La citologia della partenogenesi di Bacillus rossius

Abstract

The mediterranean stick insect Bacillus rossircs is parthcnogcnctic in tlic northern part of its rnnge and amphigonic in the southern one. Fertilized fcmnlcs give birth to both sexes, while vergin females reproduce by facultative thelytokous parthcnogcncsis. Both collecting nnd breeding have shown that in Italy sexual and vcrginnl reproductions occur and thnt among females with n high yield of parthcnogcnctic offspring there are some Kith a very low or even null hatching (nccidcntal nnd rudimentnry parthenogenesis). On the other hand some females even repeatedly mated, only producc feminine offspring, so bclinving as obligatory parthenogcnctie.The sex chromosome constitution of D. rossius is the commonly found XX, for the female, and XO, for the male. The amphigonic rcproduction is redizcd with the fusion of one sperm, out of the sevcrnl which penetrate through thc micropyle into the egg, with the haploid female pronuclcus. In both facultative nnd obligatory parthenogenetic development the egg-clcavagc begins with the haploid cliromosomc set; only nfter several days of development, when nbout a thousand cells nre formed, the chromosome doubling starts and it is brought about through an nnaphnsic restitution meclinnism, leading to an XX scx-chromosome constitution. Only chromosomically doubled cells escapc dcgcneration and contribute to the futurc embryonic devclopmcnt. The lowering of hatching or cvcn its complete failure is due to tlic rcduced emcicncy or tiic complete blockngc of the chromosome doubling mechanism. The parthenogenetic reproduction of D. rossius is therefore cytologicnlly rather primitive. The completely normal meiosis, the lntc and sometimes inemcient chromosome doubling and the absence of morphological and cytological differentiation between mono and bisexual populations support idea of n non specialized parthcnogcncsis. It must be noted, howcver, that obligntory parthcnogenetic females hare been detected as well and that, especially where the two kinds of populations occur sidc by side. a certnin degree of reproductive isolation exists. B. rossius thus constitutes an unusunlly suitable organism for the esperimcntal study of the parthenogenesis. © 1972 Taylor & Francis Group, LLC.