Ecological consequences and ontogeny of seed ... - Accueil du site

26 E. Imbert
Conclusion
Considering species with heteromorphic seed, several
characters are involved in the differentiation. This is
particularly true for heterocarpy in the Asteraceae,
where achenes differ in mass, the size of the pappus
and the structure of the pericarp. Species with seed heteromorphism
are therefore biological models which are
suitable to test some theoretical predictions about the
evolution of dispersal rates or germination strategies.
In most amphicarpic species, subterranean fruits are
produced first, while aerial fruits are only produced
when conditions are good enough (Zeide 1978; Cheplick
& Quinn 1982). Therefore, considering aerial seeds
have a better dispersal ability than subterranean ones
(which often not disperse at all), dispersal rate is higher
in good than in poor habitats. However, the interpretation
of results obtained in amphicarpic species, and
more generally in species producing both chasmogamous
and cleistogamous flowers (see Clay 1982), is
complex because the proportions of flower types are
also influenced by allocation to different reproductive
strategies (obligate self-pollination vs. open pollination).
In the Asteraceae Crepis sancta, the observed pattern
was the reverse: in bad conditions plants increase
allocation to dispersed achenes (the central ones). Conversely,
in Hypochoeris glabra (Baker & O’Dowd
1982) and Catananche lutea (Ruiz de Clavijo & Jiminez
1998) the proportion of central achenes decreases when
plant density increases (see also Imbert & Ronce 2001).
Environmental conditions also affect the proportions of
seed morphs in Atriplex triangularis (Ungar 1987) and
Atriplex sagittata (Mandák & Pysˇek 1999a). Concerning
genetic variation of seed morph proportions, some
experiments have been made with species producing
cleistogamous flowers (Clay 1982; Cheplick & Quinn
1988), and significant heritabilities have been obtained
in Crepis sancta (Imbert 2001) and Heterosperma pinnatum
(Venable & Burquez 1989), but data are scarce.
Along the same topic, the observed genetic variation
under controlled conditions or the phenotypic variation
observed in natural populations suggest that some individuals
do not present the character, i.e. some individuals
are not heteromorphic. Those individuals should
represent the basis for future research on the genetics of
the presence/absence of the character.
Although few observations have been collected on
proximal mechanisms leading to heterospermy and
heterocarpy, the importance of developmental constraints
is often suggested (Dowling 1933; Zohary
1950; McEvoy 1984; Venable 1985a). Few experiments
have examined the genetic regulation required to
produce heteromorphic fruits and seeds. In the genus
Microseris, interspecific cross-breeding showed that
the hairy character was under the control of two
epistatic loci (Bachmann & Chambers 1981; Bach-
Perspectives in Plant Ecology, Evolution and Systematics (2002) 5, 13–36
mann et al. 1984), and the expression of each locus
seems to be under the control of a third locus (Mauthe
et al. 1984). With the same materials, Mauthe et al.
(1984) have also shown that at least two loci control
for the colour of the pericarp. In the genus Spergularia,
crossing between monomorphic and heteromorphic individuals
suggested a genetic system with two loci involved
(Sterk & Dijkhuizen 1972). Developmental genetics
should help to study the ontogenetic processes.
For instance, investigation of capitulum development
of cultivated ornamental Gerbera showed that the induction
of genes contributing to organ differentiation
within the floret proceeds centripetally in the capitulum
(Yu et al. 1999; Kotilainen et al. 2000). Furthermore,
gene expression is known to vary according to
cell position. For instance, the cycloidea gene is known
to control floral asymmetry in Antirrhinum (Luo et al.
1996), and recent experiments have shown that the differentiation
between ligulate florets (in the periphery of
the capitulum) and tubulate florets (in the centre of the
capitulum) is correlated to differences in expression of
this gene in Senecio vulgaris (E. Coen, pers. comm.).
Acknowledgement. I am very grateful to Isabelle Olivieri,
Ophélie Ronce, Anders Telenius and Carl Freeman for critical
reading of the manuscript, and to Joel Mathez for his help with
the taxonomy of the cited species. Johannes Kollmann, Gregory
Cheplick and an anonymous reviewer improved the clarity of
the presentation. The final version of this manuscript was
written while I was supported by a postdoctoral fellowship
from European Community “Plant Dispersal” allocated to
B. Vosman. This is contribution ISEM 2002–011 of the “Institut
des Sciences de l’Evolution” in Montpellier.
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