~ Pollination Ecology of the Small Whorled Pogonia,
Isotria medeoloides ~
Pollination ecology is the study of the sexuality of plants. It is concerned with pollination vectors, attractants, flower morphology and the environment. Research in this field has significant bearing on evolution, systematics, conservation and propagation (Catling and Catling, 1991).
The small whorled pogonia, Isotria medeoloides (Pursh) Rafinesque, is a very rare terrestrial orchid species that may occur in a large geographical range in eastern North America. It has been listed since 1982 as an endangered species under the U.S Endangered Species Act.
Typically, members of the Orchidaceae are animal-pollinated tropical herbs, growing epiphytically in humid forests. The stamens and styles are fused into a bisexual structure, the column. The tip of the column presents pollen grains in coherent masses, the pollinia. A viscid disc, holding the pollinia on a stem is passively or actively glued to the pollinator. Most often in terrestrial orchids, the viscid substance is applied on the pollinator and the pollinia adhere to it as the animal enters deeper to the labellum. Pollination is an ďall or nothingĒ event leading to the production of up to several million miniscule seeds containing barely enough food to sustain the first three cell divisions of the embryo. A mycorrhizal association is essential to successful germination. Many fungi are suitable symbionts such as the common Rhizoctonia sp. (Nilsson 1992).
The rarity of the species cannot be attributed to human activity to any great extent. It appears to be tolerant of a wide range of conditions. Individual plants are long-lived, self-pollinating, produce abundant seed and are without major leaf or seed capsule herbivores. The paucity of herbivores was established by Mehrhoff (1983) who collected extensive data on several populations. Natural rarity presents unique problems for the reproduction and establishment of new colonies of I. medeoloides (Mehrhoff 1989a).
Range and Habitat
In the United States, the two states with the largest number of I. medeoloides are New Hampshire (28 colonies, totaling 2265 plants) and Maine (14 colonies, totaling 925 plants). The thirteen other states in its range have between 3 to 200 plants in 1 to 7 colonies (Keenan 1988). Most populations are situated in flats or at the base of south or southeasterly slopes in mesic to dry-mesic deciduous woodlands. They tend to grow on low-nutrient, acidic quartz-derived loams, and mycorrhizal association probably help them be more efficient in their use of soil nutrients and water (Ames 1922). Almost every site where they occur can be described as second-growth deciduous or deciduous coniferous forest, where an open herb layer, low to moderate amounts of shrubs, and a relatively open canopy (Mehrhoff 1989a). They have a preference for disturbed sites such as old agricultural fields and windthrow. They associate frequently with tree species Acer rubrum, Quercus rubra (Mehrhoff 1989a), and the terrestrial orchid Goodyera pubescens (Whiting and Catling 1986).
In Canada, one colony consisting of five individuals was located in 1977 in southern Ontario, but the plants failed to reappear in the spring of 1983 (Stewart 1983).
Plants of I. medeoloides do not exceed 25 cm. The leaves are five to six in number, in a whorl topping a single stem. The flowers appear singly or exceptionally in pairs and are light green to very light green, with narrow, slightly curved sepals and stubby petals arching over the labellum. The labellum is approximately the same length as the petals, nearly white and about 15 mm in length. The flower is held on a short peduncle arising from the center of the whorl (Petrie 1981). It does not possess a detectable odor, nor does it offer nectar or pollen to potential pollinators. The flowering period lasts from 2 to 13 days, and will occur from late April to mid-June depending on the location and climate. (Homoya 1977).
Reproduction and Pollination Ecology
Vegetative reproduction is rare (Thibodeau 1983). This may be a sign of intolerance for intraspecific competition in a nutrient-poor environment, or a strategy to promote establishment from seed in the few available safe sites and for an autogamous plant to increase the likelihood of genetic changes. The latter may be especially important since Mehrhoff (1983) has revealed I. medeoloides to be almost exclusively autogamous. In addition, no insect pollinator has been observed to pollinate the flowers despite extensive observations. Direct observation shows that after the flowers open, the anthers begin to droop down downward and then out of the anther socket. At this time, the pollen slips out of the pollen sac, protruding about 2 mm until it comes into contact with the stigmatic surface. Eighty-three percent of the flowers are reported to be successfully pollinated in this fashion. The process may take 48 hours (Merhoff 1983). This mechanical selfing system is evolved from a simple modification of the orchid morphology: the rostellar flap is reduced and the positioning of the anther modified (Merhoff 1989a).
