Paula Rudall, Head of Comparative Plant and Fungal Biology, reflects on how careful observation can reveal weird and wonderful structures.
In June last year, I noticed a strange flower among a group of foxgloves (Digitalis purpurea) near the Japanese Minka House in the Royal Botanic Gardens, Kew. It was an example of 'terminal peloria', a well-known floral genetic mutation that occurs spontaneously – but infrequently – in nature. I always look out for this lovely mutant when I see a group of foxgloves. Most of the flowers appear normal, but there is a huge bowl-shaped terminal flower that opens before the other flowers, despite being located above them. The terminal flower is radially symmetrical and appears to be composed of a ring of spotted petals somehow stitched together.
Peloric flowers are radially symmetrical flowers that occur in species with normally bilateral flowers – species such as foxgloves, mints, orchids or snapdragons that have a single plane of symmetry. Linnaeus was fascinated by a mutant of common toadflax (Linaria vulgaris) with radially symmetrical flowers, which he termed "Peloria", after the Greek word for monster. In his Linaria mutant, as in most genetic flower mutants, all the flowers were abnormal.
In contrast, in this particular mutant of Digitalis, only the terminal flower is different, so the location of the flower on the plant is significant. Normally, foxgloves have indeterminate inflorescences, which means that the apex of the flower spike continues to produce flowers until it becomes exhausted, sometimes achieving a considerable height in a single flowering season. However, in the mutant plant with terminal peloria, the flower stalk is terminated by the strange abnormal "bell-flower", which means that no further flowers could be produced.
My particular plant had three flowering stems, all of which were topped by a bell-flower, though one of them was imperfectly formed. Bees were happily visiting both types of flower, apparently indiscriminately.
This terminal flower mutant, sometimes called Digitalis purpurea monstrosa, has been described by botanists from the mid-nineteenth century onwards. For example, Masters (1869) documented this phenomenon in his book on Vegetable Teratology, itself a broad topic that fascinated early botanists.
Breeding experiments have shown that the terminal flower mutation in Digitalis is inherited as a simple Mendelian recessive, and can be reproduced from seed via either the peloric or normal flowers of the same plant, which are all fertile). Mutations that can be inherited and reproduce by seed could theoretically be capable of establishing new plant lineages, and would therefore be "hopeful monsters" (Goldschmidt, 1940) or "prospecies" (Bateman & DiMichele, 2002).
However, there are very few species reported that characteristically have terminal peloric flowers (one possible exception is the water mint, Mentha aquatica). Some studies have suggested that this phenomenon is more frequent in the autumn, or can be triggered by environmental shock such as a sudden increase in light intensity caused by removal of nearby plants, indicating an epigenetic effect that would typically revert back to the normal condition in a population over time.
Terminal flower mutants are known in several species of flowering plant, especially in the mint family (Lamiaceae) and related families in the order Lamiales, to which both Digitalis and Antirrhinum (snapdragons) belong. In the snapdragon, the terminal flower mutation can be induced by changes in the interaction of genes that control floral meristem identity and flower position. However, in plants where all the flowers are peloric, different genes are affected, specifically those responsible for normal expression of flower symmetry in the growing floral meristem.
Why are peloric flowers and other types of flower mutation apparently so common in plant groups such as orchids on the one hand and foxgloves and their relatives on the other? At least in part, the answer is that abnormalities are much more obvious in these groups with showy flowers that so obviously possess a single plane of symmetry, compared with the radial symmetry of most other flowers. However, terminal flower mutations can also occur in plants with radially symmetric flowers. For example, in some species of the coffee family, Rubiaceae, there are cases reported of inflorescences in which the lateral flowers all have four petals and the single terminal flower has five petals.
Thus, it always pays to keep your eyes open to look for the exceptions; indeed, it can become almost an obsession. It's useful to know that in the modern molecular era, detailed records of spontaneous heritable abnormalitie s– some dating from the earliest days of natural history – can still contribute significantly to our understanding of evolution.
- Paula -
Bateman, R. M. & DiMichele, W. A. (2002). Generating and filtering major phenotypic novelties: neoGoldschmidtian saltation revisited. Pages 109–159 in: Cronk, Q.C.B. et al., eds. Developmental Genetics and Plant Evolution. Taylor & Francis.
Bradley, D., Carpenter, R., Copsey, L., Vincent, C., Rothstein, S. & Coen, E. (1996). Control of inflorescence architecture in Antirrhinum. Nature 379: 791–797. Available online
Goldschmidt, R. (1940). The Material Basis of Evolution. Yale University Press.
Keeble, F., Pellew, C. & Jones, W. N. (1910). The inheritance of peloria and flower colour in foxgloves (Digitalis purpurea). New Phytologist 9: 68–77. Available online
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Masters, M.T. (1869). Vegetable Teratology. Ray Society, London.
Rudall P. J. & Bateman, R. M. (2003). Evolutionary change in flowers and inflorescences: evidence from naturally occurring terata. Trends in Plant Science 8: 76−82. Available online
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