Taxon of This Week: Not All Violets are Violet
By Christopher Taylor at 8/27/2007 11:35:00 am
And in a double whammy for the day, I'll head straight into this week's highlight taxa, meaning the Dactylopodolidae (I can't help it - I love the name!) lose their seat faster than an Italian government. So let's welcome in the plant family Violaceae!
Violaceae are a medium-sized family (about 800 or more species), about half of which belong to the single genus Viola (violets, pansies and small string instruments) (shown in the image above from Wikipedia. Most Violaceae are herbs, but a few are woody shrubs, trees or lianes - for instance, the South American tree Leonia triandra reaches 25m in height (see here). In fact, I get the impression that, taken genus by genus, there are actually more woody genera of Violaceae than herbaceous ones, and it is only the high diversity of the mostly herbaceous Viola that skews the ratio. Phylogenetically Violaceae are members of the rosid order Malpighiales that I've had cause to mention before as containing the gigantic-flowered holoparasite Rafflesia.
Violaceae don't appear to include anything as remarkable as Rafflesia (at least as far as I know), but they are certainly not devoid of interest. Many species of Viola produce cleistogamous flowers, i. e. the flowers never open and fertilise themselves. Often (as in Viola pubescens, shown here in a picture from Wikipedia) both cleistogamous and open flowers are produced (Culley & Wolfe, 2001), thus achieving the best of both options - the greater genetic variability obtained through outcrossing, as opposed to the more guaranteed success in setting seed of cleistogamy.
Also worth a mention are the ten or so species of Viola endemic to Hawaii, which are unique among the genus in their combination of woody stems (present in a few other species) and flowers borne in inflorescences (as opposed to singly in all other Viola). [The picture at left from the Hawaiian Plants website of Gerald Carr shows Viola chamissoniana var. tracheliifolia.] These distinctive features have lead to the suggestion that the Hawaiian species are quite basal in the genus (possibly relicts) and that they are related to basal South American Viola species that also have woody stems. However, a molecular study by Ballard & Sytsma (2000) indicates that, far from being an ancient group, the Hawaiian violets represent a single quite recent colonisation, not from South America, but from the Arctic! The sister taxon of the Hawaiian violets is the herbaceous Viola langsdorffii, found in the American Arctic and Japan. As circumstantial support for this result, Ballard and Sytsma pointed out the large numbers of migratory birds passing Hawaii from their breeding grounds in the Arctic, and that at least two Hawaiian birds appear to have recent Arctic origins - the goose Branta sandvicensis (from B. canadensis and the duck Anas wyvilliana (from A. platyrhynchos).
Ballard, H. E., Jr & K. J. Sytsma. 2000. Evolution and biogeography of the woody Hawaiian violets (Viola, Violaceae): Arctic origins, ancestry and bird dispersal. Evolution 54 (5): 1521-1532.
Culley, T. M., & A. D. Wolfe. 2001. Population genetic structure of the cleistogamous plant species Viola pubescens Aiton (Violaceae), as indicated by allozyme and ISSR molecular markers. Heredity 86 (5): 545-556.