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Figure 4 | BMC Evolutionary Biology

Figure 4

From: Reticulate evolution in stick insects: the case of Clonopsis (Insecta Phasmida)

Figure 4

Diploidization mechanism: origin of parthenogens. The proposed fast diploidization mechanism through an "intermediate triploid female" with altered meiosis and trivalent segregation. Scheme A: two complete series of chromosomes are segregated to the oocyte II, while the remaining one ends up in the first polocyte. A cytologically normal second meiotic division follows, and an anaphasic restitution (i.e. retention of the polar body) doubles the chromosome number, by the suppression of the second polocyte degeneration. As a result of this process, the oocyte has a tetraploid chromosome number, but chromosomes are in pairs and the karyotype has a perfect diploid look (hypothesis for C. soumiae parthenogen origin). Scheme B: half of the trivalents are segregating 2⇔1, the other half 1⇔2. The resulting oocyte II is therefore aneuploid, i.e. half of the chromosomes are present twice, but the other half only once. A normal second division follows, and an anaphasic restitution produces an egg that has a triploid chromosome number, but again the karyotype looks structurally diploid (hypothesis for C. gallica strains origin).

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