Autogamy may be the strategy of choice for a species composed of widely-spaced, non-clonal individuals. In fact, although cross-pollination would not be detrimental per se, a visit by a pollinator resulting in the removal of the pollinia has such as remote chance of being transferred to another flower that the event would weigh more heavily as a loss of reproductive output for an entire year (they flower only once a year) than the speculative and unlikely gain of genetic diversity. Autogamy has long been considered detrimental to genetic variability (Faegeri and van der Pijl, 1979), but there is some evidence that diversity can indeed occur in certain autogamous breeding systems (Allard et al 1968). It may also protect the species from rapid genetic changes that would later prove detrimental if the environmental changes are only transient. The tolerance of I. medeoloides for an apparent variety of site conditions may be a phenotypic sign of genetic heterozygosity (Merhoff, 1980). However, site characteristics have not yet been adequately measured and quantified; they may be far more similar than visual observation may imply.
Autogamy may evolve when pollinators become extinct, or if the range of the species comes to exceed that of its specialized pollinator. Evolutionary or learned responses from pollinators may cause them to avoid certain flowers that offer no reward like those of the genus Isotria, resulting in 'abandonment' by the pollinator (Faegeri and van der Pijl 1979). In the case of I. medeoloides, the scarcity of flowers may be at the center of selection pressure in favor of autogamy, rather than the disappearance of pollinators. A pollinator visit to a rare flower would in fact decrease the reproductive success of a plant if the probability of it carrying the pollen to another flower of the same species is too small. Reduced alliteration is a key element in pollinator avoidance of this species. The flowers are drab, offer no reward and self-pollinate promptly after opening.
Autogamy increases the probability that a seedís genotype will closely resemble its parentís. As a consequence, genetic variety within a closed period of time will decrease, whereas the genetic dissimilarity may increase when two distinct populations are compared. Increased homozygosity allows a population to be more closely adapted to its environment; negatively, it may be considered having painted itself into a corner. In general situations, autogamy is a prerequisite for successful establishment of long-distance migrants, since a single plant outside of cross-pollination distance from its congenerics is a situation that precludes the possibilitiy of exogamy. Because suitable habitats for I. medeoloides are far apart, this factor is very important for the fitness of the individuals and the very existence of the species. Facultative autogamy restricts gene flow, but a relatively long-distance dispersal like that which occurs in the tiny-seeded Orchidaceae may slowly overcome this effect. (Catling 1983). This would not be the case for an obligate autogamous species which would maintain its genetic identity through generations even in close proximity to conspecifics.
Ornduff (1969) has proposed a list of dichotomous characteristics to distinguish between autogamous and xenogamous species. Characteristics indicative of autogamy that are present in I. medeoloides, include: wide distribution, self-compatibility, few flowers per genet, short peduncle, small petals and sepals, flower funnelform, petals entire or less emarginate, colors with low contrast, greenish, whitish, or monochrome, nectaries and pollination absent, flowers scentless, anthers (pollinia) close to stigma with synchronous maturation, fewer pollen grains and ovules per flower, with fewer ovules not maturing to seeds and a large proportion of fruits maturing. The only character in the list that it not usually indicative of autogamy is diploidy, as autogamous plants rather tend towards polyploidy.
It is interesting to compare I. medeoloides, with its close relative I. verticillata. This relative is somewhat less rare and is found in larger clonal colonies. I. verticillata has larger pink scented flowers which are attractive to bees. It flowers abundantly and is pollinated by a variety of bees. Yet with all its efforts I. verticillata has a pollination rate of only 21% compared to I. medeoloides with 83%. Fruit set is only 6% (Mehrhoff 1983), and there is greater predation on both leaves and seed capsules. I. medeoloides tolerates soils with pH values between 4.0 and 4.2, while I. verticillata is comfortable in a wider range of values, from 4.2 to 5.1.
There are three main types of geographic distributions of rare species (Mayr 1963).
1) The range of the species is restricted to isolated localities yet they occur in large numbers at each locality. This is the case for I. verticillata.
2) Species found in very small numbers widely dispersed in each community where they grow.
