placed that another orchid will be fertilised when he repeats the process. The flower succeeds in its deception by flowering a few days before the female wasps emerge. If it flowered as the females were emerging the males would not be deceived, so good timing is essential.

Deception. The African water lily on its first day of blooming has hundreds of showy stamens and a nice bright flower. This attracts hover flies. On day 1 in the centre of the flower is a pool of poisonous liquid that looks like nectar. When the hover fly lands on the stamen it falls into this liquid as the stamen are very smooth and overhangs the pool. The hover fly cannot climb out as there is nothing to get a grip on. So the hover fly dies in the poisonous liquid. At night the flower closes and the liquid washes pollen from day two water lilies off the body of the dead hover fly. This pollen fertilises eggs at the bottom of the liquid. On day two it is quite safe for hover flies to visit the lily as the stamens no longer hang over a poisonous pool as the pool has gone. The mechanism is to ensure pollination between flowers of different ages, e.g. day one and day two flowers.

Deception. On an island off the Sardinian coast the dead horse plant, an arum lily, Helicodiceros sp. gives off a perfume of rotting meat. This attracts blow flies who normally lay their eggs on rotting meat on which the larvae need to feed when they hatch. The flies land on the plant and make their way to the source of the smell inside the flower. The structure inside is very elaborate, and on the way down there are guard hairs prohibiting escape. The flies are trapped inside the flower. The flies can feed on nectar and lay their eggs, however the larvae will die as there is nothing for them to feed on when they hatch. On the second day the stamens shower the captive blow flies with sticky pollen. And on the third day the guard hairs wither and the flies can escape to pollinate other Helicodiceros as the pollen rubs off their bodies as they make their way down inside the flower.

Water pollination by surface tension. Aquatic ribbon weeds have separate male and female flowers. The female puts up a long coiling stalk that reaches the surface. And the surface tension of the water cause a slight depression or dimple at this point. The flask of the male plant is under water. Oxygen bubbles force it open and this releases tiny male flowers which drift to the surface if they are not eaten by fish on the way. The male plant continues releasing flowers even at night. On reaching the surface the petals open, push down and a pollen stalk is raised. So the male flower is raised up while the female flower is waiting in a depression. The male flowers drift about, and some are eaten by fish, but if they reach within about 2 cm of the female flowers they start to slide down into the depression and the eggs are fertilised.

Reward. Dryas octapetala blooms in Greenland in the summer. The head of the flower is on a stem that rotates to follow the sun to within 2°. The sun never sets in the summer in Greenland. In the cup formed by the petals it can be as much as 10° warmer than the surrounding air, and insects need warmth as even the summer is cold in Greenland. So the pollinating insects are rewarded with pollen and nectar and central heating. The pollen will brush off their bodies when they visit the next Dryas.

Reward. Coryanthes sp., the south American bucket orchid tree epiphyte opens its flowers in the early morning and drips fluid from two glands into its bucket. The fluid gives off a scent attractive to male bees. The male bees arrive and rub off some waxy substance from the flower. Many bees are attracted and there is some fighting and pushing, also the aromatic fluid seems to have a slightly intoxicating effect. The upshot of the crowding, excitement and intoxication is that a bee will eventually fall into the bucket of fluid. There is only one way out of the bucket and that is through an escape tunnel. At the exit the tunnel closes, gripping the body of the male bee while gluing two pollen sacs to its back. The bee is trapped for ten minutes, just the amount of time it takes for the glue to dry. When other bees fall into the bucket they are allowed to pass through the tunnel unhindered, but if one is carrying pollen sacs on his back the sacs will be pulled off. This pollination is species specific, i.e. that plant can only be pollinated by a particular species of bee. This is a precarious position to be in if something happens to the pollinator.

Prey mimicry
The orchid Epipactis consimilis has small aphid-like structures which attract adult hoverflies whose larvae eat aphids. As the female hoverfly lays her egg she pollinates the flower, but when that egg hatches it will find nothing to eat.

Aggressive mimicry
Male Centris spp. bees in tropical America are fiercely territorial. Orchids in the Oncidium genus exploit the male bee's willingness to attack anything resembling another bee as their flowers are on long stalks that sway in the slightest breeze. The males repeatedly head-butt the flowers, and as they do pollinina detaches from the flowed and sticks to the forehead of the bee - perfectly placed to pollinate the next receptive flower that is head-butted.

Discuss the possible roles of "maternal dominance" and "haplodiploidy" in the evolution of social behaviour in Hymenoptera.

Eusociality arose independently in Hymenoptera with multiple origins in wasps and bees. Comparisons of social behaviour extant show that there are three possible historical pathways from solitariness to sociality.
1) A single queen lives longer and so association with offspring in increased.
2) Several unrelated queens of the same generation nest colonially with some reproductive division of labour, the association lasting for only one generation.
3) Several related queens nest colonially with increasing reproductive division, the association of queens and daughters arises through increased longevity.

Haplodiploidy or kin selection. By reducing personal survival and reproduction workers increase survival and reproduction of genes they share with other members of the colony by common descent. Individuals suffer, but colony and genes flourish. Hamilton's rule
rB - C > 0
where r = the coefficient of relatedness
B = the benefit gained by the recipient
C = the cost to the donor.
means that sisters will be more related to each other (75% on average) than to their mother (50%) the queen, and only 25% related to their brothers, so it will benefit more of a worker's genes to rear more workers.

Maternal dominance. The mother is able to neuter and/or control some offspring so as to make them help her produce more of her offspring and none or few of their own. Mother -> daughter r = 50%, Mother -> grandchild r = 25%.

It is easy to see that both haplodiploidy and maternal dominance could contribute either singly or together to the evolution of sociality. And as sociality has arisen numerous times it is unlikely that just one theory is the only contributor. It may be a varying mix of both which changes according to the circumstances each time.

1) All Hymenoptera are haplodiploid but only 6 - 8% of those so far documented are eusocial, so haplodiploidy does not inevitably lead to eusociality. Also Isoptera are not haplodiploid and yet are eusocial, so haplodiploidy is not a prerequisite to eusociality.
2) r is frequently not as simple as it seems. Many queens mate more than once diluting the relatedness. Although they may mate with brothers which dilutes it slightly less. Also the queen may manipulate the way she uses the sperm packets, or even use just one.
3) Many nests contain multiple queens especially in the tropics. This will dilute r of workers. Though if the queens are sisters this will lessen the dilution.

In both haplodiploidy and maternal dominance success depends on the queen producing all or most of the eggs. This can be achieved in tow ways: 1) bullying or 2) by pheromonal control. The first case is fairly easy to observe in the wild. A queen will nudge, bite and head butt a worker or another queen. This constant physical bullying inhibits ovarian development in the bullied. And if the bullied is manoeuvred towards the brooding area it seems to be instinctive behaviour for her to help. With pheromonal control the workers will only lay eggs when the control is incomplete or weak. In most social Hymenoptera a varying amount of eggs are laid by the workers, so it seems that the dominance of the queen is rarely complete. In Bombus atratus 90% of the males are worker produced. And in many species it is the production of males that signals the beginning of the end of the nest. In many Bombus spp. and in other eusocial species it is the workers feeding the larvae that decides whether the larvae will become queens or workers.

With maternal dominance one would expect to find sexual forms around the limit of the queen's dominance. This is found in the African weaver ant Oecophylla longinoda who completely inhibits her daughters from laying all but eggs subsequently eaten. Only in nests outside her influence or territory do the workers produce males.