Hybrids in Eleocharis
Sven-Olov Strandhede conducted numerous cross-fertilisation experiments between European taxa of Eleocharis Subseries Palustres (i.e. E. palustris/uniglumis/mamillata/austriaca**).
He also confirmed putative spontanous (i.e. natural) hybrids in Scandinavia by chromosome counts and examination. Successful cross-fertilisation was assessed by the proportions of ripe fruits set.
His results (Strandhede 1966) are summarised in diagrammatic form next⇓.
From a UK perspective, note particularly Strandhede's findings:
➢ our familiar plant (E. palustris subspecies waltersii – previously ssp. vulgaris) appears to cross with E. uniglumis both in nature and experimentally. There is more discussion of this situation below⇓.
➢ E. austriaca can hybridise with E. palustris subspecies palustris. There is however no evidence (yet) that these two forms occur anywhere near each other in UK, so the potential hybrid seems most unlikely to occur here.
➢ E. austriaca appears not to hybridise with E. palustris subspecies waltersii, either in experimental or natural conditions. Although these two forms often occur as close neighbours, the existence of hybrid plants seems unlikely – which is not the same as saying impossible. Putative examples of this cross could be tested by chromosome counts.
I have no information to hand concerning hybrids in any other of our species in the UK. The recent Hybrid Flora of the British Isles (q.v.) mentions the hybrid between E. palustris subspecies palustris and E. palustris subspecies waltersii at a single site (Port Meadow, Oxford).
[** Note: in the diagram below “pal/palustris” = palustris subspecies palustris, and “pal/vulgaris” = palustris subspecies vulgaris/waltersii; and “sterneri” is a subspecies of Eleocharis uniglumis described from the Baltic area. Similar plants have also been described more recently from southeastern Europe. See Bureš et al. (2004).]
Eleocharis palustris × uniglumis ⇑
➢ intermediate between palustris and uniglumis
➢ may exhibit a spectrum of intermediate characters between the parents
There is little ‘advice’ that I can give on this putative hybrid... other than to read Strandhede (1966 - see ref. on the spike-rush main page), which is the start- (and indeed for many aspects, the finish-)point for the palustris/mamillata/austriaca/uniglumis group in Europe.
Where the two species occur together (or even when they do not) there may be plants whose characters fall inconveniently between them. Are these the hybrid? Such plants may be “fertile” (Stace, Ed. 4) or “show little reduction in fertility” (Jermy, et al., quoting G.A. Swan) and hence the possible presence of backcrosses needs also to be considered.
Strandhede confirmed this hybrid for Scandinavia, and Max Walters named a number of UK specimens as this. But it is very vexed, and even chromosome counts may not be diagnostic. There is certainly evidence – again from Strandhede – of backcrossing and hence the possibility of "hybrid swarms", whose individuals could not be certainly named on morphology alone.
The difficulty, of course, is that in Eleocharis there are rather few morphological characters available for discrimination. Further, there is often considerable variability within the species (variation which might include introgression), blurring the distinctions between the species, let alone any intermediates.
Note that there is the possibility – yet again mooted by Strandhede - that palustris subspecies vulgaris – i.e. waltersii, our familiar plant – might itself be derived from ancient crosses between palustris subspecies palustris and uniglumis.
Gatherings in herbaria can rarely be taken further than “possible hybrid”, since (i) there cannot be enough material to give a full picture of the variation in the source population; (ii) one never has permission to demolish specimens to the extent necessary to check just how encircling the basal glumes are and whether the ‘second basal glume’ is fertile or not!
So detailed survey in the field/marsh/etc. will be necessary before any reasonable assessment can be given of what might, perhaps, be going on in the field. Morphological studies can only go so far, and chromosome counts and DNA studies would be needed to make real progress.
The recent Hybrid Flora of Britain & Ireland (BSBI, 2015) summarises the situation as follows. “In mixed populations (with E. palustris represented by subspecies vulgaris [i.e. waltersii ] ) intermediates are sometimes found; they were interpreted as hybrids by Strandhede (1965, 1966) and Walters (in Stace 1975). Those with 2n = 41, or less, closely resemble subspecies vulgaris and those with 2n = 44, or more, closely resemble E. uniglumis, and both groups might be aneuploids of those taxa rather than hybrids (Strandhede 1965).”
As far as the field situation in UK is concerned, I can offer the following:
Arthur Chater comments (pers. comm., November 2011) “I have never had it confirmed, but I have often suspected it, and especially where the two species grow together I would often not like to be definite about which species every plant belongs to.”
I have recently seen such problematic populations in the upper saltmarsh of the Solway Firth, with palustris (although uniglumis was not noted in the near vicinity), and inland in upper Lunesdale (Cumbria).
As an example of the kind of situation which presents itself: this latter site has a single dense patch of Eleocharis about 20 metres long in a marshy hollow adjacent to a stream (Broadfell Beck).
Stems were gathered on 3 August 2011 when the heads were well ripened.
Number of stems examined = 184.
Of these 184,
➢ 37/184 had two sterile basal glumes not or hardly overlapping (and on this basis would be referable to palustris)
➢ 87/184 had +/- fully encircling lowest glume, but with no flower behind the second lowest glume
➢ 60/184 had +/- fully encircling lowest glume and with a flower behind the second lowest glume
Of the 60 in the last case (i.e., with a uniglumis-type sterile encircling lowest glume and a flower behind the second),
➢ 3/60 had evidently sterile spikes, lacking any ripe nuts and with no exserted (and apparently indehiscent) anthers**
➢ 49/60 had some or many ripe nuts within the spike, but the lowest flower (i.e., that behind the second lowest glume) had not developed
➢ 8/60 had the second lowest glume with a ripe nut (and on this basis would be referable to uniglumis)
(** such sterility may be evidence of hybridity, but of course may be symptomatic of factors such as environmental stress or disease)
Hence there was every possible condition in the spectrum between those stems showing palustris characters and those showing uniglumis characters.
It proved impossible in this dense patch to determine whether there were several plants intimately intermingled, or if the observed variation was that expressed by a single variable genotype. (The stems gathered came from five ‘bunches’ gathered from across the sheet. There seemed to be as much variation within each bunch as between the bunches, although this was not tested rigorously.)
Anyone up for a genetic study?
Links to the other Eleocharis spike-rush pages (also accessible from the sidebar)
Separation of similar pairs