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Background   Coccinia grandis  is a dioecious species of Cucurbitaceae having heteromorphic sex chromosomes. The chromosome constitution of male and female plants is 22 + XY and 22 + XX respectively. Y chromosome of male sex is conspicuously large and plays a decisive role in determining maleness. Sex modification has been studied in hypogynous  Silene latifolia  (Caryophyllaceae) but there is no such report in epigynous  Coccinia grandis . Moreover, the role of organ identity genes during sex expression in  Coccinia  has not been evaluated earlier. Investigations on sexual phenotypes of  C. grandis  including a rare gynomonoecious (GyM) form and AgNO 3  mediated sex modification have added a new dimension to the understanding of sex expression in dioecious flowering plants.    Results  Morphometric analysis showed the presence of staminodes in pistillate flowers and histological study revealed the absence of carpel initials in male flowers. Though GyM plant had XX sex chromosomes, the development of stamens occurred in hermaphrodite flowers but the pollens were not fertile. Silver nitrate (AgNO 3 ) application enhanced stamen growth in wild type female flowers like that of GyM plant but here also the pollens were sterile. Differential expression of  CgPI  could be involved in the development of different floral phenotypes.    Conclusions  The three principle factors, Gynoecium Suppression (Su F ), Stamen Promoting Factor (SPF) and Male Fertility (m F ) that control sex expression in dioecious  C. grandis  assumed to be located on Y chromosome, play a decisive role in determining maleness. However, the characteristic development of stamens in hermaphrodite flowers of GyM plant having XX sex chromosomes indicates that Y-linked SPF regulatory pathway is somehow bypassed. Our experimental findings together with all other previous chromosomal and molecular cytogenetical data strongly support the view that  C. grandis  could be used as a potential model system to study sex expression in dioecious flowering plant.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-014-0325-0) contains supplementary material, which is available to authorized users.

Flower development, pollen fertility and sex expression analyses of three sexual phenotypes of Coccinia grandis