> You both consider it as possible to have different karyotypes even within
> one single species. Yes of course, this is known from different plant
> species. But the question arises whether different karyotype members of
> the same 'species' can produce fertile seeds if hybridized. If yes ok, if
> not (which is more likely in case of different chromosomal basic
> numbers)
Chromosome numbers do certainly have a great predictive value even in
large parts of _Drosera_, but especially Bryastrum and Ergaleium seem
to be groups including taxa with rapid recent development and
highly divergent chromosome numbers (growing in the SW AU regions of
mediterranean climate well known for similar phenomena in other plant
groups). In subgen. Drosera, similarly high diversification rates
(with the karyological effects mostly limited to changes in
ploidy, not involving the base number of generally 10) can be
observed in S AF. The AM and EU zones of mediterranean climate are
not or only marginally populated by _Drosera_, so these effects are
not known there.
> it is doubtful whether both parent groups might be
> considered to be members of the same 'species' or 'species and
> subspecies' sensu strictu.
I think your concept of species is perhaps too zoological here. The
interfertility criterion does not apply in quite a lot of plant
species. Many collective species or species complexes consist of
apomictic or otherwise "unusual" (i.e. regularly not interbreeding)
individuals and populations. Still it is justified to consider them
to be species in a biological sense (common descent in comparatively
recent time).
Experimental data are missing or rare in _Drosera_ so far, although
I heared of viable (apomictic?) seeds of _D.lanata_ ("2n"=19!) and
even of a hybrid with a relative with 2n=12 some time ago. This is
indeed rather incredible but it certainly merits some research.
In _Pinguicula_, hybrids between plants with 2n=22 and 2n=32 are
possible (!) and fertile (!!).
Nothing is too strange in the plant kingdom, and only little is
impossible.
> They would be rather members of a species/subspecies system in
> the broader sense (gamodeme aggregate).
Certainly.
> Drosera has many aggregate species and I guess
> experimental evidence of hybrid fertility of D. paleacea and D. subsp.
> roseana or other subspecies isn't yet available.
Not AFAIK.
> Nucleotide sequence analysis will certainly reveal interesting
> results.
Yes, perhaps.
Kind regards
Jan