A plant that seems to defy biogeographical and evolutionary rules: Dodonaea viscosa (Sapindaceae), part 1
THE PUZZLE
Dodonaea viscosa (https://www.inaturalist.org/taxa/122711-Dodonaea-viscosa and https://prota.prota4u.org/protav8.asp?g=pe&p=Dodonaea+viscosa and https://www.anbg.gov.au/gnp/interns-2007/dodonaea-viscosa.html and https://anpsa.org.au/APOL27/sep02-1.html) is biologically puzzling in at least six ways.
Firstly, it is uniquely cosmopolitan for a woody plant (https://www.mq.edu.au/__data/assets/pdf_file/0005/1275341/Plant-of-the-week-Dodonaea-viscosa-Hop-Bush-ed-2.pdf). Long before human influence, it spread naturally across several oceans and then far inland on several continents.
Secondly, this wide natural distribution was achieved despite the fact that 60 other members of the same genus remain restricted to the same continent - and in many cases the same landscapes - from which D. viscosa originated.
Thirdly, its original occurrence was on a landmass, namely the 'island continent' of Australia/Sahul (https://en.wikipedia.org/wiki/Sahul), that has never been connected to any continent by a land-bridge.
Fourthly, its diaspores seem not to be particularly adapted for long-distance dispersal.
It is true that the seed-capsules have papery wings, suggesting anemochory (https://www.dictionary.com/browse/anemochory), and the seeds survive immersion in sea water (West, 1984, https://www.publish.csiro.au/sb/BRU9840001).
However,
- the capsules tend to remain attached to the plant for months (https://plantpono.org/pono-plants/dodonaea-viscosa-%CA%BBa%CA%BBali%CA%BBi/), as opposed to falling off and blowing away,
- the seeds possess a small elaiosome (clearly illustrated in https://spapps.environment.sa.gov.au/SeedsOfSA/speciesinformation.html?rid=1513#:~:text=Seed%20collecting%3A,for%201%20to%202%20weeks. and https://spapps.environment.sa.gov.au/SeedsOfSA/speciesinformation.html?rid=51), indicating local burial by ants, and
- most populations occur far from rivers or the coast, making it unlikely that the diaspore is particularly adapted for dispersal and sowing by means of flotation.
Fifthly, D. viscosa is 'polymorphic', in a way incongruent with subspeciation/raciality/ecotypy (https://en.wikipedia.org/wiki/Ecotype and https://digital.library.adelaide.edu.au/dspace/handle/2440/119801 and https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2699.2009.02176.x).
In a given population, the leaves may take a distinctive form (e.g. https://wtlandcare.org/details/dodonaea-viscosa-subsp-cuneata/) without any geographical isolation from other forms (see 'key to the subspecies' in https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Dodonaea~viscosa).
By the same token, the leaves may remain true to form despite having been subjected to environmental pressures and 'genetic drift' on a completely different continent for hundreds of thousands of years.
Sixthly, D. viscosa has remarkably dense wood (https://www.inaturalist.org/journal/milewski/100396-a-plant-that-seems-to-defy-biogeographical-and-evolutionary-rules-dodonaea-viscosa-sapindaceae-part-2-wood-density#).
DISCUSSION
The literature lacks any satisfactory hypothesis on how D. viscosa achieved its unique spread, either ecologically or mechanistically.
The following review of fruit-form and seed-dispersal in the family Sapindaceae, more generally, highlights the puzzle.
No sapindaceous plant seems to be adapted for long-distance dispersal.
This is because
- the typical fruit-form in this family is arillate (https://en.wikipedia.org/wiki/Aril), involving seed-dispersing animals ranging from ants through birds to mammals,
- those fruits adapted - by virtue of papery wings on the mature, dry capsule - to dispersal by wind tend not to move farther than a few hundred metres from the parent individual, and
- the seeds are not particularly small in any member of the Sapindaceae.
In Dodonaea, the aril is an outgrowth of the funicle (West, page 33). It is
- always so small that it is better-described as an elaiosome (https://en.wikipedia.org/wiki/Elaiosome) than as an aril, and
- so small in D. viscosa that some authors have claimed that it is absent.
Two relevant mysteries are
- the adaptive value of the bright hues of the mature capsules of various spp. of Dodonaea, including D. viscosa (https://www.friendsgbg.org.au/uploads/images/Plants-in-Focus/Dodonaea%20viscosa.Lucy%20Pope.PiF.2018-11.1.150.PS.pdf and https://www.aboutgardendesign.com/plant-guide/item/dodonaea-viscosa-purpurea and https://resources.austplants.com.au/plant/dodonaea-boroniifolia/), and
- the natural lifespan of D. viscosa, for which no data seem available.
Please also see https://www.inaturalist.org/journal/milewski/100103-a-comparison-of-sapindaceae-in-the-mediterranean-and-adjacent-arid-climates-of-australia-and-southern-africa#.
to be continued in https://www.inaturalist.org/journal/milewski/100396-a-plant-that-seems-to-defy-biogeographical-and-evolutionary-rules-dodonaea-viscosa-sapindaceae-part-2-wood-density#...