Evolutionary Dynamics of Plant-Insects Interactions
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
What is your academic background and how long have you been studying
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.
What is your main purpose in studying the interactions between plants
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
Which is the most wide spread genus and species of orchids in France?
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.
Concerning the genus Orchis, how many species are there in this genus?
What is their geographical distribution?
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).
(= Orchis) morio
(= Orchis) ustulata
(= Aceras) anthropophora
Is France a rich region for this genus? How many species occur? Which
are the most important habitats?
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).
What did you find especially interesting in xOrchis bergonii, the hybrid
between O. simia and O. anthropophora?
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.
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?
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.
anthropophora (left), O. simia (centre) and their
hybrid xO. bergonii (right)
ON: Why is hybridization considered one of the leading mechanisms in
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
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?
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.
Photos by Schatz
Schatz B. 2006. Fine scale distribution of pollinator explains the occurrence
of the natural orchid hybrid xOrchis bergonii. Ecoscience 13 (6) (in
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,
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