Chromosome-level genome of the wood stork (Mycteria americana) provides insight into avian chromosome evolution.

Despite being quite specious (~10,000 extant species), birds have a fairly uniform genome size and karyotype (including the common occurrence of microchromosomes) relative to other vertebrate lineages. Storks (Family Ciconiidae) are a charismatic and distinct group of large wading birds with nearly worldwide distribution but few genomic resources. Here we present an annotated chromosome-level reference genome and chromosome orthology analysis for the wood stork (Mycteria americana), a species that has been federally protected under the Endangered Species Act since 1984. The annotated chromosome-level reference assembly was produced using the blood of a wild female wood stork chick, has a length of 1.35 Gb, a contig N50 of 37 Mb, a scaffold N50 of 80 Mb, and a BUSCO score of 98.8%. We identified 31 autosomal pairs and two sex chromosomes in the wood stork genome, but failed to identify four additional autosomal microchromosomes previously found via karyotyping. Orthology analyses confirmed reported synapomorphies unique to storks and identified the chromosomes participating in these fusions. This study highlights the difficulty and potential problems associated with delineating microchromosomes in reference genome assemblies. It also provides a foundation for studying karyotype evolution in the core water bird clade that includes penguins, albatrosses, storks, cormorants, herons, and ibises. Finally, our reference genome will allow for numerous genomic studies, such as genome-wide association studies of local adaptation, that will aid in wood stork conservation.