Fig. 2: Haploid male genomes indicate that crossover recombination is common.
From: Co-inheritance of recombined chromatids maintains heterozygosity in a parthenogenetic ant
a, Haploid males occur rarely in O. biroi but can be used to detect crossover recombination. Light and dark purple colouring on chromosomes indicates allelic identity and purple lettering next to chromosomes represents genotypes inferred from whole-genome sequencing. A comparison of example genotypes between two individuals illustrates that, using short-read sequencing, more crossovers are detectable between haploid genomes than between diploid genomes. This is because crossover recombination is directly observed in haploid males and inferred from segmental losses of heterozygosity in diploid females. Therefore, crossovers not followed by loss of heterozygosity cannot be detected among diploid genomes. b, The numbers of crossovers detected among all pairwise comparisons of haploid male and diploid female genomes. We sequenced four haploids and four diploids from a single stock of clonal line A, yielding six pairwise comparisons, and three haploids and three diploids from a stock of clonal line B, yielding three pairwise comparisons. c, Karyoplot illustrating all detected crossovers (dark vertical bars) and the number of sites ancestrally heterozygous in 200 kb windows (‘Detectable sites’; grey histograms). Closely spaced vertical bars (crossovers) appear as thick black blocks. Many more crossovers are detected among haploids than among diploids. d, Jittered scatterplot depicting the correlation of crossover recombination with genetic distance in two O. biroi clonal lines. Within each clonal line, each point represents the number of crossovers observed between each male and a randomly selected male from the reference colony (C16 for clonal line A; STC6 for clonal line B). To examine data spanning greater genetic distances, we sequenced one male each from two additional stock colonies from each clonal line. Pairwise genetic distances were calculated between diploid females from the respective colonies. Dotted lines depict linear models. The positive slope and near-zero y intercepts indicate that, rather than excess crossovers being a quirk of haploid male production, crossovers accumulate gradually over generations of diploid-producing parthenogenesis.
