Weird genotypes? Don't discard them, transmissible cancer could be an explanation.

Florentine Riquet,Alexis Simon,Nicolas Bierne

doi:10.1111/eva.12439

Abstract

Genetic chimerism is rarely considered in the analysis of population genetics data, because assumed to be an exceptionally rare, mostly benign, developmental accident. An unappreciated source of chimerism is transmissible cancer, when malignant cells have become independent parasites and can infect other individuals. Parasitic cancers were thought to be rare exceptions, only reported in dogs (Murgia et al., Cell, 2006, 126, 477; Rebbeck et al., Evolution, 2009, 63, 2340), Tasmanian devils (Pearse and Swift, Nature, 2006, 439, 549; Pye et al., Proceedings of the National Academy of Sciences, 2016, 113, 374), and soft‐shell clams (Metzger et al., Cell, 2015, 161, 255). However, the recent simultaneous report of four new contagious leukemias in marine mollusks (Metzger et al., Nature, 2016, 534, 705) might change the rules. By doubling up the number of naturally occurring transmissible cancers, this discovery suggests they may essentially be missed because not sufficiently searched for, especially outside mammals. We encourage population geneticists to keep in mind infectious cancer when interpreting weird genotypes in their molecular data. It would then contribute in the investigation of how widespread contagious cancer could really be in the wild. We provide an example with our own data in Mytilus mussels, a commercially important shellfish. We identified genetic chimerism in a few mussels that suggests the possible occurrence at low prevalence in European M. edulis populations of a M. trossulus contagious cancer related to the one described by Metzger et al. (Nature, 2016, 534, 705) in populations of British Columbia.

Highlights

In addition to the previously reported case in M. arenaria, hallmarks of transmissible cancers have been found in Mytilus trossulus mussels, in Cerastoderma edule cockles, and in the golden carpet-shell clam Polititapes aureus (Metzger et al, 2016)
Among 938 mussels sampled along the European Atlantic coasts, we found what we thought to be five hybrid genotypes between M. edulis and M. trossulus, one in the Wadden (a)
Metzger et al (2016)’s publication suggests transmissible cancers could be widespread in invertebrates, and genetic chimerism may be more frequent than thought

Summary

Introduction

In addition to the previously reported case in M. arenaria, hallmarks of transmissible cancers have been found in Mytilus trossulus mussels, in Cerastoderma edule cockles, and in the golden carpet-shell clam Polititapes aureus (Metzger et al, 2016). This needs to be investigated despite difficulties, in mammals that usually display low genetic diversity, and in bivalves for which host and cancerous cells are often mixed up and co-amplify

Results

Conclusion