Genome structure analysis of molluscs revealed whole genome duplication and lineage specific repeat variation

Comparative genome structure analysis allows us to identify novel genes, repetitive sequences and gene duplications. To explore lineage-specific genomic changes of the molluscs that is good model for development of nervous system in invertebrate, we conducted comparative genome structure analyses of three molluscs, pygmy squid, nautilus and scallops using partial genome shotgun sequencing. Most effective elements on the genome structural changes are repetitive elements (REs) causing expansion of genome size and whole genome duplication producing large amount of novel functional genes. Therefore, we investigated variation and proportion of REs and whole genome duplication. We, first, identified variations of REs in the three molluscan genomes by homology-based and de novo RE detection. Proportion of REs were 9.2%, 4.0%, and 3.8% in the pygmy squid, nautilus and scallop, respectively. We, then, estimated genome size of the species as 2.1, 4.2 and 1.8 Gb, respectively, with 2× coverage frequency and DNA sequencing theory. We also performed a gene duplication assay based on coding genes, and found that large-scale duplication events occurred after divergence from the limpet Lottia, an out-group of the three molluscan species. Comparison of all the results suggested that RE expansion did not relate to the increase in genome size of nautilus. Despite close relationships to nautilus, the squid has the largest portion of REs and smaller genome size than nautilus. We also identified lineage-specific RE and gene-family expansions, possibly relate to acquisition of the most complicated eye and brain systems in the three species.