Transmissible cancers are parasitic malignant cell lineages that acquired the ability to infect new hosts from the same species, or sometimes related species. First described in dogs and Tasmanian devils, transmissible cancers were later discovered in some marine bivalves affected by a leukemia-like disease. In Mytilus mussels, two lineages of Bivalve Transmissible Neoplasia (BTN), both emerged in a M. trossulus founder individual, have been described to date (MtrBTN1 and MtrBTN2). Here, we performed an extensive screening of genetic chimerism, a hallmark of transmissible cancer, by genotyping 106 SNPs of 5907 European Mytilus mussels. The genetic analysis allowed us to simultaneously obtain the genotype of hosts -M. edulis, M. galloprovincialis or hybrids- and the genotype of tumors of heavily infected individuals. In addition, a subset of 222 individuals were systematically genotyped and analysed by histology in order to screen for possible non-transmissible cancers. We detected MtrBTN2 at low prevalence in M. edulis, and also in M. galloprovincialis and hybrids although at a much lower prevalence. No MtrBTN1 or new BTN were found but 8 individuals with non-transmissible neoplasia were observed at a single polluted site on the same sampling date. We observed a diversity of MtrBTN2 genotypes that appeared more introgressed or more ancestral than MtrBTN1 and reference healthy M. trossulus individuals. The observed polymorphism is most likely due to somatic null alleles caused by structural variations or point mutations in primer-binding sites leading to enhanced detection of the host alleles. Despite low prevalence, two divergent sublineages with 10% somatic null alleles and one non-synonymous mtCOI substitution, are co-spreading in the same geographic area, suggesting a complex diversification of MtrBTN2 since its emergence and host species shift.

5907 Mytilus mussels have been sampled in two tissues (n=1749, hemolymph and mantle) or one tissue (n=4158; hemolymph for 197 or gill for 3988). Then, genotyping have been perform by KASP^TM method on 74 bi-allelic SNPs (subcontracted to LGC genomics (Hoddensdon, UK)). 

Quickly, this method puts two “allele specific” primers with two different fluorochromes in competition during a PCR amplification and gives two values of fluorescence level, one for allele X and the other for allele Y.

This dataset gives the fluorescence values for each alleles (X and Y) and the calling performed by two LGC genomics expert (for more information see the 'Material and method' section of the article and the README file).