Abstract
Heteromorphic self-incompatibility (SI) is an important system for preventing inbreeding in the genus Primula. However, investigations into the molecular mechanisms of Primula SI are lacking. To explore the mechanisms of SI in Primula maximowiczii, the pollen germination and fruiting rates of self- and cross-pollinations between pin and thrum morphs were investigated, and transcriptomics analyses of the pistils after pollination were performed to assess gene expression patterns in pin and thrum SI. The results indicated that P. maximowiczii exhibits strong SI and that the mechanisms of pollen tube inhibition differ between pin and thrum morphs. While self-pollen tubes of the pin morph were able to occasionally, though rarely, enter the style, those of the thrum morph were never observed to enter the style. The transcriptomics analysis of the pistils revealed 1311 and 1048 differentially expressed genes (DEGs) that were identified by comparing pin self-pollination (PS) vs. pin cross-pollination (PT) and thrum self-pollination (TS) vs. thrum cross-pollination (TP). Notably, about 90% of these DEGs exhibited different expression patterns in the two comparisons. Moreover, pin and thrum DEGs were associated with different Gene Ontology (GO) categories and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways following enrichment analyses. Based on our results, the molecular mechanisms underlying the pin and thrum SI in P. maximowiczii appear to be distinct. Furthermore, the genes involved in the SI processes are commonly associated with carbohydrate metabolism and environmental adaptation. These results provide new insight into the molecular mechanisms of Primula SI.
Highlights
Self-incompatibility (SI) is a common mechanism that promotes outcrossing in angiosperms
We found that the fruiting rates of pin and thrum cross-pollinations (PT and TP) were 65.11% and 71.24%, respectively, while those of self-pollinations (PS and thrum self-pollination (TS)) were zero (Figure 1B)
These results indicate that the compatible pollen of PT and TP crosses started to geminate at 4 and 8 h after pollination, respectively, while the pollen tubes of PS self-crosses rarely entered the style and those of TS self-crosses never entered the style
Summary
Self-incompatibility (SI) is a common mechanism that promotes outcrossing in angiosperms. Dimorphic heteromorphic SI has been observed in 91% of Primula species, and this species is often considered to be a model system for heterostyly [4], with the two types of flowers varying in style length, anther position, and pollen size [5]. Further studies indicated that three genes are present at the S locus as a co-adapted linkage group: G (style length and incompatibility), P (pollen size and incompatibility), and A (anther position) [2]. The Primula CYP734A50 gene was identified at the S locus and is expressed in thrum styles [13], while the GLOT, CYPT, PUMT, KFBT, and CCMT genes were shown to be present at the S locus only in the thrum morph [10]. The key genes that regulate heteromorphic SI in Primula remain unknown
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