Wednesday, January 13, 2016

Dombeya acutangula ssp. rosea flowering at Trinity College Botanic Garden

Dombeya acutangula Cav. (Malvaceae) is a taxonomically difficult taxon.  Some sources state that it is endemic to he Mascarene Islands (Mauritius, La Reunion, Rodrigues and associated islets) of the Indian Ocean, while the IUCN Redlist ( claims it is endemic to Mauritius.  Other sources give a distribution that includes Madagascar and all three of the main Mascarene islands (e.g. Le Péchon et al., 2010), while Seyani (1991) took a broader view and included specimens from Zambia, Mozambique and Tanzania in D. acutangula.  Part of the problem may lie in the variability of the taxon, with differences in leaf dissection, flower colour, pollen production and fruit set reported. 

In the Mascarene Islands, the genus Dombeya has speciated widely, with several endemic species which are often locally distributed.  The molecular work of Le Péchon et al. (2010) indicates that the Mascarene D. acutangula has colonized from Madagascar, but has not given rise to further species, though different subspecies and varieties have been described; several other Dombeya species described from Reunion but which are not currently recognized are considered to be part of the variation within D. acutangula.  The other Dombeya in the Mascarene Islands are thought to be the result of at least three separate colonisations from Madagascar.  The genus itself is taxonomically uncertain, with different sources stating between 80 and 250 species, and the relationships between Dombeya and related genera such as Trochetia and Ruizia are unclear.

Dombeya acutangula ssp. rosea Friedmann is restricted to the south-western parts of Mauritius, where it is critically endangered, with less than 50 individuals remaining; threats include habitat loss, range contraction and competition from invasive non-native species.

The photograph here is a composite of 28 different images focused on different areas and combined in focus stacking software – there is no other way to have all of the inflorescence in sharp focus.  The image shows the yellowish nectar accumulating at the base of the stamens, and the hermaphrodite flowers with stamens and a pistil: most other Mascarene Dombeya are dioecious, with separate male and female plants.  The Trinity College specimen was collected as a cutting in 1985 by John Parnell, Peter Wyse Jackson and Quentin Cronk during a TCD plant conservation expedition to Mauritius.

Dombeya is named after the French botanist Joseph Dombey, who made collections in South America for the Jardin des Plantes in Paris.  One set of his specimens was captured en route to Paris by the British and added to their own collection at the British Museum.  His return from Chile to France, with replaced specimens, took him through Cadiz, where he was promptly imprisoned and his collections again empounded, half of them this time being transferred to Madrid.

Le Péchon, T. et al. (2010).  Multiple colonisations from Madagascar and converged acquisition of dioecy in the Mascarene Dombetoideae (Malvaceae) as inferred from chloroplast an nuclear DNA sequence analysis.  Annals of Botany, 106: 343-357.

Posted: Steve Waldren

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