Whole genome resequencing of males and females of three Daphnia magna genotypes

Mechanisms of sex determination (SD) differ widely across the tree of
life. In genotypic sex determination (GSD), genetic elements determine
whether individuals are male or female, while in environmental sex
determination (ESD), external cues control the sex of the offspring. In
cyclical parthenogens, females produce mostly asexual daughters, but
environmental stimuli such as crowding, temperature or photoperiod may
cause them to produce sons. In aphids, sons are induced by ESD, even
though GSD is present, with females carrying two X chromosomes and males
only one (X0 SD system). In contrast, although ESD exists in Daphnia, the
two sexes were suggested to be genetically identical, based on a 1972
study on Daphnia magna (2n=20) that used three allozyme markers. This
study cannot, however, rule out an X0 system, as all three markers may be
located on autosomes. Motivated by the life cycle similarities of Daphnia
and aphids, and the absence of karyotype information for Daphnia males, we
tested for GSD (homomorphic sex chromosomes and X0) systems in D. magna
using a whole genome approach by comparing males and females of three
genotypes. Our results confirm the absence of haploid chromosomes or
haploid genomic regions in D. magna males as well as the absence of
sex-linked genomic regions and sex-specific single nucleotide
polymorphisms. Within the limitations of the three populations studied
here and the methods used, we suggest that our results make the
possibility of genetic differences among sexes in the widely used Daphnia
model system very unlikely.