Subtelomeric repeat expansion in Hydractinia symbiolongicarpus chromosomes.

Despite the striking conservation of animal chromosomes, their repetitive element complements are vastly diverse. Only recently, high quality chromosome-level genome assemblies enabled identification of repeat compositions along a broad range of animal chromosomes. Here, utilizing the chromosome-level genome assembly of Hydractinia symbiolongicarpus, a colonial hydrozoan cnidarian, we describe an accumulation of a single 372 bp repeat unit in the subtelomeric regions. Based on the sequence divergence, its partial affinity with the Helitron group can be detected. This sequence is associated with a repeated minisatellite unit of about 150 bp. Together, they account for 26.1% of the genome (126 Mb of the 483 Mb). This could explain the genome size increase observed in H. symbiolongicarpus compared with other cnidarians, yet distinguishes this expansion from other large cnidarian genomes, such as Hydra vulgaris, where such localized propagation is absent. Additionally, we identify a derivative of an IS3EU-like DNA element accumulated at the putative centromeric regions. Our analysis further reveals that Helitrons generally comprise a large proportion of H. symbiolongicarpus (11.8%). We investigated Helitron presence and distributions across several cnidarian genomes. We find that in Nematostella vectensis, an anthozoan cnidarian, Helitron-like sequences were similarly accumulated at the subtelomeric regions. All these findings suggest that Helitron derivatives are prone to forming chromosomal extensions in cnidarians through local amplification in subtelomeric regions, driving variable genome expansions within the clade.