Dandelion Sex

Just got back from a trip to the Florida Keys; I hope to post something about that soon. In the mean time, heres a short essay I wrote for my class, where we have been discussing the evolution of sex and reproductive modes. I was examing a population of dandelions found in Europe. You might think them a dry subject, but they are really fascinating if you investigate a little.

The common dandelion (Taraxacum officinale) are found throughout western
and central Europe (Verduijn et al. 2004; Meirmans et al. 2003; Van Der
Hulst et al. 2000).  Sexual reproducing diploid forms often coexist with
apomictic tripoloid, and in some rare cases tetraploid, lineages
(Verduijn et al.  2004; van Dijk 2003).  While mixing occurs, apomictics
dominate the northern regions of Europe while sexual lineages dominate
central and southern latitudes.  Several hypotheses have been proposed
for this distribution.  One, apomictics perform more successfully when
there are limited biotic interactions; their reproductive mode gives
them an advantage.  It is also thought that apomictics do better in
marginal, fluctuating environments.  Sexual lineages perhaps do better
in the central and southern climes due to more complex ecosystems;
recombination allows them to keep up with the evolutionary arms race or
fill specialized niches (van Dijk 2003).

Early studies of T. officinale prompted many questions concerning the
evolution of the species reproductive modes.  Despite the fact that
pollen plays no role in seed production for the apomictics, the plant
continues to produce pollen, nectar, and functionless yellow petals (van
Dijk 2003).  Maynard-Smith (1978) came to the conclusion that this
hinted at a relatively recent origin for apomictic reproduction and that
evolutionary adaptation in asexuals is slow.

Since then, more progress has been made to understand the population
dynamics of the species.  Hybridization can occur between sexual and
asexual populations.  Haploid egg cells of sexual diploids can be
fertilized by diploid pollen of apomictic triploid/tetraploid lineages
(van Baarlen et al. 2000).  Crossing between these apomictic clones and
sexual lineages will lead to genetic variation and thus provide adaptive
opportunities for changing environments (van Dijk 2003).  Crossing also
results in the production of novel polyploid lineages (Merimans et al.
2003).  It has even been suggested that the long term success of
apomixis largely depends on the plants ability to cross with sexual
relatives (van Dijk 2003).

References

Maynard-Smith, J.  1978.  The Evolution of Sex.  Cambridge University Press.

Meirmans, P.G., Vlot, E.C., Den Nijs, J.C.M, and S.B.J. Menken.  2003.
Spatial ecological and genetic structure of a mixed population of sexual
diploid and apomictic triploid dandelions.  J. Evol. Biol. 16: 343-352.

van Baarlen, P., van Dijk, P.J., Hoekstra, R.F., and J.H. de Jong.
2000.  Meiotic recombination in sexual diploid and apomictic triploid
dandelions (Taraxacum officinale L.).  Genome 43: 827-835.

van der Hulst, R.G.M., Mes, T.H.M., den Nijs, J.C.M., and K. Bachmann.
2000.  Amplified fragment length polymorphism (AFLP) markers reveal that
population structure of triploid dandelions (Taraxacum officinale)
exhibits both clonality and recombination.  Molecular Ecology 9: 1-8.

van Dijk, P.J.  2003.  Ecological and evolutionary opportunities of
apomixis: insights from Taraxacum and Chondrilla.  Phil. Trans. R. Soc.
Lond. B 358: 1113-1121.

Verduijn, M.H., van Dijk, P.J., and J.M.M. van Damme.  2004.  The role
of tetraploids in sexual-asexual cycle in dandelions (Taraxacum).
Heredity 93: 390-398.