Investigation and screening of chrysanthemum resources to identify self-compatible mutants

Chrysanthemum (Chrysanthemum × morifolium) is an important horticultural crop with great ornamental and economic value. Most chrysanthemum cultivars are self-incompatible, which limits the creation of pure chrysanthemum lines which are the material basis for breeding and theoretical research. Here, artificial self-pollination and natural cross-pollination were used to analyze the self-compatibility (SC) and cross-compatibility (CC) of 85 chrysanthemums cultivated by our research group. By conducting a phenotypic study, observing of pollen germination in vitro and in vivo and gene expression analyses, we explored the reasons for the self-compatibility of chrysanthemum and identified self-compatible mutants. It was found that there were wide variations in self-compatibility among different chrysanthemum cultivars and the SC index ranged from 0 to 5.64, which could classify the chrysanthemum cultivars into three types: self-compatibility (SC index ≥ 1), self-incompatibility (SI, 1 > SC index > 0) and high self-incompatibility (SC index = 0). Seven chrysanthemum cultivars with self-pollinated progenies (‘Hongguan’, K183−8, A22, B12, D87, D76 and S8) were selected, and the pollen germination rates in vitro and in vivo were not significantly related to SC. The pollen tube of ‘Hongguan’ could pass through its own stigma and grow toward the embryo sac along the style, and there was no obvious inbreeding depression in self-pollinated progenies of ‘Hongguan’. In addition, we found that one S-locus receptor kinase (SRK), two S-locus cysteine-rich protein (SCR), two exocyst complex component EXo70A1 (Exo70A1) and one thioredoxin h-like 1 (THL1) were important for regulating self-compatibility of C. × morifolium. Our results not only laid a material foundation for creating pure lines through continuous self-pollination and analyzing the regulation mechanism underlying SC variation but also provided the possibility for theoretical research on the important ornamental traits of chrysanthemums in the future.