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Figure 6 | BMC Genetics

Figure 6

From: A single mutation results in diploid gamete formation and parthenogenesis in a Drosophila yemanuclein-alpha meiosis I defective mutant

Figure 6

Working model integrating the cytological observations and the genetic analysis of yem1 mutants to account for the formation of the diploid eggs. In a wild type oocyte chromatids arms of the homologues undergo exchange. The kinetochores of the sister chromatids whose orientation is monopolar are maintained by meiosis I specific cohesion. This configuration is necessary and sufficient for metaphase I arrest in the mature oocyte. In a recombination defective mutant (mut-rec), homologues are not maintained any longer through their arms: no tension is generated. Meiosis resumes prematurely in the oocyte before ovulation (precocious anaphase) and the homologues fail to segregate properly. These defects do not affect sister chromatids, neither their centromeric cohesion nor their kinetochores monopolar orientation. In an oocyte that is mutant for yem-alpha (yem1/Df3450), meiosis arrests at metaphase I. Defective orientation of sister kinetochores may occur to some extent (such as bi-orientation at meiosis I). In a recombination defective background, yem1/Df3450 oocytes resume meiosis prematurely. When bi-oriented sisters lose centromeric cohesion before undergoing poleward migration a single division occurs. If conditions are met (segregation of an appropriate set of chromosomes), viable diploid eggs may form with a single or two X chromosomes. The exceptional progeny being parthenogenetic they are essentially females (egg with 2 maternal X chromosomes). A fraction of sterile males developed from eggs with a single maternal X chromosome. This suggests the possibility that these eggs resulted from an equational division with some mixed segregation occurring too.

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