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November 26, 2014
 
 
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Mihai Costea

Diversity and evolution of gynoecium in Cuscuta (dodders, Convolvulaceae) in relation to their reproductive biology: two styles are better than one. Pl. Syst. Evol. 296: 51–76.


M. Wright, M. Welsh, M. Costea

published: 2011 | Research publication | Cuscuta

The gynoecium of 122 species and 14 varieties of Cuscuta (dodders) was investigated with light and scanning electron microscopy techniques to asses its diversity and evolution and to provide a morphological foundation for understanding the different reproductive strategies encountered in the genus. Data were optimized into a consensus tree constructed from three large-scale molecular phylogenies of the genus based on nuclear ITS and plastid trnL-F sequences. The number of styles combined with the stigma shape appear to be the only floral/fruit characters that allow a separation of Cuscuta subgenera. In addition, gynoecium morphology is useful to delimit species in certain clades. The one style gynoecium of subg. Monogynella is mostly likely ancestral, while gynoecia with two styles are derived in subgenera Cuscuta and Grammica. Gynoecia with two styles encountered the latter subgenera provide a greater morphological complexity and flexibility for various reproductive strategies. In subg. Cuscuta, both the equal styles and stigmas continue to elongate and modify their position after the flowers open, until pollination takes place. In subg. Grammica, the two unequal styles may create a spatial separation of the sexes in the flower, herkogamy, while the two stigmas mature sequentially and have a differential timing of their receptivity for pollen. A nectary consisting of a ring of modified stomata at the base of the ovary, the equivalent of the hypogynous nectary disc present in many Convolvulaceae, was observed for the first time in all Cuscuta species. The vasculature of the styles is reduced, represented mostly by phloem; xylem is present only in subg. Monogynella. Certain gynoecial characters, such as the papillae diameter, stigma surface area, stigma width, and style width were moderately correlated with the pollen volume, pollen polar-equatorial ratio and tectum perforation. Gynoecium features suggest that Cuscuta is allied with the “bifid clade” (Dicranostyloideae) in Convolvulaceae.

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revised Jun 17/11

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