The role of floral development in studies of legume evolution

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Abstract
Comparative floral development is useful to test hypotheses of phylogeny and taxonomic relationships among legumes because it provides a greater understanding of character states. It can reveal homologies, as in the cupule, an anomalous and varied structure among species of Psoralea that originates by fusion of two or more small bracts. Convergence is demonstrated by comparing differing developmental pathways of specialized floral features such as dorsiventral heterostameny, eccentric corolla, porate stamens, and enantiostyly, among species of Cassia, Senna, and Chamaecrista. Naturally occurring homeosis can be identified, as in the example of stamens developing from uncommitted primordia in petal sites in Saraca declinata. Floral development among species in a genus is generally very similar, with any differences usually occurring only terminally. In contrast, ontogenies among related genera show more differences, and those differences may be accelerated, appearing earlier in ontogeny than interspecific differences. Floral development in Ceratonia siliqua, in the same tribe as the Cassia group, diverges strongly from the latter, lacking petals and having a unique type of inception of stamens. Developmental evidence supports removing Ceratonia from Cassieae. Key words: development, evolution, flower, legumes, ontogeny, phylogeny.

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