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Evolutionary transitions of style polymorphism Lithodora (Boraginaceae)

Floral polymorphisms imply the presence of multiple states or morphs for some flower features. Traditionally it has been considered two types of floral polymorphisms, continuous (eg., floral tube length) and discrete (eg., heterostyly or dioecy). Floral polymorphisms are perfect systems where to test evolutionary hypothesis and recent advances in molecular techniques have allowed testing such hypothesis.

Fig1. Evolutionary transitions predicted by Lloyd & Webb's model (1992)

Heterostyly is a floral polymorphism well known since Darwin's description in The different forms of flowers on plants of the same species (1887). Moreover, it is very common among angiosperms (it has evolved in 28 families of angiosperms). Heterostylous populations present two floral morphs that differ in the reciprocal position of sexual whorls in the flowers. One morph has long styles and short anthers (L-morph) and the other, sort styles and long anthers (S-morph). Together with this reciprocal herkogamy, it is common that heterostylous species present an incompatibility system that prevents from self-pollinization and pollinization among the plants of the same morph.

Fig2. Evolutionary transitions observed in tribe Lithospermeae, particularly in genus Lithodora s.l. (Boraginaceae)

In this study ancestral reconstruction of stylar polymorphism was carried under parsimony and maximum likelihood methods in tribe Lithospermeae and in particular in genus Lithodora (Boraginaceae). A detailed morphometric study confirmed different types of stylar polymorphism in the genus. It was also found a new type of stylar polymorphism called relaxed distyly, in which anthers height is variable within a flower (each anther being at a different height) and that contrast to regular distyly (constant anther height within the flowers).

Monomorphism is ancestral to distyly and evolution from monomorphism to distyly involves an intermediate state of stylar dimorphism (in which flowers present two morphs for the style length-long and short- but the anthers are at the same height in both) as predicted by some evolutionary model proposed for heterostyly. As a novel result, we found the evolution from distyly to the new type of polymorphism, relaxed distyly, which could be a result of a relaxation in the selective forces maintaining the distyly.

Fig3. Pollinators appear to be the selective agents driving the evolution

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