Origin of Secondary Somatic Embryos and Genetic Stability of the Regenerated Plants in Hevea brasiliensis

Micropropagation by secondary embryogenesis had been established in Hevea brasiliensis. However, origin and development of secondary embryos and genetic stability of the regenerated plants remained unclear Visual and cytological observations found secondary embryos mainly arose from epidermal cells of cotyledons and might originate from single cells. The number of pro-embryos (45.00) peaked after 20 days of cultivation for cell differentiation. At the maturation stage, the sum of immature embryos including globular, heart-like, and torpedo-liked secondary embryos (22.50) was approximately equal to the number of cotyledonary embryos (21.75), which was much lower than that ofpro-embryos (45.00), indicating a great number of pro-embryos were not able to convert into cotyledonary embryos under the current culture conditions. Evaluation of the genetic stability through chromosome number counting and EST-SSRs fingerprint, showed the chromosome number of the regenerated plants was similar to the mother tree (2n=36), the variation rates ofEST-SSRs loci and the regenerated plants initially increased and then decreased with multiplication cycles, and the variation rate of EST-SSRs loci was considered low (<2.61%), indicating the genome remained stable during multiplications. These results could further improve secondary embryogenesis system and inquire into its potential applications in H. brasiliensis.