An odd case of evolutionary convergence between a cycad and a mat-rush
(writing in progress)
https://en.wikipedia.org/wiki/Macrozamia
https://en.wikipedia.org/wiki/Lomandra
Here we have a strange case of evolutionary convergence, mimicry, the archaic vs the modern, genetic plasticity, and puzzling adaptation.
A species involved, first described as recently as 1991, is endangered with extinction owing to development of its limited habitat near Bundaberg, Queensland, Australia.
Macrozamia lomandroides (https://www.inaturalist.org/taxa/135828-Macrozamia-lomandroides) seems to be convergent in growth-form with Lomandra longifolia (https://www.inaturalist.org/taxa/83375-Lomandra-longifolia).
The former is restricted to a tiny range in southeastern Queensland whereas the latter is widespread in eastern Australia (and probably to be found now also in e.g. New Zealand because it has proved to be such a useful plant e.g. along roadsides).
I have tended to think of cycads, including Macrozamia, as ‘living fossils’. It therefore comes as a surprise to find at least one species of Macrozamia playing what looks like a ‘modern game’ in its deceptive resemblance to a thoroughly successful plant belonging to a completely different lineage of plants. To put this in a somewhat exaggerated way, here we seem to have a case of a ‘dinosaur plant’ breaking the mold and emulating a vulgar plant which is so modern that it is virtually a weed.
Lomandra longifolia forms a tussock, as do so many other graminoid plants worldwide. By contrast, Macrozamia has large pinnate leaves. Macrozamia has converged in appearance with Lomandra not by reducing its pinnate leaves but simply by twisting the rachis so that the pinnae become differently arranged, more or less into the form of a tussock.
There is also a complex involvement of spinescence in the relationship between the two genera. Both Macrozamia and Lomandra possess spinescence in at least some species. In the case of Macrozamia, the spinescence is of the tips of the pinnae plus the tips of the cone-scales. Even those spp. of Macrozamia which lack leaf-spinescence retain spinescence in both male and female cones (and M. lomandroides retains these reproductive spines albeit in reduced form). In the case of Lomandra, we likewise see spinescence of the leaf-tip, plus spinescence of the inflorescence/infructescence.
I myself recently encountered leaf-spinescence in Lomandra in the form of the species L. confertifolia, on a rocky outcrop (Mount Tinbeerwah) in the Sunshine Coast. Thus I know from personal experience that at least one species of Lomandra qualifies as leaf-spinescent. And the spinescence of the inflorescence/infructescence of several spp. of Lomandra is well-known (with one species being given the specific epithet ‘hystrix’). In the case of Lomandra longifolia, the teeth on the leaf-tips do not seem to qualify as leaf-spines because of limited sclerophylly, but the reproductive spines nevertheless occur, which is on the unusual side for a graminoid plant.
A remarkable point of evolutionary convergence: as far as I know, M. lomandroides is the only species in its genus that has several ‘small sharp teeth’ at the tip of the pinna, and arranged asymmetrically to boot.
So what is going on here?
Why has M. lomandroides converged with Lomandra, in ways both gross (obvious) and subtle (detailed)? Is this a case of ‘protective mimicry’ comparable with what we see in mistletoes in Australia? And is it mere coincidence that both parties in this relationship have parallel forms of spinescence, at least at the generic level?
Please follow the captions below, in addition to the more detailed description of M. lomandroides which I provide immediately below.
From D. L. Jones (2002), pages 344-345: “Macrozamia lomandroides...mature leaves...the leaflets appearing crowded and whorled because of the strongly twisted rhachis...rhachis twisted strongly in more than 2 spirals...leaflets...thick and leathery...stiffly erect to spreading...the apex asymmetrical and with 1-6 sharp teeth on the upper margin...lower leaflets not reduced in size and not spine-like...male cones...with an apical, spine-like appendage 0.1-0.8 cm long...female cones...with an apical, spine-like appendage 0.2-3 cm long...”
The following shows how ‘graminoid’ the appearance of Macrozamia lomandroides can be. This plant seems to have a tussock form. Who would immediately realise that it is in fact a cycad?
Macrozamia lomandroides:
https://wetlandinfo.des.qld.gov.au/wetlands/ecology/components/species/?macrozamia-lomandroides#prettyPhoto[1]/2/
Here is some background reading:
http://www.friendsoflanecovenationalpark.org.au/Flowering/Flowers/Lomandra_longifolia.htm
The following shows the leaf-tip of Lomandra longifolia. This species is not, as far as I know, leaf-spinescent, because as I understand it the teeth shown below do not qualify as ‘pungent’. I.e. the leaves, though tough, are not lignified enough to provide the support needed to make the ‘teeth’ pungent. However, the odd thing is that the teeth on the leaf-tip are several and asymmetrical, which can equally be said for the peculiar teeth on the tip of the pinnae of Macrozamia lomandroides, which seem unique in the genus Macrozamia.
Lomandra longifolia:
https://keyserver.lucidcentral.org/weeds/data/media/Html/lomandra_longifolia.htm
The following shows the teeth on the tips of the leaves of various spp. of Lomandra. I know from recent personal experience that the teeth in the case of L. confertifolia are indeed ‘pungent’, i.e. L. confertifolia qualifies as a leaf-spinescent plant even though L. longifolia does not.
http://www.saveourwaterwaysnow.com.au/03_enews/newsletter.asp?ID=21
The following shows how widespread Lomandra longifolia is in its natural distribution. This commonness perhaps makes it an effective ‘model’ to mimic, as an anti-herbivore tactic.
The following shows the distribution of the various spp. of Macrozamia including M. lomandroides.
(writing in progress)
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