Genetic relationships of Michelia compressa (Magnoliaceae) with Michelia species and its improvement by interspecific hybridization

Abstract

The genus Michelia encompasses important plants for landscaping and timber, with a wide global distribution. However, the genetic interrelations among species within the genus are not well understood. This study aims to clarify the genetic connections between Michelia compressa (Maxim.) Sarg. (Magnoliaceae) and other species in the genus to probe the potential improvement by interspecific hybridization. The work progresses in three phases: understanding M. compressa’s genetic architecture and its relevance to other Michelia species, identifying natural hybrids among progeny, and developing novel varieties through hybridization. First, the genome from M. compressa was analyzed to confirm its genetic background. The result shows it had 17 pairs of metacentric and 2 pairs of submetacentric chromosomes (2n = 2x = 38, 34m + 4sm), belonging to Stebbins type 2B, which was consistent with the Michelia uniform karyotype formula. Second, transcriptomic data were used to design ten pairs of simple sequence repeat (SSR) primers. The set of ten SSR primers exhibited a high level of polymorphism and were found to be efficient for genotyping. The average number of alleles (Na) per locus was 14.6, with an average expected heterozygosity (He) of 0.860, observed heterozygosity (Ho) of 0.447, and polymorphic information content (PIC) of 0.847. These universal primers were then employed to determine genetic relationships among Michelia species and create SSR fingerprints for 20 Magnoliaceae species. The cluster analysis results revealed that Magnoliaceae was classified into three branches, while Michelia was classified into five branches. M. compressa exhibits a close phylogenetic relationship with both M. maudiae and M. cavaleriei var. platypetala. Third, according to the fingerprinting information, all 19 progeny from open-pollinated M. compressa, i.e., M. ‘Zhongshanhanxiao’, were confirmed to be true hybrids resulting from natural crosses. The putative progenitors of M. ‘Zhongshanhanxiao’ are believed to be M. maudiae and M. cavaleriei var. platypetala. M. compressa demonstrates a significant hybrid affinity with both M. maudiae and M. cavaleriei var. platypetala. The SSR primers developed were used to identify hybrids, particularly in the M. compressa × M. maudiae and M. compressa × M. cavaleriei var. platypetala combinations, where a substantial proportion of true hybrids were found. Further analysis suggests that the probable progenitors of M. ‘Zhongshanhanxiao’ are either M. maudiae or M. cavaleriei var. platypetala. This research yields significant insights into the genetic relationships of M. compressa, as well as the prospects for genetic enhancement via interspecific hybridization. Furthermore, it establishes a theoretical framework for conserving and innovating Magnoliaceae germplasm resources.