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Discovery of supernumerary B chromosomes in Drosophila melanogaster.

Bauerly E, Hughes SE, Vietti DR, Miller DE, McDowell W, Hawley RS - Genetics (2014)

Bottom Line: These B chromosomes also contain centromeres and are primarily composed of the heterochromatic AATAT satellite sequence.Although the AATAT sequence comprises the majority of the 4(th) chromosome heterochromatin, the B chromosomes lack most, if not all, 4(th) chromosome euchromatin.Presumably as a consequence of their heterochromatic content, these B chromosomes significantly modify position-effect variegation in two separate reporter systems, acting as enhancers of variegation in one case and suppressors in the other.

View Article: PubMed Central - PubMed

Affiliation: Stowers Institute for Medical Research, Kansas City, Missouri 64110.

ABSTRACT
B chromosomes are small, heterochromatic chromosomes that are transmitted in a non-Mendelian manner. We have identified a stock of Drosophila melanogaster that recently (within the last decade) acquired an average of 10 B chromosomes per fly. These B chromosomes are transmitted by both males and females and can be maintained for multiple generations in a wild-type genetic background despite the fact that they cause high levels of 4(th) chromosome meiotic nondisjunction in females. Most curiously, these B chromosomes are mitotically unstable, suggesting either the absence of critical chromosomal sites or the inability of the meiotic or mitotic systems to cope with many additional chromosomes. These B chromosomes also contain centromeres and are primarily composed of the heterochromatic AATAT satellite sequence. Although the AATAT sequence comprises the majority of the 4(th) chromosome heterochromatin, the B chromosomes lack most, if not all, 4(th) chromosome euchromatin. Presumably as a consequence of their heterochromatic content, these B chromosomes significantly modify position-effect variegation in two separate reporter systems, acting as enhancers of variegation in one case and suppressors in the other. The identification of B chromosomes in a genetically tractable organism like D. melanogaster will facilitate studies of chromosome evolution and the analysis of the mechanisms by which meiotic and mitotic processes cope with additional chromosomes.

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Related in: MedlinePlus

B chromosomes in Drosophila melanogaster are smaller in size than the 4th chromosomes. Larval neuroblasts from 3rd instar larvae of (A) a normal wild-type female karyotype, (B) a male from the mtrm126 stock displaying 12 B chromosomes, and (C) a WT+B female displaying 5 B chromosomes. As measured during cytological examination, the B chromosomes are ∼0.36 µm in size, while the 4th chromosomes averaged 0.56 µm. Arrows indicate the location of 4th chromosomes.
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fig4__B: B chromosomes in Drosophila melanogaster are smaller in size than the 4th chromosomes. Larval neuroblasts from 3rd instar larvae of (A) a normal wild-type female karyotype, (B) a male from the mtrm126 stock displaying 12 B chromosomes, and (C) a WT+B female displaying 5 B chromosomes. As measured during cytological examination, the B chromosomes are ∼0.36 µm in size, while the 4th chromosomes averaged 0.56 µm. Arrows indicate the location of 4th chromosomes.

Mentions: Cytological examination of the B chromosomes in neuroblast metaphases (Figure 4) revealed them to be ∼64% the size of the 4th chromosome. However, given the difference in intensity of staining between the B and 4th chromosomes, it is possible that a simple measurement overestimates the size of the B chromosomes. We assessed the number of B chromosomes by two methods: counting CID foci in prometaphase I oocytes (Figure 1) and direct counting of B chromosomes in larval neuroblasts by careful analysis of z-stacks from acquired images (Figure 4). These two methods were employed because counting CID foci is difficult in the highly compacted configuration of a prometaphase I spindle, especially when we observe single DAPI masses that appear to be capped by CID foci on both ends (Figure S3), a complication not encountered in larval neuroblast imaging.


Discovery of supernumerary B chromosomes in Drosophila melanogaster.

Bauerly E, Hughes SE, Vietti DR, Miller DE, McDowell W, Hawley RS - Genetics (2014)

B chromosomes in Drosophila melanogaster are smaller in size than the 4th chromosomes. Larval neuroblasts from 3rd instar larvae of (A) a normal wild-type female karyotype, (B) a male from the mtrm126 stock displaying 12 B chromosomes, and (C) a WT+B female displaying 5 B chromosomes. As measured during cytological examination, the B chromosomes are ∼0.36 µm in size, while the 4th chromosomes averaged 0.56 µm. Arrows indicate the location of 4th chromosomes.
© Copyright Policy - open-access
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4286233&req=5

fig4__B: B chromosomes in Drosophila melanogaster are smaller in size than the 4th chromosomes. Larval neuroblasts from 3rd instar larvae of (A) a normal wild-type female karyotype, (B) a male from the mtrm126 stock displaying 12 B chromosomes, and (C) a WT+B female displaying 5 B chromosomes. As measured during cytological examination, the B chromosomes are ∼0.36 µm in size, while the 4th chromosomes averaged 0.56 µm. Arrows indicate the location of 4th chromosomes.
Mentions: Cytological examination of the B chromosomes in neuroblast metaphases (Figure 4) revealed them to be ∼64% the size of the 4th chromosome. However, given the difference in intensity of staining between the B and 4th chromosomes, it is possible that a simple measurement overestimates the size of the B chromosomes. We assessed the number of B chromosomes by two methods: counting CID foci in prometaphase I oocytes (Figure 1) and direct counting of B chromosomes in larval neuroblasts by careful analysis of z-stacks from acquired images (Figure 4). These two methods were employed because counting CID foci is difficult in the highly compacted configuration of a prometaphase I spindle, especially when we observe single DAPI masses that appear to be capped by CID foci on both ends (Figure S3), a complication not encountered in larval neuroblast imaging.

Bottom Line: These B chromosomes also contain centromeres and are primarily composed of the heterochromatic AATAT satellite sequence.Although the AATAT sequence comprises the majority of the 4(th) chromosome heterochromatin, the B chromosomes lack most, if not all, 4(th) chromosome euchromatin.Presumably as a consequence of their heterochromatic content, these B chromosomes significantly modify position-effect variegation in two separate reporter systems, acting as enhancers of variegation in one case and suppressors in the other.

View Article: PubMed Central - PubMed

Affiliation: Stowers Institute for Medical Research, Kansas City, Missouri 64110.

ABSTRACT
B chromosomes are small, heterochromatic chromosomes that are transmitted in a non-Mendelian manner. We have identified a stock of Drosophila melanogaster that recently (within the last decade) acquired an average of 10 B chromosomes per fly. These B chromosomes are transmitted by both males and females and can be maintained for multiple generations in a wild-type genetic background despite the fact that they cause high levels of 4(th) chromosome meiotic nondisjunction in females. Most curiously, these B chromosomes are mitotically unstable, suggesting either the absence of critical chromosomal sites or the inability of the meiotic or mitotic systems to cope with many additional chromosomes. These B chromosomes also contain centromeres and are primarily composed of the heterochromatic AATAT satellite sequence. Although the AATAT sequence comprises the majority of the 4(th) chromosome heterochromatin, the B chromosomes lack most, if not all, 4(th) chromosome euchromatin. Presumably as a consequence of their heterochromatic content, these B chromosomes significantly modify position-effect variegation in two separate reporter systems, acting as enhancers of variegation in one case and suppressors in the other. The identification of B chromosomes in a genetically tractable organism like D. melanogaster will facilitate studies of chromosome evolution and the analysis of the mechanisms by which meiotic and mitotic processes cope with additional chromosomes.

Show MeSH
Related in: MedlinePlus