Characterization and transcriptome analysis of candidate genes involved in hybrid incompatibility of Rhododendron

Abstract

Abstract One of the great unsolved mysteries of evolutionary biology concerns the genetic mechanisms underlying the reproductive isolation among diverging lineages. Hybrid incompatibility can result in reproductive isolation and speciation, showing a reduced fitness in hybrids in comparison with their parents. Therefore, identifying the candidate genes contributing to hybrid incompatibility will provide important information for uncovering the regulation mechanism of reproductive isolation. Although the genetics of hybrid incompatibility has been examined in model organisms, it has been described in relatively few instances in non-model plant. In this study, RNA-Seq was performed on the gene expression profiles of unpollinated ovary (CK) of Rhododendron delavayi as female parent, normal (T1) and aborted (T2) zygotic embryo of R. cyanocarpum and R. sinofaconeri as male parents, respectively, at 40 days after cross-pollination when the development of the two zygotic embryos were differentiated obviously. A total of 5,177 differentially expressed genes (DEGs) were identified by pairwise comparison. Venn analysis found 146 uniquely DEGs in T2-vs-T1, many of which were involved in immune responses, including one natriuretic peptide receptor 1 (NPR1), three accelerated cell death 6 (ACD6) genes and 11 disease resistance proteins. Furthermore, KEGG enrichment analysis showed that only the plant-pathogen interaction pathway were simultaneously enriched in DEGs of CK-vs-T1, CK-vs-T2, and T1-vs-T2, suggesting their important role in hybrid incompatibility. Further analysis found that 12 disease resistance protein, three nucleotide-binding site leucine-rich repeat protein (NLR), two endoplasmin homolog and one mitogen-activated protein kinase 2 (MAPK2) were included in the plant-pathogen interaction pathway. Of them, 13 genes were upregulated in aborted zygotic embryo compared with normal zygotic embryo. We inferred that NPR1, ACD6, disease resistance protein and NLR can activate autoimmune response, in turn, result in hybrid incompatibility in Rhododendron. The study provides a basis for revealing the regulation mechanism of hybrid incompatibility and could benefit genetic breeding of Rhododendron.