Key Concepts -- Lecture 12 (Caryophyllales) IB 168 Spring 2009 Pp
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Key Concepts -- Lecture 12 (Caryophyllales) IB 168 Spring 2009 pp. 238—249 in Simpson Caryophyllales -- Well-supported monophyletic group. 1) Synapomorphies of Caryophyllales (diagnostic characteristics): a) Betalains -- alkaloid-like vacuolar pigments that impart bright red or purple (betacyanins) to yellow (betaxanthins) coloration to floral and sometime vegetative parts (anthocyanins – common vacuolar pigments of flowering plants – are lacking in Caryophyllales, except in Caryophyllaceae and a few other taxa (not covered in our class), which lack betalains). b) Curved embryos. c) Perisperm, tissue of the megasporangium, serves as nutritive tissue for embryo, not endosperm, which is present in minimal amounts. d) Free-central placentation or basal placentation of compound ovary, commonly; also, styles usually free or only partially fused (toward base). e) Secondary growth is anomalous (with alternating bands of xylem and phloem or concentric rings of vascular bundles in stems). f) True petals missing (petaloid parts derived from staminodes or sepals). Other unusual features: - Pollen is often with > 3 germination pores ("golf-ball" pollen) - Often with C4 or CAM photosynthesis -- atmospheric carbon is fixed by an enzyme (PEP carboxylase) that has exceptionally high affinity for carbon dioxide (therefore can minimize opening of stomata and resultant water loss) and functions well at high temperatures; associated with hot, dry environments. CAM is associated with succulents, which open their stomata (air-exchange pores) only at night, when evaporative loss of water should be lower. 2. Understanding of relationships within Caryophyllales has changed dramatically with molecular phylogenetic data: Polygonaceae Droseraceae insectivory Nepenthaceae Simmondsiaceae Caryophyllaceae betalains lost?? curved Amaranthaceae embryo, perisperm succulence Cactaceae betalains?? "Portulacaceae" "Core" Caryophyllales Phytolaccaceae Nyctaginaceae succulence Aizoaceae a) Because Caryophyllaceae lacks betalains and has anthocyanins, like other angiosperms outside Caryophyllales, Caryophyllaceae was once thought to be sister group to rest of Caryophyllales, but not so. Now, phylogenetic evidence indicates that Caryophyllaceae + Amaranthaceae are sister to other members of Caryophyllales. Amaranthaceae have betalains, which suggests that perhaps Caryophyllaceae has lost betalains and regained anthocyanins (possibly betalains and anthocyanins are alternative chemical states of a common biosynthetic pathway). b) Succulents occur in several families of Caryophyllales and until recently thought to be a monophyletic group; now evidently polyphyletic. Now, phylogenetic evidence suggests that Aizoaceae is more closely related to the non- succulent families Phytolaccaceae and Nyctaginaceae than to the succulent families Cactaceae and Portulacaceae (not all members of Portulacaceae are succulent). c) Comparison between Aizoaceae and Cactaceae, in particular, shows good examples of how a more careful analysis of morphological features can reveal that features once thought to be homologous are in fact homoplasious: i) Inferior ovary -- In Aizoaceae, the inferior ovary is surrounded by (and fused to) a hypanthium (fused bases of perianth and stamens -- which are leaf homologues); in Cactaceae, surrounded by the receptacle (which is the tip of the stem on which the flower is borne). In cacti, the outside of the ovary has clusters of spines, which are nodes -- nodes only occur on stems, not on leaves, so the tissue surrounding the ovary cannot be hypanthium. Also, the vascular strands running through the tissue outside the ovary is decurved, as might be expected if the ovary has become “developmentally depressed” into the receptacle. ii) Petal-like parts -- In Aizoaceae, the petaloid parts appear to be derived from staminodes (sterile stamens); in Cactaceae, petaloid parts appear to be derived from sepals. iii) Succulence -- Members of Aizoaceae are leaf succulents; members of Cactaceae are stem succulents. In general, features associated with life in extreme environments have evolved repeatedly in Caryophyllales, including C4 or CAM photosynthesis, succulence, salt secretion, and different means of carnivory or insectivory (although only one origin of carnivory or insectivory may have occurred). .