Tasmanian Plants

A rather recent trend in molecular science has been to use the technique to extort genes to reveal the history of how a plant has extended it’s geographical distribution throughout time.

I have written about how researcher James Worth used molecular techniques to pinpoint the locations (refugia) where Myrtle Beeches (Nothofagus cunninghamii) survived during the last glacial period. Just earlier this year, researchers Paul Nevill, Gerd Bossinger and Peter Ades published a paper in the Journal of Biogeography doing the same for the Mountain Ash (Eucalyptus regnans).

As in James Worth’s Myrtle Beech study, the researchers looked for variations at specific locations in the chloroplast DNA in Mountain Ash individuals distributed throughout the species natural geographical range. Different individuals may exhibit specific sequences which may differ from region to region and these are known as haplotypes.

A large amount of haplotypes found in a population an any given area would suggest that the area is a glacial refugium as we would expect a species to have persisted for longer periods of time in a refugium, thereby accumulating genetic changes. Conversely, places with low diversity of haplotypes could be construed to have been colonized after the glacial period ended, as there wouldn’t have been time enough for a high diversity of haplotypes to develop.

The results of the study showed that Mountain Ashes of the Northeast and Southeast of Tasmania has a high diversity of haplotypes, many of which were unique to the region. This suggests that the Northeast and Southeast of Tasmania harbored refugia that sheltered Mountain Ashes during the glacial period. By contrast, the central parts of Tasmania had a lower diversity of haplotypes. Another way of interpreting this was that there was fixing of haplotypes in that region, suggestive of a more recent colonization of the area following the end of the glacial period.

One consideration that remains to be addressed is the ease with which Eucalypts hybridize. E. regnans for example may hybridize with E. oliqua (Stringybark) and E. delegatensis (Gum-topped Stringybark). Hybridization may result in chloroplast sharing between species and a more comprehensive study will probably be needed to ensure that all these factors are taken into consideration.

For now it seems we are getting closer toward reading the the silent tale of survival that the ancestors of the Mountain Ashes in the Northeast and Southeast have etched in the genes of their descendants.

Students of mosses (muscologists) have their agendas to see the Globe Moss when they come to Tasmania.  For students of liverworts (a.k.a hepaticologists), Tasmania houses yet another bryological treasure – a genus of liverworts known as Treubia.

Worldwide, Treubia has consists roughly 6 members of a largely southern hemispheric distribution. Proudly, Tasmania has two members of Treubia, T. tasmanica and T. lacunosa. These are among the most unmistakable of liverworts. Ironically, the appearance of Treubia has puzzled bryologists for decades since famed plant morphologist Karl von Goebel described the genus in 1891.

Liverworts in general, can be divided into two broad groups based on their appearance. These are thallose liverworts with flattish bodies without clear stems or leafy liverworts, which usually have clearly defined stems and leaves.

Treubia on the other hand doesn’t fit very well in either. It would not be too accurate to claim that Treubia has a stem. That would mean it is thalloid. However, from this ‘thallus’ arises many flaps of what look like leaves.

Tasmania’s very own early bryologist Leonard Rodway said of  Treubia tasmanica in 1911:

Many authorities try to avoid the breaking down of established systems by treating the lateral expansions as lobed portions of lateral wings. This seems a distorted description of the apparent structure, and does not tend to a clear understanding of the evolution of the hepatics.

Some bryological giants like Rudolf Schuster and George Scott interpret Treubia to be the midway point between the primitive thalloid way of life in liverworts to the more advanced leafy upgrade. To draw an analogy with animals, Treubia would be to liverworts what velvet worms (Onychophorans) are to invertebrates.

How does this ‘halfway house’ theory fit in with what is known of the molecular phylogeny of liverworts?

Molecular work has shown Treubia to be one of the most basal groups of liverworts, related to yet another morphologically enigmatic group of liverworts of the genus Haplomitrium (which incidentally is alleged to occur in Tasmania as well). Together, Treubia and Haplomitrium form a group that diverged early from the the course of the liverwort evolutionary stream.

Lending strong support to the antiquity of Treubia is the fossil record, with Treubia-like fossils being among the earliest liverwort fossils known. Treubia can really be considered to be a ‘living fossil’ like the Wollemi Pine.

In my interpretation, the leaf-like morphology of Treubia is hence an innovation of it’s own and not an attempt to bridge the thalloid to leafy condition. Could the Treubia lineage then represent an independent attempt to make leaves?

We may not live to see the descendants of the Treubia lineage, for bryophytes features in general do not evolve very fast. But still, Treubia remains a reminder of the innovation and possibilities that even the ancient can strive toward. It is indeed a liverwort that that epitomizes the legacy of Gondwana!

We don’t look one bit alike, but we are family.

That happens to be the story of a rather obscure group of bryophytes and exemplifies how drastically molecular technology is changing how bryophyte taxonomists study and classify this fascinating group of plants.

Whenever I visit dimly lit gullies in wet forest I always try to look out for bryophytes, one of which is a rather nondescript moss that used to be known as Echinodium hispidum. It was the only one of it’s genus in Tasmania and the nearby New Zealand has another species, E. umbrosum.

A very limpid way of describing this moss without getting into a tirade of alien sounding botanical terms would be to say that it is branched, has spirally arranged leaves that are widened toward the base. A look under the microscope will reveal the nature of the leaves.

Strangely, it is probably the combination of it being rather nondescript and it’s preference for dimly-lit gullies that enables almost instantly recognition of the species for the trained eye.

