First Plastome Sequences of Two Endemic Taxa of Orbea Haw. from the Arabian Peninsula: Comparative Genomics and Phylogenetic Relationships Within the Tribe Ceropegieae (Asclepiadoideae, Apocynaceae)
Alharbi SA. Biology, January 2026.
Read the paper Free to read · doi:10.3390/biology15030223
What this paper is about
The first complete chloroplast genomes for two Arabian members of Orbea, a group of stem succulents in the same subfamily as milkweed, compared against thirteen published genomes from across the tribe.
Both have the standard structure and the same 114 genes. The work is about where these plants sit in the family tree rather than about milkweed as such.
Summarised here from the abstract below. Not the authors’ words.
Abstract
Orbea is a morphologically diverse lineage within the subtribe Stapeliinae, yet plastome evolution in Arabian taxa remains insufficiently characterized. This study reports the first complete chloroplast genomes of Orbea sprengeri subsp. commutata and O. wissmannii var. eremastrum and investigates plastome structure, sequence variability, and phylogenetic relationships across tribe Ceropegieae. Chloroplast genomes were assembled, annotated, and compared with 13 published plastomes representing major Ceropegieae lineages. Both Arabian plastomes displayed the typical quadripartite structure and identical gene content of 114 unique genes, including 80 protein-coding genes, 30 transfer RNA genes, and four ribosomal RNA genes. However, O. wissmannii var. eremastrum exhibited pronounced structural divergence, possessing the largest plastome recorded for the tribe (170,054 bp), an 8.9 kb expansion of the inverted repeat regions, and an 8.4 kb inversion spanning the ndhG-ndhF region. Comparative analyses revealed conserved gene order across Ceropegieae but identified six highly variable loci (accD, clpP, ndhF, ycf1, psbM-trnD, and rpl32-trnL) as potential DNA barcodes. Selection pressure analyses indicated strong purifying selection across most genes, with localized adaptive signals in accD, ndhE, ycf1, and ycf2. Phylogenomic reconstruction consistently resolved the two Arabian Orbea taxa as a distinct clade separate from the African O. variegata. This study fills a gap in Ceropegieae plastid genomics and underscores the importance of sequencing additional Orbea species to capture the full extent of genomic variation within this diverse genus.
Abstract reproduced from the published paper, which is licensed CC BY. Copyright remains with the authors.
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Milkweed USA. (2026). First Plastome Sequences of Two Endemic Taxa of Orbea Haw. from the Arabian Peninsula: Comparative Genomics and Phylogenetic Relationships Within the Tribe Ceropegieae (Asclepiadoideae, Apocynaceae) (listing). Retrieved DATE, from https://milkweedusa.com/research/first-plastome-sequences-of-two-endemic-taxa-of-orbea-2026/
"First Plastome Sequences of Two Endemic Taxa of Orbea Haw. from the Arabian Peninsula: Comparative Genomics and Phylogenetic Relationships Within the Tribe Ceropegieae (Asclepiadoideae, Apocynaceae) (listing)." Milkweed USA, 2026, https://milkweedusa.com/research/first-plastome-sequences-of-two-endemic-taxa-of-orbea-2026/. Accessed DATE.
Milkweed USA. "First Plastome Sequences of Two Endemic Taxa of Orbea Haw. from the Arabian Peninsula: Comparative Genomics and Phylogenetic Relationships Within the Tribe Ceropegieae (Asclepiadoideae, Apocynaceae) (listing)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/research/first-plastome-sequences-of-two-endemic-taxa-of-orbea-2026/.
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title = {First Plastome Sequences of Two Endemic Taxa of Orbea Haw. from the Arabian Peninsula: Comparative Genomics and Phylogenetic Relationships Within the Tribe Ceropegieae (Asclepiadoideae, Apocynaceae) (listing)},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/research/first-plastome-sequences-of-two-endemic-taxa-of-orbea-2026/},
note = {Accessed DATE}
}