Aberrant spermatogenesis and the peculiar mechanism of sex determination in Symphypleonan collembola (Insecta)

Light and electron microscopy evidence have been obtained to describe the p eculiar spermatogenesis in the collembolan species Sminthurus viridis and A llacma fusca (Sminthuridae), In these two species, the two sexes differ for the lack of two chromosomes (the sex chromosomes) in males (males, 2n = in ; females, 2n = 12), While oogenesis seems to proceed normally, spermatogen esis is peculiar because the two daughter cells of the first meiotic divisi on have different chromosome numbers (six and four). The cell receiving fou r chromosomes degenerates, while the cell receiving six chromosomes complet es meiosis and produces identical spermatozoa (n = 6), At fertilization, pr onuclei with six chromosomes fuse together to form zygotes with 2n = 12, Ma le embryos must lose two sex chromosomes during the first zygotic mitosis, as all male cells have 2n = 10 chromosomes. The sex chromosome system of th ese species can be identified as X1X1X2X2:X(1)X(1)0. Electron microscopy ob servations show that the same peculiar spermatogenesis occurs also in two o thers species of the same family, Caprainea marginata and Lipothrix lubbock i, The peculiar sex determination system described is similar but not ident ical to what is observed in other insect orders, and it may represent an ev olutionary step toward parthenogenesis, It is suggested that this peculiar spermatogenesis is common to all Symphypleona.