Figure 1.
—Haplodiploidy, PSR and polyploid phenomena occurring in N. vitripennis. (A) Normally, haploid males produce haploid sperm via a nonreductive abortive meiosis (HOGGE and KING 1975 ). Diploid females produce haploid embryos via meiosis (KING and RICHARDS 1969 ; BULL 1982 ). Unfertilized embryos develop parthenogenetically as haploid males. Fertilized embryos develop as diploid females. (B) In the N. vitripennis polyploid strain (WHITING 1960 ), diploid males produce diploid sperm. Haploid eggs that are fertilized by these diploid sperm develop into triploid females. The fecundity of these triploid females is severely reduced because most of their eggs are aneuploid and inviable (WHITING 1960 ). These embryos are indicated as "x" embryos. The remaining viable eggs consist of haploid and diploid eggs. If unfertilized, these eggs develop as haploid and diploid males, respectively. If fertilized by haploid sperm, they develop as diploid and triploid females, respectively. The gray shading indicates the method of maintaining this polyploid strain. Diploid females (st/st) were crossed with diploid males (st +/+ oy). In the next generation, the resulting triploid daughters (st +/+ oy/st +) were allowed to produce sons parthenogenetically. Wild-type, diploid sons (st +/+ oy) were selected and used to repeat the first generation of maintenance crosses. (C) Males that harbor PSR (indicated as "*") produce haploid sperm bearing the paternal autosomes and PSR. Following fertilization, the paternal autosomes condense abnormally and are lost (WERREN et al. 1987 ). The fertilized embryo develops as a haploid male carrying the maternal autosomes and PSR. Unfertilized eggs develop as non-PSR males.
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