Increased seed survival and seedling emergence in a polyploid plant invader.
OBJECTIVE
Because seeds have essential functions in the life cycle of plants, even subtle changes in their characteristics may have important demographic consequences. In this study, we examined whether potential changes in seed characteristics as a result of polyploidy or postintroduction evolution may have contributed to the invasion of Centaurea stoebe (Asteraceae). This plant occurs as diploid and tetraploid cytotypes in its native range in Europe, whereas only tetraploids have been found and become invasive in North America. Specific comparisons among these three "geo-cytotypes" allow us to explore hypotheses of preadaptation resulting from polyploidy (European diploids vs. European tetraploids) and postintroduction evolution (European tetraploids vs. North American tetraploids).


METHODS
Using seeds collected from plants of each geo-cytotype grown in a common maternal environment, we compared seed mass, morphology (achene and pappus size), dispersal potential (falling velocity, seed roughness), survival, germination, and seedling emergence in a combination of laboratory, greenhouse and field experiments.


RESULTS
We found increased seed mass in North American tetraploids compared with European tetraploids. Seed morphology and dispersal potential were largely similar in all geo-cytotypes. Seed survival under field conditions was higher in native and invasive tetraploids compared with diploids. Germination in the laboratory was similar among all geo-cytotypes, but seedling emergence under field conditions was higher in invasive tetraploids than in the other geo-cytotypes.


CONCLUSIONS
Our findings suggest that a combination of preadaptation due to polyploidy (increased seed survival) and further postintroduction evolution in North American tetraploids (increased seed mass and seedling emergence) may have contributed to their invasion.
Department of Biology, Ecology and Evolution, University of Fribourg, Chemin du Musée 10, 1700 Fribourg, Switzerland. min.hahn@gmx.ch
Min A
MA
Hahn
Tabea
T
Lanz
Dominique
D
Fasel
Heinz
H
Müller-Schärer
D018533:Biomass; D031179:Centaurea / Q000033:anatomy & histology / Q000235:genetics* / Q000502:physiology; D004171:Diploidy; D005060:Europe; D040961:Genetic Drift; D018525:Germination; D058865:Introduced Species; D009656:North America; D036226:Seedlings / Q000033:anatomy & histology / Q000235:genetics / Q000502:physiology; D012639:Seeds / Q000033:anatomy & histology / Q000235:genetics* / Q000502:physiology; D057891:Tetraploidy
Asteraceae; Centaurea stoebe; dispersal; invasive species; polyploidy; postintroduction evolution; seed ecology; seed viability; seedling establishment; spotted knapweed