Hemiparasites obtain nutrients and inorganic salts from host plants through haustoria, a habit that has evolved independently at least 12 times in angiosperms. Cuscuta represents one of the 12 angiosperm orders that have transitioned independently from autotrophs to parasites.
Morphologically, Cuscuta species are stem-hippophores, with roots and leaves completely degenerated or very weakly photosynthetically active or lost. The loss of plastid genes and genome functional degeneration have been reported multiple times in Cuscuta. However, the evolutionary associations between its gene and genome functional degeneration and evolution have not yet been clarified.
In a study published in Plant Molecular Biology, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and their collaborators conducted de novo assembling of 29 new plastomes including 20 samples from seven Cuscuta species and nine autotrophic species of Convolvulaceae. They aimed to decipher the mechanism of plastome evolution in Cuscuta and its autotrophic plant relatives of Cuscutaceae.
The results showed that that the structural variation of plastomes in Convolvulaceae was diverse, with nine types of structural rearrangements and six types of inverted repeat (IR) border expansion-contraction. The structural variations were closely related to parasitic life-form transition and might have been aggravated by IR border expansion-contraction and large repeat fragments. The plastome degeneration of Cuscuta species was progressive, with massive gene loss occurring only in three species from the Ceratophorae group of Grammica subgenus.
Overall, the parasitic habit of Cuscuta promoted the plastome genes to be subjected to relaxed selection constraints, and the accumulation of microvariations in a large number of plastome genes led to the degeneration of plastomes.
"Our study provides new evidence towards?a better understanding of plastomic evolution, variation, and reduction in the genus Cuscuta," said YU Wenbin of XTBG.

Contact
YU Wenbin Ph.D Principal Investigator
Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Mengla, Yunnan 666303, China

E-mail:yuwenbin@xtbg.ac.cn
First published: 15 April 2024