To make things worse, I will merge some of my opinions (as my knowledge on
these hybrids is rather limited) into your discussion. ;-)
1. _D.intermedia * rotundifolia_
The distribution and identity of the supposed hybrid _D.intermedia *
rotundifolia_ (D.*beleziana) is subject to some debate. E.g. DIELS did not
believe that the type of D.(*?)beleziana is really a hybrid (he thought it
might be a form of _D.intermedia_). Experiments performed in the UK to
produce the hybrid artificially (in either direction) have failed, and the
hybrid does apparently not exist (spontaneously) in Britain. If we
interpret these data to one extreme, it could even be supposed that this
hybrid does not exist (?) in Europe. I do not know the situation in America
(which seems to be somewhat different!).
2. Origins of _D.anglica_
>> I think I have
>> found D. rotundifolia X D. intermedia here in Michigan. However, I am not
>> sure how to differentiate this hybrid from D. X anglica (D. rotundifolia X
>> D. linearis).
>
> Hmm, I don't know what else this hybrid might be called, but I
> think there are some not inconsiderable problems in viewing it
> as conspecific with Drosera anglica HUDS.
(...)
> It would also set an unusual (unprecedented?) exception for fertile
> Drosera hybrids from comparatively unrelated species
(...)
> there is the problem of how the species
> comes to be found widely distributed around the northern hemisphere,
> for the most part in the complete absence of D. linearis - a species,
> if anything more tolerant of varied growing conditions than D.
> anglica (certainly in my own experience as far as PH, water quantity
> and quality are concerned.)
(...)
> I suppose it is possible that the North American plants are in fact
> a separate but extremely similar taxon derived from "an accident of
> cell division" [ibid], but in the absence of any published evidence
> supporting this (unless anyone knows of such research?)
(...)
> Perhaps the three species evolved together from a more primitive
> common ancestor? My understanding of genetics and palaeobotany are
> woefully inadaquate to assess the probable course of evolution in
> this rather complex case, but whichever path was taken, the end
> result is to my mind a distinct species. Any thoughts anyone? Jan?
It is unusual but not unprecedented in genera whose interspecific hybrids
are usually sterile that some of these become stabilized and fertile. These
can be called hybridogenic species (i.e. natural taxa of polyphyletic
origin!). Such processes can be observed e.g. in the rather well known (and
difficult) genus _Mentha_ (Lamiaceae, the obvious hybrids like e.g.
_M.*piperita_ being sterile), some of the species of which (e.g.
_M.spicata_) are thought to represent such fertile hybridogenic taxa.
_Drosera collinsiae_ is perhaps another example.
Rather certainly, one of the supposed "parents" of _D.anglica_, viz.
_D.linearis_ did never occur outside N America. All recent Eurasian species
of _Drosera_ (_D.anglica_, _D.intermedia_, _D.rotundifolia_) can be assumed
to have conquered most of their present range only recently during
Pleistocene. So I would suppose that _D.anglica_ existed +/- in its present
form (in N America) before it spread to Europe and Asia. If it is of
hybridogenic origin, the hybridization and speciation event(s) must have
taken place before the spread of _D.anglica_.
I do not think that _D.linearis_ is a more tolerant species than
_D.anglica_, at least as long as both species are compared throughout their
respective global ranges. _D.linearis_ is restricted to a comparatively
small (temperate to subarctic) area in N America, preferring more alkaline
soil conditions. _D.anglica_ has (together with its supposed "ancestor"
_D.rotundifolia_) one of the largest ranges in the whole genus (almost
throughout the Holarctis, which it has conquered in a comparatively short
period of time, v.s.), tolerating various (usually more acid soil) habitat
conditions. I know some growers (I do not grow any of these myself) who
have considerably more difficulties to grow _D.linearis_ than to grow
_D.anglica_.
_D.linearis_ and _D.rotundifolia_ have 2n=20 chromosomes, _D.anglica_ has
2n=40.
So apparently _D.anglica_ is younger than the other two, i.e. the three
species did probably not evolve together from a more primitive common
ancestor but _D.anglica_ did evolve from (a?) more primitive ancestor(s?)
like _D.rotundifolia_ or _D.linearis_.
An interesting caryological paper by KONDO (I am very sorry that I have
forgotten the place of publication; I will try to find out but it may take
some time!) demonstrates that indeed half of the chromosomal complement of
_D.anglica_ may have stemmed from _D.rotundifolia_, the other half being
evidently of different origin. So the hybrid hypothesis is not too
far-fetched.
3. _D.anglica * rotundifolia_
>> To make matters worse, Schnell indicates that D. rotundifolia X
>> D. anglica plants exist (D. X obovata). The latter is confusing, since I
>> thought fertile D. anglica was an amphidiploid? Perhaps D. rotundifolia
>> manifests several ploidy levels?
The rather well-known and not disputed hybrid _D.anglica * rotundifolia_
(D.*obovata) is widespread and quite frequent throughout the common ranges
of the two parent species. It is sterile and has 2n=30 chromosomes
(obviously triploid, 3x). This hybrid originates spontaneously when the two
parents meet.
The easiest way to distinguish it (rather tentatively) from _D.intermedia *
rotundifolia_ is to compare the base of the peduncles. In _D.a.*r._ the
peduncle emerges in a straight (erect) line or with a very short curve +/-
from the centre of the rosette (like in both parent species). In _D.i.*r._
it emerges with a wide curve (ascending) obviously laterally from the
rosette (like in _D.intermedia_, from which this hybrid is -v.s.- difficult
to distinguish).
Kind regards
Jan