Recently, chromosome-scale assembled genomes of
two closely related orchids were published: one is a partially mycoheterotrophic orchid,
Platanthera zijinensis, and the other is a fully mycoheterotrophic orchid, Platanthera
guangdongensis. The two species are only found in one location in Zijin County, Guangdong
Province, China, where they live alongside each other but in quite different habitats, with
P. zijinensis mainly growing on light-abundant rocky hills and P. guangdongensis in the
adjacent forest with a dense canopy. Also, P. zijinensis has leaves and a root system with
tubercles, whereas P. guangdongensis has neither leaves nor roots, but has a tuber
(vertical, underground stem), the organ to which the mycorrhizal fungi, critical to its
holomycoheterotrophic lifestyle, are attached. Comparing the high-quality genomes of P.
zijinensis and P. guangdongensis, together with other sequenced orchids, would hence enable
us to study the evolution of initial, partial and full mycoheterotrophy in orchids. The two
sequenced genomes of P. zijinensis and P. guangdongensis would also fill the gap between
sequenced orchid genomes of photosynthetic orchids on one side and non-photosynthetic
orchids on the other side, paving a way to identify possible genes related to
mycoheterotrophy in Orchidaceae. The genes related to mycoheterotrophy at different
developmental stages of both photosynthetic and non-photosynthetic orchids could also
provide insights into the origin and evolution of mycoheterotrophy in Orchidaceae.
P. zijinensis and P. guangdongensis both have a karyotype of
2n = 2X = 42 chromosomes. To
completely sequence the genomes, we generated a total of 441.16 Gb and 414.20 Gb data for P.
zijinensis and P. guangdongensis, respectively, with multiple insert libraries using PacBio
technologies. With k-mer analyses we estimated a genome size of 4.15 Gb for the P.
zijinensis genome with a heterozygosity of 1.80% and a genome size of 4.27 Gb for the P.
guangdongensis genome with a heterozygosity of 1.89%, thus indicating relatively high
heterozygosity levels for both genomes. The total length of the genome assembly was 4.19 Gb
with a contig N50 value of 1.77 Mb for the P. zijinensis genome and 4.20 Gb with a contig
N50 value of 1.57 Mb for the P. guangdongensis genome.
Genomes of leafy and leafless Platanthera orchids illuminate the evolution of
mycoheterotrophy. Li et al. 2022. Nature Plants.
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