New Insight into the Phylogeny and Taxonomy of Cultivated and Related Species of <i>Crataegus</i> in China, Based on Complete Chloroplast Genome Sequencing

Hawthorns (Crataegus L.) are one of the most important processing and table fruits in China, due to their medicinal properties and health benefits. However, the interspecific relationships and evolution history of cultivated Crataegus in China remain unclear. Our previously published data showed C. bretschneideri may be derived from the hybridization of C. pinnatifida with C. maximowiczii, and that introgression occurs between C. hupehensis, C. pinnatifida, and C. pinnatifida var. major. In the present study, chloroplast sequences were used to further elucidate the phylogenetic relationships of cultivated Crataegus native to China. The chloroplast genomes of three cultivated species and one related species of Crataegus were sequenced for comparative and phylogenetic analyses. The four chloroplast genomes of Crataegus exhibited typical quadripartite structures and ranged from 159,607 bp (C. bretschneideri) to 159,875 bp (C. maximowiczii) in length. The plastomes of the four species contained 113 genes consisting of 79 protein-coding genes, 30 tRNA genes, and 4 rRNA genes. Six hypervariable regions (ndhC-trnV(UAC)-trnM(CAU), ndhA, atpH-atpI, ndhF, trnR(UCU)-atpA, and ndhF-rpl32), 196 repeats, and a total of 386 simple sequence repeats were detected as potential variability makers for species identification and population genetic studies. In the phylogenomic analyses, we also compared the entire chloroplast genomes of three published Crataegus species: C. hupehensis (MW201730.1), C. pinnatifida (MN102356.1), and C. marshallii (MK920293.1). Our phylogenetic analyses grouped the seven Crataegus taxa into two main clusters. One cluster included C. bretschneideri, C. maximowiczii, and C. marshallii, whereas the other included C. hupehensis, C. pinnatifida, and C. pinnatifida var. major. Taken together, our findings indicate that C. maximowiczii is the maternal origin of C. bretschneideri. This work provides further evidence of introgression between C. hupehensis, C. pinnatifida, and C. pinnatifida var. major, and suggests that C. pinnatifida var. major might have been artificially selected and domesticated from hybrid populations, rather than evolved from C. pinnatifida