The research aims to explore the cross compatibility between different Sesamum species and analyze the biological characteristics of interspecies hybrid progeny so as to supply the foundation for efficient application of wild sesame species.

A sesame cultivar Yuzhi 11 (S. indicum, 2n=26) and 4 wild Sesamum species including S. latifolium (2n=32), S. calycinum (2n=32), S. angustifolium (2n=32), and S. radiatum (2n=64) were applied to construct interspecies cross combinations using diallel hybridization method by artificial pollination in the field. Embryo rescue method was also used to obtain interspecific hybrid F₁. Interspecific hybrid compatibility was compared based on hybrid capsule formation rate. Botanical characters of hybrids were observed during flowering and mature stages. Pollen fertility was assessed using Alexander staining method. Chromosome number and karyotype characteristics of root somatic cells of hybrids were observed using smear chromosome preparation technique. Specific and polymorphic SSR primers in Sesamum were used to analyze the molecular difference in interspecific hybrids.

Twenty positive and reciprocal cross combinations were constructed for the 5 Sesamum species. A total of 2091 flowers were pollinated and 370 hybrid capsules were harvested. As to the female parents with more chromosomes, hybrid capsules were more easily obtained. The cross compatibility among the 5 Sesamum species significantly varied from 1.18% (S. radiatum×S. calycinum) to 63.33% (S. calycinum×S. angustifolium). F₁ plants of 9 combinations produced hybrid seeds, while the ratio of pollen sterility of F₁ progeny ranged from 35.21%-100.00%. The cross S. calycinum×S. angustifolium presented the highest sterility ratio to 87.68%. Hybrid progeny exhibited the obvious heterosis over parents in plant height, plant type, and some key agronomic traits. As to the positive and reciprocal hybrid F₁ derived from sesame cultivar and the wild species, leaf shape, flower shape, and flower color showed partial characters of both parents. The cross compatibility between sesame cultivar (n=13) and the 3 Sesamum species with chromosome group n=16 ranked as S. angustifolium>S. calycinum>S. latifolium. The cross compatibility between wild species S. radiatum (n=32) and the 3 species with n=16 ranked as S. calycinum>S. angustifolium>S. latifolium. Among the 5 Sesamum species, the genetic relationship between S. calycinum and S. angustifolium is relatively closest. The chromosome number of root tip cells of some hybrid plants is consistent with the theoretical value calculated from the parents. Screening results of the 3 pairs of polymorphic SSR primers indicated that 99.66% of obtained F₁ plants are true hybrid. Chromosome karyotype and SSR marker screening results reflected the genetic difference and characters of Sesamum species.

Among the 5 Sesamum species, the cross compatibility varies significantly and the heterosis of interspecific hybrid is obvious. Of which only S. calycinum and S. angustifolium have the relatively closest genetic relationship and could be directly applied for elite germplasm creation and interspecific hybrid breeding in Sesamum. Reproductive isolation barriers exist in other cross combinations. Some techniques including embryo rescue and molecular marker application should be used to achieve the utilization of wild Sesamum species for sesame breeding.