We investigated the maturation divisions of two obligate parthenogenetic natural interspecific hybrids: the diploidB. whiteiand the triploidB. lynceorum. B. whiteishows two maturation divisions which produce 4 viable haploid nuclei. At the telophase II two non-sister nuclei come into contact and fuse to restore the diploid chromosome number and the maternal genetic structure. On the other hand, in the triploidB. lynceorum, pachytenic oocytes undergo an extra DNA doubling which allows the formation of a somatic number of autobivalents. Two divisions in rapid succession result in a pronucleus with the somatic number of unistranded chromosomes and two quickly degenerating polar bodies: this mechanism maintains the maternal cytological and genetic structure in the offspring.