The origins of asexual brine shrimps

Determining how and how often asexual lineages emerge within sexual
species is central to our understanding of sex-asex transitions and the
long-term maintenance of sex. Asexuality can arise “by transmission” from
an existing asexual lineage to a new one, through different types of
crosses. The occurrence of these crosses, cryptic sex, variation in ploidy
and recombination within asexuals greatly complicates the study of
sex-asex transitions, as they preclude the use of standard phylogenetic
methods and genetic distance metrics. In this study we show how to
overcome these challenges by developing new approaches to investigate the
origin of the various asexual lineages of the brine shrimp Artemia
parthenogenetica. We use a large sample of asexuals, including all known
polyploids, and their sexual relatives. We combine flow cytometry with
mitochondrial and nuclear DNA data. We develop new genetic distance
measures and methods to compare various scenarios describing the origin of
the different lineages. We find that all diploid and polyploid
A. parthenogenetica likely arose within the last 80,000 years
through successive and nested hybridization events that involved
backcrosses with different sexual species. All
A. parthenogenetica have the same common ancestor and therefore
likely carry the same asexuality gene(s) and reproduce by automixis. These
findings radically change our view of sex-asex transitions in this group,
and show the importance of considering asexuality “by transmission”
scenarios. The methods developed are applicable to many other asexual
taxa.