The genus Echinodium was erected in 1866 and was honored a family status of it’s own, the Echinodiaceae in 1909. Within bryological circles however, the family and genus is of some interest because of it’s anomalous distribution of it’s members: out of 6 species, two are found in Australasia (Australia and New Zealand) and four in Macronesia.

Earlier in 1986, taxonomist Steven Churchill was starting to sense that something was quite amiss with the species of Echinodium. He included all 6 species under Echinodium but was prudent enough to suggest the genus could potentially contain species that are not related to each other.

However, observant as Churchill was with the light microscope, the ‘molecular microscope’ was about to throw a spanner into the works.

In a recent study in 2008, Michael Stech and colleges, compared the specific DNA sequences of the six Echinodium species with species of other moss genera and found robust evidence that the six species of Echinodium did not form cohesive group. The Macronesian species largely remained in the Echinodiaceae but the two Australasian species were actually found to be more closely related with a totally different genus of mosses, Thamnobryum, a member of a totally different moss family, the Neckeraceae.

With such definitive prove of the new generic relationships, Stech and colleges renamed Echinodium hispidum to Thamnobryum hispidum.

Now, while mosses are simply mosses to some, anyone who would take even just a cursory look at the now T. hispidum and compare it to other species of Thamnobryum will find it hard to reconcile this new relationship.

For instance, the commoner Thamnobryum species in Tasmania, T. pumilum, is somewhat dendroid (shaped-like a tree); the leaves are flattened in a single plane; and the plants tends to produce thin wiry branches in addition to normal ones.

No familial resemblance whatsoever between the two species.

Molecular-based taxonomies of bryophytes have lagged behind that of vascular plants but whatever little that has been done is already revealing some rather surprising and revolutionary information that is eroding the very foundations of established taxonomies of the 20th century.

There is always more than meets the microscope when it comes to studying bryophytes!

The Myrtle Beech (Nothofagus cunninghamii) is one of Tasmania’s icon trees, and is the dominant component of  Tasmania’s cool temperate rainforest. Where these dendrons attain their finest stature in some parts of Tasmania’s verdant Northwest and Northeast, they assemble grand cathedral or callidendrous (meaning ‘beautiful tree’) rainforests, which has for generations captured the imagination and awe of Tasmanians.

Back 18,000 years ago, when glaciations in Tasmania were at their maximum (called the Last Glacial Maximum and henceforth abbreviated LGM), practically the whole of the island would have been unsuitable for the development of cool temperate rainforest, except in pockets of areas in the west. Such areas where plants survived during glacial periods are called refugia.

In the present day, the Northeastern part of Tasmania has sizeable patches of Myrtle Beech rainforest. Yet, geomorphological and pollen-based data suggests that the entire Northeastern area was too arid during the LGM to support rainforest. The question thus arises whether Myrtle Beech trees had survived there in refugia during the LGM or whether they were dispersed from refugia from the west after the LGM?

The immediate problem with the latter suggestion is that Myrtle Beech seeds disperse poorly over long distances, making it unlikely for seed to cross over 150 km from western refugia.

Tackling this conundrum was the topic of Dr James Worth’s honours research and part of his doctorate studies. The efforts of James and his fellow investigators have culminated in a recent publication in the scientific journal New Phytologist.

From his extensive fieldwork, James collected Myrtle Beech leaves from over 340 trees across the distributional range of the species, which includes both Tasmania and Victoria. Using molecular techniques, James then extracted the chloroplast DNA from these individuals and compared their DNA sequences.

James discovered that a common signature in the DNA (a chloroplast DNA sequence that is called a haplotype) that exists for Myrtle Beech trees in Victoria and in numerous areas of Tasmania. The western part of Tasmania however, had an additional and significantly large suite of other endemic haplotypes, suggesting a complex evolutionary history of Myrtle Beeches in that area, and perhaps survival in numerous refugia, which is within expectations.

Myrtle Beech haplotype distribution. White circles and black circles represent the widespread and endemic western haplotypes in the left and right map respectively. Red circles represent the unique Northeastern haplotypes. MA = Mt Arthur; MB = Mt Barrow; BL = Ben Lomond; MM = Mt Maurice; MV = Mt Victoria; BT = Blue Tiers

In the Northeast, trees from two regions bore the common haplotypes, some in the western extreme (Mt Barrow), and some in the eastern extreme (Blue Tiers). In between was a central region (areas in the vicinity of Mt Victoria, Mt Arthur and Mt Maurice) in which a unique haplotype was discovered.

At least for this central region, the presence of the unique haplotype is strong evidence that there must have been refugia for the Myrtle Beech in that area.

James concluded that the Myrtle Beech withstood the aridity of the last glacial period within multiple regions in apparently inhospitable climates.

Whether cathedral rainforest actually existed in refugia in the Northeast during those times is questionable but if the conditions then were simply untenable for rainforest, Myrtle Beech trees could still have survived, being, as we are currently able to observe, able to occur as a compact shrub in harsh highland environments.

This is where the true virtues of the Myrtle Beech comes to light. If Myrtle Beech did not survive through the last glacial, there would be no rainforest to speak of. Yet, Myrtle Beech did more than just survive through the LGM. Fossils suggests that it has been around for at least 780000 years. Myrtle Beeches have therefore survived through numerous cycles of glaciation.

The resilience of this iconic temperate tree throughout the ages has unquestionably shaped Tasmania’s modern biota.