Paternal genome elimination, monogenic reproduction, and the evolutionary genetics of atypical sex chromosome systems

Sex chromosomes differ from autosomes in both their ploidy and transmission genetics. Consequently, selection, mutation, and drift may act differently upon them, driving distinct patterns in genetic divergence, diversity, and gene content. Recently, researchers have begun to consider a wider set of organisms with non-standard inheritance and sex-determination systems, however in many cases we lack theory which extends to such cases. One such example is paternal genome elimination (PGE), an unusual reproductive system which has independently evolved in two fly families, the fungus gnats (Sciaridae) and gall midges (Cecidomyiidae), and one order of springtails (Symphypleona). Under PGE, males receive but do not transmit a paternal genome, such that the autosomes and X chromosomes exhibit the same transmission genetics, but with different somatic ploidy. This makes them uniquely suited to test hypotheses about the role of haploid selection in males. Additionally, repeatedly throughout these groups a novel sex determination system – monogeny – has evolved, whereby females produce broods of exclusively one sex. The genetic basis of monogeny partitions the X chromosome into three segments, all displaying distinct inheritance patterns. Here we develop a series of theoretical models adapted to the genetics of these groups, generating testable predictions as to the relative genetic diversity within populations, and divergence between populations. Our results suggest that these species are excellent systems with which to test many fundamental principles in evolutionary genetics.