The Evolutionary Dynamics of Plant-Insects Interactions


Schatz Bertrand
Bertrand Schatz is PhD on the Foraging System of a Neotropical and researcher at the National Centre of Scientific Research in the Centre for Functional and Evolutionary Ecology, in Montpellier (France).








ON: What is your academic background and how long have you been studying orchids?
SB: I’m a researcher at the CNRS (National Centre of Scientific Research) in the CEFE laboratory (Centre for Functional and Evolutionary Ecology) situated in Montpellier (France). Before that, I completed my PhD thesis in Toulouse (France) on the foraging system of a neotropical ant, and I was financed by the European Commission to perform a post-doc in Brighton (England) on the navigation system of a Mediterranean ant. I have steadily shifted my research towards ant-plant relationships, and more generally towards insect-plant relationships. I am studying pollination ecology in orchids for the past three years. I am now in charge of the section on “insect-orchid relationships” for SFO (French Society of Orchidophily) and work in collaboration with National Parks for plant conservation.

ON: What is your main purpose in studying the interactions between plants and insects?
SB: My general purpose is the understanding of the evolutionary dynamics of plant-insects interactions based on comparative studies of several biological models. The nature and the specificity of these interactions, as well as the encounter and the conflicts among their participants, are investigated within a unified framework that includes behavioural, chemical and evolutionary ecology. My main biological models comprise the insect community linked with the pollination of the Mediterranean orchids, the hymenoptera community (pollinators, parasites and ants) associated with the mutualism between figs and fig wasps as well as between ants and plants. With regard to the orchids, I am interested in the variety of strategies (nectar reward, visual and sexual mimicry) displayed by the various genera to attract insects for pollination and by the associated biological traits (odours emitted, hybridisation, inflorescence morphology). More information is available at my website (see http://www.cefe.cnrs.fr/coev/B_Schatz.htm).

ON: Which is the most wide spread genus and species of orchids in France?
SB: In France, there are about 150 species of orchids, belonging to 30 genera. Four genera are dominant with more than 15 species each (Ophrys, Orchis, Dactylorhiza and Epipactis), seven genera have 2-4 species and there are also several other genera with only a single species. 18 orchid species are nationally protected while several other species are regionally protected. In the first is the famous lady’s slipper orchid (Cypripedium calceolus), which is the symbol of the SFO (French Society of Orchidophily), and the endemic species Ophrys aymoninii which is only present in a particular region in the south of France. All French orchids are terrestrial, and are more abundant in the South.

Cypripedium calceolus
Ophrys aymoninii

ON: Concerning the genus Orchis, how many species are there in this genus? What is their geographical distribution?
SB: Distributed from the North of Africa to the West of Asia, the genus Orchis is one of the commonest genera of terrestrial orchids in the Mediterranean region. It comprises about 60 species, and about 70 intrageneric hybrids have been listed. The centres of speciation for this genus are the South of France and Italy on one side with Greece and Crete on the other side. This genus is generally pollinated by various Hymenoptera, but information about pollinators of Orchis spp. are often scarce and anecdotal. A recent review showed that confirmed pollinators are known for only 15 species of Orchis (Schatz, 2005a; 2006). Recent molecular analysis demonstrates that the species within the genus Orchis should actually be divided into three genera; some species remain in Orchis, while others belong to either Anacamptis or Neotinea. However, I will use the ‘old’ nomenclature here, except in the case of Aceras anthropophorum which is now renamed Orchis anthropophora (Schatz, 2006).

Anacamptis (= Orchis) morio
Neotinea (= Orchis) ustulata
Orchis (= Aceras) anthropophora

ON: Is France a rich region for this genus? How many species occur? Which are the most important habitats?
SB: In France, there are 22 Orchis species, which occupy various ranges of habitats from sea level to 1000 m in altitude (up to 2400m for certain species). Some species (O. papilionacea papilionacea, O. longicornu and O. pauciflora) are only present in Corsica, while a majority are present along the Mediterranean coast (O. provincialis, O. lactea, O. conica, O. olbiensis, O. champagneuxii) and others are found in most French regions (O. anthropophora, O. mascula, O. purpurea, O. militaris, O. morio morio, O. ustulata). Most orchids occur in calcareous soils, while certain Orchis species display a preference for humid zones or for special habitats (sometimes acidic) such as mountain forests in the Alps or in the Pyrenees. In this genus, five species are protected at the national level in France (Orchis coriophora fragrans, O. longicornu, O. pauciflora, O. collina and O. spitzelli).

Orchis coriophora fragrans
O. longicornu

ON: What did you find especially interesting in xOrchis bergonii, the hybrid between O. simia and O. anthropophora?
SB: The two parental species present several advantages for research: 1) they are abundant in the South of France, 2) they are present in populations with a large variation in number and composition, 3) they emit different volatile odours, 4) they attract a relatively large number of pollinators, and 5) they are well pollinated (rate of pollination higher than 80%). The study of the hybridization between these two parental species has been studied in different steps (Schatz, 2006). First, hand pollinations confirmed that autogamy (pollination within the same flower), geitonogamy (pollination between different flowers from the same plant) and allogamy (pollination between flowers from different plants) were effective in all three taxa, but the absence of fruit set on with bagged inflorescences confirmed that insect visits were required for pollination. Second, fruit set of the hybrid was strongly pollinator-limited whereas fruit set was not pollinator-limited in either parental species. Third, I also identified the suite of confirmed pollinators among all species observed visiting the inflorescences of both parental species during controlled periods of the day. These data allow me to quantify the relative importance of different insect species for the pollination of both parental species, and to demonstrate that a particular species of beetle is the sole pollinator shared by the two parental species. I also observed this beetle effecting interspecific pollination (both ways) between O. simia and O. anthropophora, which confirms that this species is responsible for hybridization. Moreover, the particular habitat preferences of this beetle allow predicting the natural occurrence of the hybrid xO. bergonii; extensive observations on surrounding areas indicate that these findings can be generalised to an ecologically similar area of about 1600 km2. To my knowledge, this is a novel demonstration that the spatial distribution of a natural hybrid in plants can be predicted by the local occurrence of a pollinator that effects hybridization. Finally, comparisons among the populations where hybrids occur allow determining the threshold value of the minimal number of parental individuals required for the presence of at least one hybrid in a population. Taken together, these results show the value of this model for studying the conditions required for hybridisation by taking into account the ecology of the parental species in relationship with their pollinators.

