Automictic parthenogenesis in the diploid-triploid stick insect Bacillus atticus and its flexibility leading to heterospecific diploid hybrids

Summary

Bacillus atticus carius is a complex of diploid and triploid all-female parthenogens. In diploids the first meiotic divisions are regular and segregate two haploid nuclei which fuse back to restore an unreduced condition. Second meiotic divisions yield a polar body and a pronucleus which starts embryo development. In triploids univalent, heterologous and multivalent chromosome associations occur, leading to unbalanced segregating nuclei, as evidenced by DNA measurements. Fused prophase II nuclei reconstitute a triploid nucleus which undergoes metaphase II to originate a polar body and the pronucleus. These findings explain both the heterozygosity pattern and the clonal maintenance of cytotypes in the progeny. They also suggest, in line with allozyme and NOR's findings, possible interracial crosses at the origin of the polyclonal B. atticus complex. Fertilised egg batches of B. atticus females mated to B. grandii or B. rossius males had a lower hatching than virgin egg controls, indicating that sperm-egg interaction occurs and that the ensuing hybrid embryos suffer from bringing together heterospecific gene pools. Nonetheless, a low number of diploid hybrids—via fertilisation by sperm of haploid ova which had resumed a normal meiosis—or triploid hybrids—via sperm genome addition to diploid ova—were obtained. On the whole, the maintenance of some recombination during prophase I and the ability to produce diploid and triploid hybrids demonstrates that these parthenogenetic strains are neither invariant nor reproductively isolated from related taxa, as also occurs in other unisexuals.

Key words:

• Genome addition
• hybridization
• meiotic flexibility
• polyploidy
• unisexual diversification