A review of current progress in our research on sex determination in Diospyros

This review summarizes current progress in our research on sex determination in Diospyros species. We used next generation sequencing technologies and Diospyros lotus, a close relative of the cultivated species Diospyros kaki, as our model. We compared floral initiation and developmental patterns of male and female flowers between D. kaki and D. lotus because the sexual systems of D. kaki differ from those of most other Diospyros species. The sexual system of D. lotus and most Diospyros species is dioecious while D. kaki is polygamous. After confirming that floral initiation patterns were similar in D. lotus and D. kaki, we used D. lotus as a model to study sexuality in Diospyros. We crossed D. lotus ‘Kunsenshi Female’ with D. lotus ‘Kunsenshi Male’ to produce an F ₁ population (the KK F ₁ population) that segregated distinct male and female phenotypes. Using 63 F ₁ offspring, a bulked segregant analysis with amplified fragment length polymorphisms showed that the sexuality of D. lotus is controlled by a single locus or haploblock in a heterogametic male (XY) system. The genomes of D. kaki bearing male flowers contained the maleness DNA marker developed from D. lotus, suggesting that the genetic control of sexuality is similar in D. lotus and D. kaki. We conducted Illumina HiSeq paired-end sequencing of the genomes of 57 individuals from the KK F ₁ population. We cataloged and assembled male-specific subsequences (male-specific k-mers) from the Illumina HiSeq genomic reads data from male and female pools. These analyses enabled us to isolate the male-determining regions on the Y-chromosome in D. lotus. Integrating transcriptome and evolutionary analyses led to the identification of one Y-specific sex-determinant candidate gene, OGI. OGI encodes a small RNA that in turn triggers transitive RNA interference on a feminizing gene, MeGI, located on the autosomal or pseudoautosomal region of the sex chromosome to produce male flowers. These data indicate that the genetic control of maleness expression in Diospyros, including D. kaki, is based on the OGI/MeGI system.