ON: In your lecture, you talked about the comparison you did on floral morphology and the volatile compounds emitted by flowers of the three taxa. Could you develop this question a little? Why are only few insects observed on the inflorescences of hybrids, whereas insects were more numerous on inflorescences of the two parental species?
SB: The hybrids display an intermediate floral morphology measurements for several parameters (size of labellum, length of spur, width of heat, number of colour spots, general colour). However, the hybrid displayed higher values for several vegetative parameters (number of flowers, total height, number and length of leaves) (see photo) (Schatz, 2005b). As a result, the three taxa have different morphologies which are likely to induce differences in the visiting behaviour of the pollinating species. Moreover, the main important factor that explains why I did not observe effective pollinators on the hybrid is certainly the different volatile odours emitted by these three taxa. After results (to be published) obtained by the non-destructive head-space technique, we have found that the volatile bouquet of the hybrid is outside the range of the volatile bouquet emitted by the two parental species. My hypothesis is that the different morphology and, mainly, the different volatile compounds emitted explain the relatively low rate of insect visits observed in the case of the hybrid.

Orchis anthropophora (left), O. simia (centre) and their hybrid xO. bergonii (right)

ON: Why is hybridization considered one of the leading mechanisms in plant evolution?
SB: In addition to sympatry (co-existence in the same population) and overlapping flowering periods, hybridization in animal-pollinated species requires that the two parental species share at least one common pollinator. Following Grant’s hypothesis of ethological isolation, pollinator specificity could act as an effective isolating mechanism between sympatric plant species. However, numerous observation in the fields as well as the existence of hybrids demonstrate that these barriers to cross-pollination are at best only partially effective and hybrids occur when parental species share one or more species as pollen vectors (Schatz, 2006). This is particularly true within Orchidaceae where an important number of hybrids have been identified and, thus constitutes one of their major facilitators of speciation.
The potential distribution of natural hybrids is usually deduced based on the habitat where parental species are abundant and sympatric, also called potentially ‘hybridogenous’ populations. However, despite the obvious role of insects as pollen vectors in hybridization, the potential importance of the distribution patterns and microhabitat preferences of pollinators in determining the occurrence of hybrids has not been well studied. Here, the hybridization between O. simia and Orchis anthropophora provide one of the rare examples in which the ecological conditions of the hybridisation were studied. This predictive ability about the spatial distribution of this natural hybrid depended on two conditions, a good knowledge of the confirmed pollinators of the plants studied and the determination, under natural conditions, of the minimal number of individuals of parental species above which the hybrid is found to occur. This latter condition reflects the natural history of local hybridization, which is conditioned at least in part by i) the relative proportions of the two parental species in each patch, ii) the ratio between intraspecific and interspecific pollinations performed by the species of pollinator(s) effecting hybridization and iii) the physiological compatibility of the two parental species for hybridization.

ON: Why do you consider that orchids constitute an ideal biological model for the study of the behavioural and chemical ecology of the interactions established with their pollinators?
SB: Relatively few model systems in pollination biology have been studied with equal emphasis on plant and animal partners. However, the pollination ecology of orchids provides a wide range of strategies developed to attract insects for pollination. The pollination ecology of orchids can be then better understood by comparative studies concerning the efficiency of these different strategies and of the environmental conditions for their occurrence. Several authors have emphasized that there is an urgent need for more rigorous ecological observations of pollination in Orchidaceae to generate less speculative coevolutionary studies. Here, the close observation of all insects visiting flowers was very important groundwork, since it allowed the determination of the confirmed pollinators of each parental species, of those insect species effecting hybridization, and of the relative importance of the latter among all pollinators. The study of the confirmed pollinators allowed a better understanding of the ecological conditions favouring the hybridization between the two orchid species studied here, suggesting the interest of similar studies on plants belonging to other families (Schatz, 2006). As illustrated in the case of this study, the generation of new insights in pollination biology will be greatly accelerated if we can build lasting bridges between zoology and behavioural ecology.

References
Schatz B. 2006. Fine scale distribution of pollinator explains the occurrence of the natural orchid hybrid xOrchis bergonii. Ecoscience 13 (6) (in press)
Schatz B. 2005a. Reproduction sexuée : pollinisation, fécondation et hybridation. In : Les orchidées de France, Belgique et Luxembourg. (2ème édition) (Ed. M. Bournérias), Collection Parthénope, Biotope (in press).
Schatz B. 2005b. Comparative analysis between two orchids and their hybrid. 18th World Orchid Conference, March 11-20, 2005, Dijon, France, (in press).

Photos by Schatz Bertrand  


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