Evolution of Petal Epidermal Micromorphology In Legumease

The legume flower is highly variable in symmetry and differentiation of petal types. Most papilionoid flowers are zygomorphic with three types of petals: one dorsal, two lateral and two ventral petals. Mimosoids have radial flowers with reduced petals while caesalpinioids display a range from strongly zygomorphic to nearly radial symmetry. Despite this variation in petal types within the family, it is still unknown if the three different petals within this family have a different identity (a particular epidermal type) that differenciate them. Using SEM analysis of a large representation of the family I identified six major epidermal types. This analysis revealed that papilionoids have the highest degree of variation of epidermal types along the dorsiventral axis within the flower. Each petal type is almost characterized by a unique micromorphology. While caesalpinoids lack a strong micromorphological variation along this axis and usually have only a single major epidermal type within a flower.


Classification of the major epidermal types in Leguminosae. (A, G, M) tabular rugose cells with granulose sculpture (TRG) in Calliandra haematocephala (Mimosoideae). (B and H) tabular rugose cells with striation (TRS) in Wisteria sinensis (Papilionoideae) and (I) in Lotus japonicus (Papilionoideae). (C and I) tabular flat cells with striations (TFS) in W. sinensis and (P) in Lotus japonicus. (D, J, P) papillose conical cells (PCS) in the dorsal petal of Lotus japonicus. (E, K, Q) papillose knobby cells (PKR) in the dorsal and lateral petals of Robinia pseudoacacia (Papilionoideae). (F and L) papillose lobular cells (PLS) in the dorsal petal of Lathyrus venetus and (R) in Lathyrus sylvestris (Papilionoideae). This latter epidermal type was only observed in these two species. All images correspond to the adaxial side of the petal. Scale bar 50 µm (A-F), 100 µm (G-L) and 20 µm ((M-R).

Micromorphological variation witihin the dorsiventral axis within the flower (where each petal type has a specific micromorphology) evolved only once in papilionoids and it has been lost at least three times within this group. Tha highest level of micromorphological variation is observed in Loteae and genitioids. The genetic basis of petal identiy has been analyzed in the model legume Lotus japonicus (Loteae) and in Pisum sativum (IRLC clade). Ech group hs a particular distritbution of epidermal types, in Loteae each petal types has a specific micromorphology, while in most members of the IRLC clade have lost this micromorphological variation, but minor epidermal types still differentiate each petal.


Schematic representation of the phylogenetic relationships within Leguminosae showing the typical distribution of the major epidermal types observed. This figure is intended to summarise the main patterns but it should be noted that rare variant patterns may occur in clades as well as those listed. The general epidermal surface observed within the flower is given in relation to the dorsiventral axis within the flower, using the representation: dorsal/lateral/ventral petal. Clades with an asterisk contain lineages with loss of papillose cells (PCS and PKR). The numbers following the asterisk indicate the number of losses under parsimony and ML, respectively. — lack of this type of petal (Tree according to Wojciechowski et al., 2004; Lavin et al., 2005; Lewis et al., 2005)

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