Abstract
Castor (Ricinus communis L) is an ideal model species for sex mechanism studies in monoecious angiosperms, due to wide variations in sex expression. Sex reversion to monoecy in pistillate lines, along with labile sex expression, negatively influences hybrid seed purity. The study focuses on understanding the mechanisms of unisexual flower development, sex reversions and sex variations in castor, using various genotypes with distinct sex expression pattern. Male and female flowers had 8 and 12 developmental stages respectively, were morphologically similar till stage 4, with an intermediate bisexual state and were intermediate between type 1 and type 2 flowers. Pistil abortion was earlier than stamen inhibition. Sex alterations occurred at floral and inflorescence level. While sex-reversion was unidirectional towards maleness via bisexual stage, at high day temperatures (Tmax > 38 °C), femaleness was restored with subsequent drop in temperatures. Temperature existing for 2–3 weeks during floral meristem development, influences sexuality of the flower. We report for first time that unisexuality is preceded by bisexuality in castor flowers which alters with genotype and temperature, and sex reversions as well as high sexual polymorphisms in castor are due to alterations in floral developmental pathways. Differentially expressed (male-abundant or male-specific) genes Short chain dehydrogenase reductase 2a (SDR) and WUSCHEL are possibly involved in sex determination of castor.
Highlights
Castor (Ricinus communis L) is an ideal model species for sex mechanism studies in monoecious angiosperms, due to wide variations in sex expression
High polymorphism of sex expression in castor ranges from genotypes having spikes which are completely pistillate to completely staminate, with interspersed staminate flowers or in DPC and capsule formation (ISF), monoecious with apical ISF, or apically non-interspersed, or with terminal hermaphrodite flower and having monoecious sex variants showing variation in percentage as well as the relative position of male and female flowers within the g enotype[10,12,13]
Sex determination stage and an intermediate bisexual stage were delineated and first time reported in both male and female flowers in castor
Summary
Castor (Ricinus communis L) is an ideal model species for sex mechanism studies in monoecious angiosperms, due to wide variations in sex expression. The hereditary instability affecting sex, manifested as sex reversal in pistillate lines is a widely prevalent phenomenon in castor, which negatively influences hybrid seed purity. Such reversions may be early or late, when it occurs respectively at lower or higher than quaternary orders of spikes, later reversions resulting in more females in the p rogenies[10,15]. For maintenance of pistillate lines, interspersed staminate flowers are induced during summer and hybrid seed production is carried out during rabi season[16] This method requires less roguing of sex-revertant spikes, but is highly season-dependent. More than 3000 differentially expressed genes were identified between monoecious and pistillate genes, possibly involved in sex determination in c astor[29]
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