Analysis of inbreeding depression in the tea plant (Camellia sinensis) based on the self-compatible cultivar ‘Ziyan’

•Strong inbreeding depression of fitness traits was observed in the selfing progeny.•DNA marker-based paternal analysis is efficient to determine self-compatibility.•Self-compatible tea plants may be useful breeding and genetic researches. Most tea plants (Camellia sinensis) are considered self-incompatible (SI). It is difficult to identify self-compatible (SC) tea mutants through controlled hand-pollination. ‘Ziyan’ (ZY), a purple tea cultivar, was previously found to be SC by pedigree relationship reconstruction of its offspring using DNA markers. To assess the inbreeding depression in C. sinensis, in this study we identified 29 selfing progeny plants (SP) of ZY and compared their fitness traits and contents of secondary metabolites with the out-crossing progeny (OP, n = 38 plants) and the clonal mother ZY (n = 3 plants). The results showed that fitness traits, including root-collar diameter, matured leaf area, and fruit number of SP, were significantly lower than those of OP and ZY. Notably, the average fruit number of SP was only 9.0 % of ZY and 7.3 % of OP. The spring bud flush time of SP was also significantly later than the OP as indicated by the sprouting index on March 30th, 2020. However, the average content of anthocyanin of SP was 3.67 times that of OP. The total catechin and caffeine contents in SP were lower than those in OP and ZY, but less significantly. Together, the results indicated a strong inbreeding depression of fitness traits in the first selfing generation of ZY, while the impacts on secondary metabolites were not consistent. In addition, a possible homozygous lethal allele was found on LG03. The findings of this study have useful implications for inbred line-based breeding and researches in C. sinensis.