Seasonality, Microspore Developmental Stages, and Genotypes Affect Plantlet Regeneration and Dihaploid Induction Efficiency in Papaya Anther Culture

Papaya, a trioecious plant, has a potentially low ability for the regeneration of plantlets from anther culture because of the semilethality of microspores in male and hermaphrodite plants. In this study, seasonality, microspore developmental stages, and genotypes were investigated in regard to the enhancement of dihaploid induction in male papaya anther culture. Embryo or callus induction rates tended to vary according to season. Out of 62 female plants established, 47 triploid plants were induced from microspores at uninucleate to uninucleate-mitotic stages, while the 15 diploid plants were induced from microspores at tetrad to uninucleate stages in ‘KC4-WI’. Embryo (0-1.5%) or callus (0-5.0%) induction rates were low in different 5 different strains used for culture. To clarify the derivatives of calli which did not differentiate to plantlets, the sex diagnostic PCR analysis was applied to calli. Of the 19 calli, 15 were identified as females from microspores. However, we could not identify whether the male calli originated from anther wall with heterogenic sex genes (M1m) or from microspores with homogenic sex genes (M1M1) that might not manifest lethality at callus differentiation. To further clarify the derivatives, the genotype analyses for male calli are currently in progress.　Based on these results, we suggest that the following factors are important for improving dihaploid plantlet induction rates, i.e., (1) cultivation of anther-donor plants at approximately 26°C; (2) culture of anthers with microspores at the tetrad–uninucleate stage; and (3) investigation of genotypes with high embryo induction ability.