A team of scientists from Hong Kong, the United Kingdom and the United States has sequenced and assembled the chromosomal-level genomes of two very different millipede species: the orange rosary millipede Helicorthomorpha holstii and the rusty millipede Trigoniulus corallinus. Their results provide important insights into arthropod evolution and highlight the genetic underpinnings of unique features of millipede physiology.
The subphylum Myriapoda, composed of millipedes and centipedes, is a fascinating but poorly understood branch of life, including species with a highly unusual body plan and a range of unique adaptations to their environment.
Centipede is Latin for ‘100 feet,’ but centipedes have between 30 and 354 legs, and no species has exactly 100 legs.
In contrast, millipede is Latin for ‘1,000 feet,’ and while millipedes include the ‘leggiest’ animal on Earth, no species has as many as 1,000 legs, with the true number varying between 22 and 750 legs.
Myriapods were among the first arthropods to invade the land from the sea, during an independent terrestrialization from early arachnids and insects, which occurred during the Silurian period approximately 400 million years ago.
Today, the Myriapoda consists of approximately 16,000 species, all of which are terrestrial.
“Currently, just two myriapod genomes are available: the genome of the centipede Strigamia maritime and a draft genome of the millipede Trigoniulus corallinus,” said first author Dr. Zhe Qu from the Chinese University of Hong Kong and colleagues.
“Consequently, the myriapods, and particularly the millipedes, present an excellent opportunity to improve understanding of arthropod evolution and genomics.”
The researchers sequenced the genomes of two millipede species — Helicorthomorpha holstii and Trigoniulus corallinus — from two different orders, each distributed widely throughout the world.
They also analyzed the gene transcripts (transcriptomes) at different stages of development, and the proteins (proteomes) of the toxin-producing ‘ozadene’ glands.
They found that the two species have genomes of vastly different sizes — Helicorthomorpha holstii’s genome is 182 million base pairs (Mb), while the genome of Trigoniulus corallinus is 449 Mb.
This is was due mainly to Trigoniulus corallinus’ genome containing larger non-coding regions (introns) within genes and larger numbers of repetitive ‘junk’ DNA sequences.
Homeobox genes play central roles in body plan formation and segmentation during animal development, and the authors found lineage-specific duplications of common homeobox genes in their two species, which differed as well from those found in the previously published millipede genome.
None of the three, however, displayed the massive duplications seen in the homeobox genes in the centipede genome.
They made further discoveries about the organization and regulation of the homeobox genes as well.
Many millipedes bear characteristic glands on each segment, called ozadene glands, which synthesize, store, and secrete a variety of toxic and noxious defensive chemicals.
The team identified multiple genes involved in production of these chemicals, including genes for synthesizing cyanide, as well as antibacterial, antifungal, and antiviral compounds, supporting the hypothesis that ozadene gland secretions protect against microbes as well as predators.
The results provide new insights into evolution of the myriapods, and arthropods in general.
“The genomic resources we have developed expand the known gene repertoire of myriapods and provide a genetic toolkit for further understanding of their unique adaptations and evolutionary pathways,” said senior author Dr. Jerome Hui, a researcher at the Chinese University of Hong Kong.
The study was published in the journal PLoS Biology.
Z. Qu et al. 2020. Millipede genomes reveal unique adaptations during myriapod evolution. PLoS Biol 18 (9): e3000636; doi: 10.1371/journal.pbio.3000636