3)Species occurring as a few hundred individuals at scattered localities, or restricted to a small geographic area or a single population. This is the case for I. medeoloides.
Herbivory is less intense on I. medeoloides than in I. verticillata, and this may be a consequence of its greater rarity (Mehrhoff 1983). The probability of a herbivore finding the next plant of the same species decreases with the distance between plants. An acquired taste is also less likely to be developed by the local herbivores. Some herbivores may even avoid unfamiliar vegetation (Drury 1980).
The most highly modified floral structure of the Orchidaceae, the labellum, is strongly associated with animal pollination. This suggests that in general, autogamy would be derived from xenogamy in this family (Catling 1983). It is therefore possible to speculate that I. medeoloides evolved from I. verticillata. Mutations or local selection pressures may have allowed I. medeoloides to survive in more acidic soils where there are even fewer competitors. If this new habitat was or became infrequent, autogamy may have been the best possible strategy to maximize fitness. One may suggest that homozygosity acts as a form of selection against deleterious recessive genes. The presence of inferior specimens would be a prerequisite for the maintenance of the population of recessive genes; when suitable habitats are exceedingly rare, there is little room to keep these disadvantageous alleles along with the better ones, even in the presence of heterozygous superiority. Not only would they compete for sites with the better specimens, bet in marginal situations with low nutrient availability, they may never even reach flowering size. Although it is as vital for I. medeoloides to evolve superior individuals as for any other species, this cannot be accomplished by the acquisition of heterozygous vigour. Genetic uniformity may sometimes be of greater value than diversity.
Deception pollination occurs frequently in orchids. If loss of pollinator reward results in reduced attraction and visitation, why would deceit evolve? Temperate woodland terrestrial orchid populations are often sparse, they are confined to some of the most difficult growing conditions, such as deep shade and acid soils. Resources are so scarce that many species cannot tolerate any neighbors in and around their root zones. The savings in carbohydrates and amino acid resulting from a deceitful flower is especially important in a marginal environment. It has been suggested that when population densities are insufficient to maintain pollinator interest, these vital resources are better allocated to vegetative growth or seed production. In the extreme, production of an unattractive, rewardless, and autogamous flower with productive chlorophyll rather than metabolically costly anthocyanin pigments yields additional economy (Mehrhoff 1989b).
A terrestrial orchidís environment is indeed very marginal. Soils were found to contain less than 20 lb of phosphate per acre. These amounts are so low that they would result in crop failure in agricultural land. However, these orchids are obligate mycorrhizal, and grow slowly. These amounts may be adequate. If more phosphorous was present, plants of faster growing species would be more competitive (Stickey 1967). However, competition for light is more important than competition for nutrients. Is has been assessed that plants growing in the brighter areas of the woods had greater whorl diameter, a parameter directly correlated to the probability of seed production (Mehrhoff 1989b).
No limiting factors to the survival and success of I. medeoloides have been identified so far, such as pollination, seed dispersal, predation or disease susceptibly. The fruit set is high (57%) compared to the relatively common Cypripedium acaule whose reported production is a scant 20 capsules from 895 flowers over a 10-year period (Nilsson 1992). Where declines in community vitality have occurred, they could be directly attributed to increases in the herb cover surrounding the plants. This trend can be expected to occur in unstable habitats such as an opening in the tree canopy (Mehrhoff, 1989a). The number of appropriate microsites for germination and establishment of orchid seeds is limited, and the seed production might suffice to saturate those available sites. This is made possible by the production of a large number of extremely light weight, passively distributed seeds (Calvo 1993). From a management perspective, perhaps the most important aspect of I. medeoloides conservation is not the survival of existing colonies at any cost, but the monitoring of the establishment of new colonies in where suitable sites occur. Colonies of plants thriving in disturbances are expected to be transient as the environment progresses from one seral stage to the next.
We have seen that autogamy is a successful reproductive strategy in a species with a wide and sparse population such as I. medeoloides. In this particular case, removal of pollen masses by a pollinating vector greatly reduces fitness and is an undesirable event. Mechanisms for prompt self-pollination and reduction of attractiveness insure a very low probability of insect visitation. Autogamy is efficient for this species, yields a good pod set and there is yet no evidence that this breeding system is a sign of a degeneration or a road to an evolutionary dead end.
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