Abstract
BackgroundCoccinia 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 AgNO3 mediated sex modification have added a new dimension to the understanding of sex expression in dioecious flowering plants.ResultsMorphometric 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 (AgNO3) 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.ConclusionsThe three principle factors, Gynoecium Suppression (SuF), Stamen Promoting Factor (SPF) and Male Fertility (mF) 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 materialThe online version of this article (doi:10.1186/s12870-014-0325-0) contains supplementary material, which is available to authorized users.
Highlights
Coccinia grandis is a dioecious species of Cucurbitaceae having heteromorphic sex chromosomes
There is no doubt that the ‘female suppressing’ functions of the Y chromosome in male C. grandis is an initial event towards the establishment of the sexual dimorphism
The process of stamen initiation occurs in wild type female and GyM plants even in the absence of Y chromosome but the arrest of further development of stamens suggests a possible interference in ‘Stamen Promoting’ functions (SPF)
Summary
Coccinia grandis is a dioecious species of Cucurbitaceae having heteromorphic sex chromosomes. Investigations on sexual phenotypes of C. grandis including a rare gynomonoecious (GyM) form and AgNO3 mediated sex modification have added a new dimension to the understanding of sex expression in dioecious flowering plants. Some dioecious plant species have fertile bisexual relatives [10], which are excellent system for sex chromosome study. The occurrence of sex chromosomes in dioecious plants is surprisingly rare and only 19 species are known to have heteromorphic sex chromosomes [10]. The heteromorphic sex chromosomes are well-studied in Silene latifolia (Caryophyllaceae), in which male and female plants carry XY and XX sex chromosomes respectively [11]. Sex determination is controlled by a pair of recently evolved sex chromosomes, Y controlling male and YH controlling hermaphrodite [18]. Sex chromosome study in different dioecious plant species provides an insight for better understanding of plant sex chromosome evolution
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