Seed dispersal is a complex process in which several genetic and environmental factors play a role. Amongst others, factors like the presence and size of pappus, achene weight, plant height and wind conditions determine together the average distance that a seed will disperse. Presence and size of a pappus may affect the dispersal distance of a seeds as seeds with larger pappus have a higher terminal velocity and as a result may float longer in the air. Also, a larger pappus may improve the adherence to, e.g., the fur of animals that function as dispersal vector. Plant height can strongly affect dispersal distance, as taller plants release the seeds higher up in the air, where currents are stronger and therefore seeds can travel further. Under specific conditions selective pressure may act upon dispersal-related traits. This seems for instance to be the case for island populations, were selection can occur relatively fast, within a few generations (Cody and Overton 1996). A prerequisite for selection to take place is the presence of genetic variation. Also important is the number of genes involved in expression of the dispersal-related trait, as it can be expected that traits that are determined by only a few major genes, can evolve very fast.
As a model system, we have analyze the molecular basis of two characters related to dispersal capacity by mapping the major gene(s) involved through QTL (Quantitative Trait Loci) analysis in Chichorium. A segregating population was be constructed between C. endivia and C. calvum and offspring was characterized for pappus length and plant height to determine the number of QTLs involved in these two different dispersal-related traits.
QTLs involved in pappus length
C. endivia has a pappus of more than 0.5 mm (which is up to 25% of the achene length), while C. calvum has (virtually) no pappus. In the F2 population a whole range of different pappus lengths was observed, indicating that pappus length is a quantitative character. To our surprise we detected only two QTLs for pappus length, which together explained 62% of the variation in pappus size (Table 1). Both loci have a strong additive effect when present in homozygote state. Also it turns out that the locus on linkage group 10 is a dominant characteristic. Although the rudimentary pappus of C. endivia is unlikely to be involved in wind dispersal of the seed, this study shows that the presence or absence of a pappus may be governed by just a few QTLs and perhaps only a few genes. This means that it is conceivable that selection can takes place in a relatively short period of time (in the order of a few generations in a small population).
QTLs involved in plant height
The C. endivia parent was substantially larger than C. calvum (Table 2). The F2 plants were on average larger than both parents, indicating a heterosis effect. In the F2 again a whole spectrum of different plant lengths was found and five QTLs were detected, clearly indicating the polygenic nature of this character. These results are in line with observations of others (tomaat Fulton et al 1997 TAG). Alleles having a positive effect on plant height were found in C. endivia as well as in C. calvum. In four of the five loci dominance was in the positive direction. Among the five pairs of QTL loci, pair-wise interactions were observed. All these data indicate that plant height is a complex trait, in which several genes are involved; therefore selection in either direction will most likely take much more time than selection for pappus size.