The Morphological Development Of The Petal Diversity Of Ranunculaceae Plants

The phenotypic changes of the living things are closely related to the developmental repatterning which is one of the major causes for morphological diversity as well as the important process of the evolution. Flower is the most important morphological innovation in angiosperms. The diversity of morphology of flowers and floral organs is the main result of adaptation and also an important foundation for speciation. Unlike the flat and non-secretory petals in most of eudicots, the petals in Ranunculaceae, which is the most important representative family in basal eudicots, exhibit a variety of three-dimensional structure and has a nectary. The petals of different genera show great diversity in morphology, including tubular, bilabial, cup-shaped, flat, concaved or scaled at the base, with or without spur or succate. Because the petal primordia of different petals are the same, the great divergence of mature petals might be the result of ontogenetic repatterning. To prove this hypothesis, the development of petals of 26 species from 22 genera, which represent all ten petalous clades in Ranunculaceae, were observed with scanning electronic microscope. The main results are as follows:1) Among 14 clades revealed by the phylogenetic studies of Ranunculaceae, the genera in the basal most clade, including Glaucidium, and the second basal most clade including Hydrastis, are apetalous. The genera in the third basal most clade, including Coptis and Xanthorhiza, have flat petals which have nectar tissue on the upper surface. The genera in 9 of the other 11 clades, which is the core Ranunculaceae, are petalous and the petals in those clades are of bilabiate or the derived form from the bilabiate ones. The ontogeny of petals can be divided into five major stages. Stage 1:petal primordium formed a blade and a basal stalk (the stalk is unapparent in some species), the blade will develop into the lower lip of petal. Stage 2:a depression appears in the ventral central of the balde, the secretory tissue will appear in the depression develop into, and the depression will develop into a spur in some genera in Stage 4. Stage 3:two (or sometimes one) bulges arise at the lower edge of the depression, the bulges will develop into the upper lip of petal in Stage 4. The petals get a primary bilabiate morph at this stage. Stage 4:two bulges fuse one another, or each of them fuse with one margin of the blade, or the depression develops into a spur. Stage 5:the mature petal formed through different growth speed of the upper lip, the lower lip and the fused part.2) The obvious morphological developmental repattering begins from Stage 2, and the forming of petal diversity is crucial at Stage 4 and Stage 5. At Stage 4, two bulges may fuse one another at first and then fuse with the margins of the blade, or each of the bulges may fuse with one margin at first and then fuse one another, or the bulges stop further development but the depression deepens to form the spur. If only one bulge appears at Stage 3, then the bulge will fuse with two margins of the blade directly. At Stage 5, the fused bulges (the upper lip), or the fused part of the bulge and the blade margin, or the blade (the lower lip) undergo different growth speed that may result in different morphology of the mature petals. Based on the observation on the developmental stages, we suggested that the forming the bilabiate petals might result in the deeply hiding of the nectar which is the main reward for the nectar sucking pollinators. The deeply hiding of the nectar might increase the feeding difficulty and thus prolong the staying time for the pollinators and to increase the efficiency of pollination and adapt to different types of pollinator.3) The evolutionary analysis of the ontogeny repatterning events at different stages based on the phylogenetic framework showed that the depression appeared in Coptis clade at first and then two bulges arisen at the lower edge of the depression in core Ranunculaceae. The appearance of the bulges is an innovation in core Ranunculaceae that let the morphological divergence of the petals and the diversification of the family.