Egg maturation and parthenogenetic recovery of diploidy in the scorpion Liocheles australasiae (Fabricius) (Scorpions, Ischnuridae)

In the scorpion Liocheles australasiae, egg maturation and parthenogenetic recoveries of chromosome number and nuclear DNA content were examined by hi stological, karyological observations and quantitative measurements of DNA. The primary oocyte becomes mature through two successive maturation divisi ons. The first maturation division takes place in the primary oocyte to pro duce a secondary oocyte and a first polar body. The second maturation divis ion soon occurs in the secondary oocyte, in which the nucleus is divided in to a mature egg nucleus and a second polar body nucleus, not followed by cy toplasmic fission. The first polar body, in one case, was successively divi ded into two second polar bodies; in the other case it was not divided. In either case, these polar bodies remained attached to the early embryo. The fate of these polar bodies during further embryogenesis were studied. In th e karyological analysis, the chromosome number was divided into two groups, one from 27-32, the other was 54-64. The former was presumably the metapha se chromosome number at the meiotic division; the latter was presumably the metaphase chromosome number at the mitotic division. DNA content in the di ploid nucleus of the primary oocyte, doubled before the maturation division s, was reduced through the maturation divisions by one-half in the nuclei o f the secondary oocyte and the first polar body and by one-fourth in the nu clei of the egg and the second polar bodies. The first reduction of DNA con tent corresponded to halving the number of the chromosomes in the first mat uration division and the second to the nuclear division in the secondary oo cyte. These reductions represent a common process of egg maturation, except the final production of the mature egg with two haploid nuclei, an egg nuc leus, and a second polar body nucleus. These two nuclei, which were formed apart in the mature egg, drew near to fuse into a zygote nucleus. The chrom osome number and nuclear DNA content were doubled in the zygote and each bl astomere in embryos, supporting the hypothesis that the egg nucleus fuses w ith the second polar body nucleus and this conjugation initiates subsequent embryonic development. (C) 2001 Wiley-Liss, Inc.