Self-(in)compatibility in Tunisian apple accessions [Malus domestica. Borkh]: S-genotypes identification and pollen tube growth analysis.

Abstract

Self-incompatibility studies have revealed a potential use of Tunisian apple resources for crop improvement and modern breeding programs and a likely correlation between the pollen tube growth and flowering period. Apples [Malus domestica. Borkh] exhibit an S-RNase-based gametophytic self-incompatibility (GSI) system. Four primer combinations were used to S-genotype eighteen Tunisian local apple accessions and twelve introduced accessions that served as references. Within the Tunisian local accessions, S2, S3, S7, and S28 S-alleles were the most frequent and were assigned to 14 S-genotypes; among them, S7S28, S3S7, S2S5, and S2S3 were the most abundant. PCA plot showed that population structuring was affected by the S-alleles frequencies and revealed a modern origin of the Tunisian varieties rather than being ancient ones. Nonetheless, the results obtained with 17 SSR markers showed a separate grouping of local Tunisian accessions that calls into question the hypothesis discussed. Pollination experiments showed that the pollen started to germinate within 24h of pollination but 48h after pollination in the "El Fessi" accession. The first pollen tubes arrived in the styles within 36h of pollination in two early flowering accessions known as "Arbi" and "Bokri", and after 72h of pollination in late flowering "El Fessi" and 48h after pollination in remaining accessions. The first pollen tube arrests were observed in accessions "Arbi" and "Bokri" within 84h of pollination, within 108h of pollination in "El Fessi" and within 108h of pollination in remaining accessions. In the apple accession called "Boutabgaya," the pollen tubes reached the base of the style within 120h of pollination without being aborted. Nevertheless, the self-compatible nature of "Boutabgaya" needs more studies to be confirmed. However, our results revealed the malfunction of the female component of the GSI in this accession. To conclude, this work paved the path for further studies to enhance the insight (i) into the relation between the flowering period and the pollen tube growth, (ii) self-compatible nature of "Boutabgaya", and (iii) the origin of the Tunisian apple.