Résumé
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.