In land plants, most chloroplast genomes are single, circular, double-stranded DNA sequences 100-220 kb in size, with a quadripartite structure including one large single-copy (LSC) region, one small single-copy (SSC) region, and a pair of inverted repeat (IR) regions. Available plastomes of the Lauraceae show similar structure and varied size, but there has been no systematic comparison across the family. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and Kunming Institute of Botany (KIB) conducted a study to understand the dynamics and evolution of plastome structure in magnoliids. They focused on the plastomes of the important family Lauraceae and the related families Calycanthaceae (Laurales), Chloranthaceae (Chloranthales), Magnoliaceae (Magnoliales), Piperaceae (Piperales), and Winteraceae (Canellales). 

The researchers compared compare 47 plastomes, 15 newly sequenced, from 27 representative genera. All of the complete plastome sequences of Lauraceae from all five orders (Canellales, Chloranthales, Laurales, Magnoliales, and Piperales) and related families yielded a fully resolved tree. 

The sizes of the fifteen newly sequenced Lauraceae plastomes differed greatly, from 114,623 bp in the hemiparasitic vine, Cassytha capillaris, to 158,530 bp in Beilschmiedia tungfangensis, as a result of the loss of one IR copy and six ndh genes in Cassytha. 

Comparative genomic analysis indicated that missing segments of DNA in Lauraceae plastids mainly drive the genome contraction events. 

Based on their phylogenetic analysis and structural comparisons, different loss events occurred in different lineages of the Laurales, and fragment loss events in the IR regions have largely driven the contraction of the plastome in the Lauraceae. 

Those independent loss events show that in the Lauraceae the plastomes of Neocinnamomum, Cassytha, the core group, and the basal group could share a common ancestral genome structure like that of Caryodaphnopsis henryi, but have subsequently evolved independently with different loss patterns. 

The study entitled “Evolutionary Comparisons of the Chloroplast Genome in Lauraceae and Insights into Loss Events in the Magnoliids” has been published in Genome Biology and Evolution.