Chromosome elimination and sex determination in springtails (Insecta, Collembola)

A post-zygotic mechanism of sex determination is described in the two symph ypleonans Dicyrtomina ornata (Nicolet) and Ptenothrix italica Dallai. The p rocess consists of the loss of two sex chromosomes from the male embryo. At the end of the first meiotic division of spermatogenesis, a second chromos ome elimination occurs, allowing half the secondary spermatocytes, later tr ansformed into spermatids, to receive a complete haploid set of chromosomes . The secondary spermatocytes, which receive an incomplete set of chromosom es, degenerate. Males of the two collembolan species, therefore, produce a reduced number (50%) of spermatozoa. Females of D, ornata have 2n = 12 and males 2n = 10 chromosomes; females of P. italica have 2n = 14 and males 2n = 12 chromosomes. In both species, oogenesis proceeds normally and chromoso mes pair and form chiasmata in meiotic prophase. The adaptive significance of this post-zygotic mechanism of sex determination is discussed. The mecha nism seems to be a characteristic feature of the suborder Symphypleona. The neanurid Arthropleona Anurida maritima (Guerin), which was studied for com parative analysis, has 2n = 8 chromosomes and normal spermatogenesis produc ing haploid nuclei with four chromosomes. (C) 1999 Wiley-Liss, Inc.