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.

Decoupled maternal and zygotic genetic effects shape the evolution of development

Evolutionary transitions from indirect to direct development involve changes in both maternal and zygotic genetic factors, with distinctive population-genetic implications, but empirical data on the genetics of such transitions are lacking. The polychaete Streblospio benedicti provides an opportunity to dissect a major transition in developmental mode using forward genetics. Females in this species produce either small eggs that develop into planktonic larvae or large eggs that develop into benthic juveniles. We identify large-effect loci that act maternally to influence larval size and independent, unlinked large-effect loci that act zygotically to affect discrete aspects of larval morphology. The likely fitness of zygotic alleles depends on their maternal background, creating a positive frequency-dependence that may homogenize local populations. Developmental and population genetics interact to shape larval evolution.

New species of Dispio Hartman, 1951 and Streblospio Webster, 1879 (Polychaeta, Spionidae) from the coast of the Iberian Peninsula

Re-assessment of spionid specimens from Iberian Peninsula initially assigned to Dispio uncinata Hartman, 1951 and Streblospio benedicti Webster, 1879 led to the recognition of two new species from the spionid genera Dispio and Streblospio from coasts around the Iberian Peninsula. Dispio elegans sp. nov. is characterised by having an oblanceolate-shaped prostomium. In addition, the first two notopodial postchaetal lamellae are serrated with digitiform papillae, the anterior neuropodial lamellae are smooth; all branchiae are almost completely fused to the notopodial lamellae, but with the tips free; the notochaetae on chaetiger 1 are smooth, alimbate capillaries; the ventral chaetae located in the position of the sabre chaetae on chaetigers 1–2 are smooth, alimbate capillaries, but becoming granulated on chaetiger 3; and the middle and posterior chaetigers are granulated and reticulated. Streblospio padventralis sp. nov. is characterized by lacking dorsal papillae between the branchiae of chaetiger 1; the hooks have 4–5 pairs of small teeth; sabre chaetae are present from chaetiger 3; the pygidium has two ventral lappets; and brooding structures are present in the coelomic cavities. We suggest raising the status of Streblospio benedicti japonica Imajima, 1990 to full species level as S. japonica Imajima, 1990. A key for Streblospio species is provided.

Dimorphic development inStreblospio benedicti: genetic analysis of morphological differences between larval types

The marine polychaeteStreblospio benedictiexhibits two distinct larval types, making it a model for the study of developmental evolution. Females produce either large eggs or small ones, which develop into distinct lecithotrophic or planktotrophic larvae with concomitant morphological and life-history differences. Here, we investigate the inheritance of key morphological traits that distinguish the larval types. We use genetic crosses to establish the influence of maternal and zygotic differences on larval phenotypes. We find a large maternal effect on larval size and the number of larval chaetae, while the number and length of these chaetae are also strongly influenced by zygotic genotype. Interestingly, the distribution of larval phenotypes produced by these crosses suggests traits intermediate to the two parental types should not be uncommon. Yet, despite gene flow between the types in natural populations, such intermediates are rarely found in nature, suggesting that selection may be maintaining distinct larval modes.