Abstract
Nemertea is a phylum consisting of 1300 mostly marine species. Nemertea is distinguished by an eversible muscular proboscis, and most of the species are venomous. Genomic resources for this phylum are scarce despite their value in understanding biodiversity. Here, we present genome size estimates of Nemertea based on flow cytometry and their relationship to different morphological and developmental traits. Ancestral genome size estimations were done across the nemertean phylogeny. The results increase the available genome size estimates for Nemertea three-fold. Our analyses show that Nemertea has a narrow genome size range (0.43–3.89 pg) compared to other phyla in Lophotrochozoa. A relationship between genome size and evolutionary rate, developmental modes, and habitat was found. Trait analyses show that the highest evolutionary rate of genome size is found in upper intertidal, viviparous species with direct development. Despite previous findings, body size in nemerteans was not correlated with genome size. A relatively small genome (1.18 pg) is assumed for the most recent common ancestor of all extant nemerteans. The results provide an important basis for future studies in nemertean genomics, which will be instrumental to understanding the evolution of this enigmatic and often neglected phylum.
Highlights
The new estimates show that the 2C values in Nemertea range between 0.43 pg in Emplectonema gracile (Johnston, 1837) (Hoplonemertea: Monostilifera) and 3.89 pg in Lineus acutifrons Southern, 1913 (Pilidiophora: Heteronemertea)
Our analyses showed that the evolution of genome size in Nemerteans is related to reproductive and developmental type as well as habitat, as differences in several trait evolutionary parameters were revealed
The genome size estimates increase the available data for nemerteans three-fold and the phylum-wide genome size range increases from almost five-fold to over ninefold
Summary
Nemertea (ribbon worms) is a clade of worm-shaped animals comprising approximately 1300 species, with close phylogenetic affinities to Lophotrochozoa [1,2,3]. The most species-rich lineage within Nemertea are Hoplonemertea that possess a proboscis that is armed with one large (Monostilifera) or several smaller (Polystilifera) calcareous stylets [6]. Another commonly encountered clade, Pilidiophora, is characterized by a unique, helmet-shaped larval type, the pilidium [7,8]. Hoplonemertea and Pilidiophora comprise a monophyletic lineage termed Neonemertea [9,10]. Palaeonemertea, as currently defined, represents a third clade comprising morphologically diverse species that is sister to Neonemertea [10,11,12]
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