Genetic diversity and paternity analysis of Mytilaria laosensis hybrids using microsatellite markers provide insight into the breeding system

Mytilaria laosensis is an important evergreen broadleaf tree species in Southeast Asia and can provide excellent raw materials for pulp and fiberboard industrial production due to its fast growth and desirable wood properties. It produces hundreds of hermaphrodite flowers in a single inflorescence and controlled pollination through emasculation is difficult. In this study, we adopted hand-assisted pollination to investigate the seed set ratio and hybrid efficiency. A total of 65 cross combinations including 723 inflorescences were performed by using five female and eight male trees. The fruit-set rate varied greatly from 6.67% to 84.62% among different groups with an average of 23.68%. Moreover, new microsatellite markers were developed through transcriptome sequencing and ten polymorphic markers were screened out to examine the genetic variation and hybrid purity of 742 progeny. An average of 14.7 alleles was detected per locus. The parent population showed the highest expected heterozygosity of 0.77, while other hybrid populations exhibited different heterozygosity values from 0.46 to 0.71. Both principal coordinate analysis and phylogenetic tree revealed that these groups could be divided into five clusters and each cluster represented progeny which shared the same female parent. Paternity test showed that 18 offspring were sired by the designated pollen parents, while all other offspring were all heterozygous with pollens from non-target father. The results implied that outcrossing might be the predominant breeding system and self-incompatibility might exist in M. laosensis. This study provides useful reference for molecular-assisted hybrid breeding research and a practicable approach for new cultivar development in tree species.