LAUSR

The genome size (C-value) of a species indicates the total amount of DNA included in its haploid chromosomes. It is one of the fundamental characteristics that relate closely to the evolution of organisms. Information on molluskan C-values is limited despite the abundance of morphological diversity. Therefore, to better understand molluskan genome evolution, we conducted a comprehensive survey of the C-values and AT content of 146 species in four molluskan classes by flow cytometry. The difference in AT content was found to be related to an increase in C-value in gastropods and cephalopods. For cephalopods, a positive correlation of AT content with the increase in chromosome number and C-value was observed from Nautilus pompilius and N. macromphalus (2n = 52) to the specialized groups of octopus (2n > 60) and squid (Sepiida and Teuthida; 2n = 92–112). In contrast, a negative correlation between C-value and AT content in gastropods was demonstrated from the limpet group Patellogastropoda (2n = 16–18) to the specialized group Neogastropoda (2n = 26–72). Our data suggest at a likely positive correlation between AT content and C-value in the cephalopods lineages but a negative correlation between them in the gastropod lineages. Furthermore, the reduction of the C-value in the Bivalvia lineage could not have been caused by the selective deletion of AT-rich regions. Our study adds valuable data to the currently limited knowledge base of molluskan C-values, which will provide important information for future genome projects in mollusks.