The genome of the poecilogonous annelid Streblospio benedicti

Streblospio benedicti is a common marine annelid that has become an important model for developmental evolution. It is the only known example of poecilogony, where two distinct developmental modes occur within a single species, that is due to a heritable difference in egg size. The dimorphic developmental programs and life-histories exhibited in this species depend on differences within the genome, making it an optimal model for understanding the genomic basis of developmental divergence.

Studies using S. benedicti have begun to uncover the genetic and genomic principles that underlie developmental uncoupling, but until now they have been limited by the lack of availability of genomic tools. Here we present an annotated chromosomal-level genome assembly of S. benedicti generated from a combination of Illumina reads, Nanopore long reads, Chicago and Hi-C chromatin interaction sequencing, and a genetic map from experimental crosses.

At 701.4 Mb, the S. benedicti genome is the largest annelid genome to date that has been assembled to chromosomal scaffolds, yet it does not show evidence of extensive gene family expansion, but rather longer intergenic regions. The complete genome of S. benedicti is valuable for functional genomic analyses of development and evolution, as well as phylogenetic comparison within the Annelida and the Lophotrochozoa.

Despite having two developmental modes, there is no evidence of genome duplication or substantial gene number expansions. Instead, lineage specific repeats account for much of the expansion of this genome compared to other annelids.