Perianth and Floral Evolution
Petals and sepals
There are many ways in which sterile parts of the flower (sepals and petals = perianth) can assume different forms in order to adapt to specific pollinators. Flowers are machines for reproduction – see how all the parts work to accomplish reproduction
These sterile leaves of the flower (perianth) may not be in two distinct (petal and sepal) whorls. If petals and sepals cannot be distinguished morphologically, then use the general term: tepals
Perianth parts are sometimes missing (e.g. Ranunculaceae, petals may be missing, sepals may be petaloid, petals may take the form of nectaries - German honigblatter or "honey leaves") - the practical sessions will allow you to examine a range of Ranunculaceae flowers.
- Lilium: outer whorl and inner whorl petaloid: i.e. two whorls of tepals (and other petaloid monocots)
- Ranunculaceae: sepals can be petaloid with petals absent
- Carex (sedge): the perianth is completely absent (the protective role is provided by a bract) – in this case a single ovary is enclosed in a perigynium [utricle], which is a bract
- Asteraceae: Sepals present but modified into a PAPPUS, which can take the form of bristles, feather-like organs or scales. These serve for fruit dispersal (a pappus is also found some other groups, such as Valeriana in the Valerianaceae)
- Some Asteraceae: pappus (=calyx) may be absent but petals (corolla) present (also some Valerianaceae - Plectritis)
Concept of "pollination syndrome" – perianth a major contributor to this - a combination of characteristics that collectively result in effective pollination by a particular pollinator.
Differentiation of perianth into: calyx (sepals) and corolla (petals)
- an undifferentiated perianth of tepals is primitive; morphologically and anatomically differentiated sepals and petals are advanced.
- there can be a "division of labour" if you have differentiated parts
- however, perianths of undifferentiated tepals occur in some otherwise advanced families like the Cactaceae (eg. Schlumbergera),
- petals and sepals may exist in separate whorls, but look alike morphologically, as in the Liliaceae. Division between differentiated petals/sepals and two whorls of tepals may be somewhat arbitrary.
Differentiation of floral whorls controlled by MADS-box genes
Primitive MADS-box expression is apparently less finely regulated, therefore less differentiation of parts.
Eudicot: ABC system
A, AB, BC, C = sepal, petal, stamen, carpel
Basal group and monocots (e.g. Lilium, Tulipa): B class genes more broadly expressed
AB, AB, BC, C = petal, petal (i.e. tepals), stamen, carpel
How did this change in MADS-box gene regulation occur? Not really known. In eudicots B-class co-regulated by UFO (unusual floral organs) – UFO regulation in basal angiosperms has not been determined. Note: ancestral form of monocot flower very difficult to determine – basal branch Acorus (a rush-like plant) has very reduced flowers – few useful morphological features
Gene expression patterns have been used to infer whether organs are sepals or petals,
e.g. Asteraceae – pappus thought to be derived from calyx – and this is supported by MADS box gene expression.