Maternal inheritance of primary sex ratios in the dark-winged fungus gnat Lycoriella ingenua

Sex determination mechanisms in insects are extraordinarily diverse, although most species have zygotic genotypic sex determination where sex is established by sex chromosomes upon fertilisation. Dark-winged fungus gnats (Diptera: Sciaridae) are a large and speciose family of flies where sex determination is a result of an unusual interplay of zygotic, maternal, and environmental factors. This causes some species to produce broods that deviate considerably from the standard 1:1 sex ratio. An early study suggested that these primary sex ratios may be heritable from mother to daughter, but this observation has not been corroborated and the genetic basis for this trait remains unknown. Other studies have found that in some species, there is an additional temperature effect on the primary sex ratio, but again the mechanism is unknown. Here, we perform sibling crosses and temperature-shift experiments in the common mushroom pest Lycoriella ingenua and find evidence for highly variable and heritable primary sex ratios, but no significant environmental effect. We discuss the consequences of our findings for understanding the mechanisms that produce these unusual sex ratios, and the evolution of sex determination more broadly in this clade.