Yongsung Lee 1,5, Chung Hyun Cho 1,5, Chanyoung Noh 2,5, Ji Hyun Yang 1, Seung In Park 1, Yu Min Lee 1, John A. West 3, Debashish Bhattacharya 4, Kyubong Jo 2,* & Hwan Su Yoon 1,*
1Department of Biological Sciences, Sungkyunkwan University, Suwon 16419, Korea.
2Department of Chemistry, Sogang University, Seoul 04107, Korea.
3School of Biosciences 2, University of Melbourne, Parkville, Victoria 3010, Australia.
4Department of Biochemistry and Microbiology, Rutgers University, New Brunswick 08901, USA.
5These authors contributed equally: Yongsung Lee, Chung Hyun Cho, Chanyoung Noh.
*Corresponding author: correspondence to Kyubong Jo or Hwan Su Yoon
Eukaryotic organelle genomes are generally of conserved size and gene content within phylogenetic groups. However, significant variation in genome structure may occur. Here, we report that the Stylonematophyceae red algae contain multipartite circular mitochondrial genomes (i.e., minicircles) which encode one or two genes bounded by a specific cassette and a conserved constant region. These minicircles are visualized using fluorescence microscope and scanning electron microscope, proving the circularity. Mitochondrial gene sets are reduced in these highly divergent mitogenomes. Newly generated chromosome-level nuclear genome assembly of Rhodosorus marinus reveals that most mitochondrial ribosomal subunit genes are transferred to the nuclear genome. Hetero-concatemers that resulted from recombination between minicircles and unique gene inventory that is responsible for mitochondrial genome stability may explain how the transition from typical mitochondrial genome to minicircles occurs. Our results offer inspiration on minicircular organelle genome formation and highlight an extreme case of mitochondrial gene inventory reduction.