Journal
GENOME BIOLOGY AND EVOLUTION
Volume 11, Issue 10, Pages 2789-2796Publisher
OXFORD UNIV PRESS
DOI: 10.1093/gbe/evz187
Keywords
plastome; parasitism; mycoheterotrophy; gene loss; Parasitaxus
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Funding
- Large-Scale Scientific Facilities of the Chinese Academy of Sciences [2017-LSF-GBOWS-02]
- open research project of Cross-Cooperative Team of the Germplasm Bank of Wild Species, Kunming Institute of Botany, Chinese Academy of Sciences
- CAS Light of West China Program [Y8247411W1]
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Plastid genomes (plastomes) of parasitic plants undergo dramatic reductions as the need for photosynthesis relaxes. Here, we report the plastome of the only known heterotrophic gymnosperm Parasitaxus usta (Podocarpaceae). With 68 unique genes, of which 33 encode proteins, 31 tRNAs, and four rRNAs in a plastome of 85.3-kb length, Parasitaxus has both the smallest and the functionally least capable plastid genome of gymnosperms. Although the heterotroph retains chlorophyll, all genes for photosynthesis are physically or functionally lost, making photosynthetic energy gain impossible. The pseudogenization of the three plastomeencoded light-independent chlorophyll biosynthesis genes chIB, chIL, and chiN implies that Parasitaxus relies on either only the light-dependent chlorophyll biosynthesis pathway or another regulation system. Nesting within a group of gymnosperms known for the absence of the large inverted repeat regions (IRs), another unusual feature of the Parasitaxus plastome is the existence of a 9,256 bp long IR. Its short length and a gene composition that completely differs from those of IR-containing gymnosperms together suggest a regain of this critical, plastome structure-stabilizing feature. In sum, our findings highlight the particular path of lifestyle associated reductive plastome evolution, where structural features might provide additional cues of a continued selection for plastome maintenance.
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