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Chromosome-Scale Genome of Aldabra Giant Tortoise

The first chromosome-level genome assembly for the Aldabra giant tortoise (Aldabrachelys gigantea) is reported in GigaScience this week. The animal, which can weigh up to 650 pounds and live 100 years or longer, is one of only two giant tortoise species left in the world and currently faces extinction threats due to climate change and its limited distribution. In order to help conservation efforts, a team led by researchers from the University of Zurich used PacBio high-fidelity sequencing and chromosome conformation capture sequencing to produce a 2.37-Gbp assembly of A. gigantea with a scaffold N50 of 148.6 Mbp, which resolved into 26 chromosomes. RNA sequencing-assisted gene model prediction revealed 23,953 protein-coding genes and 1.1 Gbp of repetitive sequences, while synteny analyses among turtle genomes showed high levels of chromosomal collinearity even among distantly related taxa. The researchers assessed the utility of the reference genome for species conservation by performing low-coverage whole-genome resequencing for 30 wild and two zoo-housed tortoises, inferring the genetic structure of the wild population and the likely origin of the zoo-housed individuals. In addition to its use in preserving A. gigantea populations, the work is expected to inform comparative genomics studies focused on the genetic underpinnings of aging and gigantism given the tortoise's large body size and long life span, the study's authors add.