The fourth chromosome: targeting heterochromatin formation in Drosophila. Drosophila melanogaster chromosomes

Transcription

1 The fourth chromosome: targeting heterochromatin formation in Drosophila Drosophila melanogaster chromosomes modified from TS Painter, 1934, J. Hered 25:

2 DNA packaging domains HATs Euchromatin Less condensed Chromosome arms Unique sequences; gene rich Replicated throughout S Recombination during meiosis Heterochromatin Highly condensed Centromeres and telomeres Repetitious sequences; gene poor Replicated in late S No meiotic recombination Transcriptional activators Heterochromatin Protein 1 complex Hyper-acetylated histone tail Hypo-acetylated histone tail; methylated H3/K9 But- - the banded 1.2 Mb of the dot chromosome containes 82 genes- a normal gene density for the Drosophila chromosome arms - and a 10-fold higher concentration of repetitious elements - suggesting interspersed euchromatin and heterochromatin 2

3 HP1 HP1: a banded pattern on chromosome 4 HP2 Merge HP1 3

4 HP1 distribution is coincident with H3-mK9 but opposed to H4-acK8 P element transposon: variegating phenotype hsp26-plant +490 T hsp70-white 4

5 Chromosome four has interspersed heterochromatic and euchromatic domains 2-M C-12 2-M390 39C-42 *Many variegating reporters lie within genes. Chromosome four has interspersed heterochromatic and euchromatic domains This variegation is suppressed by loss of HP1. 5

6 Mobilization of the P element Local transposition Local deletion Local duplication Local Deletions Produce a Switch in Eye Phenotype 6

7 Local deletions induce a change in chromatin structure 39C-X 39C-12 4-M382 4-M348 4-M325 4-M33 4-M1226 Prt D XbaI P XbaI 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 100% 81% 74% 79% 63% 67% 42% 39C-X 39C-12 M382 M348 M325 M33 M1226 SalI D XbaI P XbaI SalI probe 5 kb A shift in histone acetylation correlates with shift in eye phenotype 39C-12 BEST Hcf CG2052 7

8 Heterochromatic vs euchromatic domains Heterochromatin Barrier? Euchromatin HATs HP1 complex Methylated histone H3 tail Transcriptional activators Acetylated histone tail Duplications also cause a switch in eye phenotype 8

9 Local duplications change the distance between the P element and 1360 remnants Variegating P inserts lie within 10 kb of a copy of element 1360 Distance to Nearest 1360 Element (Kb) M193 5-M340 2-M626 6-M180 5-M345 5-M275 6-M397 6-M350* 5-M93 5-M29 39C-12 5-M244 5-M263 6-M528 4-M979 4-M325 6-M365 6-M421 4-M993 6-M156 4-M637 2-M530 6-M534 4-M348 4-M271 4-M529 4-M779 4-M703 6-M114 4-M33 2-M010 4-M344 4-M M300 2-M802 3-M48 2-M59A.R 4-M382 6-M350** 9

10 Model: element 1360 (hoppel) as an initiator of heterochromatin A Model for Targeting Heterochromatin Formation 10

11 sirna generated from 1360 Conclusions The fourth chromosome of D. melanogaster is largely heterochromatic But 82 genes in 1.2 Mb- normal gene density Ten-fold higher levels of repetitious sequences Incomplete transposition of the P element on the fourth chromosome Results in local deletions and duplications Can cause a switch in phenotype Argues against a fixed boundary Supports an equilibrium model Suggests competition between alternative packaging states, summed by nucleosome modification Proximity to a 1360 associated with heterochromatin formation A role for RNAi? - mutations in RNAi machinery impact silencing, levels of H3-mK9 - observe 22 bp dsrna from 1360 suggests RNAi may target assembly of HP1-associated heterochromatin 11

12 The dot chromosome of D. virilis is not associated with HP1 Our research goal: To compare finished sequence from the dot chromosomes of D. melanogaster with D. virilis 12

13 Selection of target genes Small (dot) chromosome of many Drosophila species is known to have several of the same genes (Podemski, 2001*) Sequencing has recently been completed for D. pseudoobscura, a species my diverged from D. melanogaster Spring 2003, Rachel Shevchek did a BLAST comparison using cdna sequences from all the genes from the dot chromosome of D. melanogaster to the genomic sequence of D. pseudoobscura from the Baylor website She looked for >200bp chunks of genes that were very highly conserved (>80%) and designed PCR primers using Primer3 ( Library screened summer 2004 by Elmer Kellman & Libby Slawson; identified fosmids used in Bio 4342 in spring *Podemski, Ferrer, Locke (2001) Whole arm inversions of chromosome 4 in Drosophila species, Chromosoma 110, In situ hybridizations Ten fosmids confirmed by in situ hybridization to the polytene chromosomes of D. virilis Done in the lab of Dr. Mary- Lou Pardue at MIT Will not tell us what gene the fosmid contains, but will tell us where some of the DNA from the fosmid is localized *example in situ, M-L Pardue 13

14 Comparison of repeat densities in D. virilis (Dv) and D. melanogaster (Dm) Percentage of Repetitious DNA (%) Dv Dm Simple Repeats DNA Transposons Retrotransposons Dm Dv 0 Dot Chromosomes Long Arms 14

15 Project Participants Students 04 Jim Bogenpohl, Seth Bloom, James Dee, Emiko Morimoto, Jenny Myoung, Andrew Nett, Fatih Ozsolak, Mandy Tittiger, Andrea Zeug Faculty and Staff Sarah Elgin, Prof Biology Elaine Mardis, Co-Director, GSC & Asst Prof Genetics Chris Shaffer, Biology, Senior Teaching Fellow Staff of the Genome Sequencing Center Jeremy Buhler, Asst Prof Computer Science Michael Brent, Asso Prof Computer Science 04 TA s Libby Slawson and Colin Malone Thanks to Rachel Shevchek, Elmer Kellman, Carolyn Craig, Mary Lou Pardue (MIT), David Lopatto (Grinnell) and to many other WU faculty & staff for guest lectures., Big Questions As we complete more of the D. virilis dot chromosome, will the same conclusions hold? Gene identity, synteny, evidence of rearrangements? Size of genes, gene density? Levels and kinds of repetitious sequences? Given sequence data from other Drosophila species, can we do a better job in defining genes? What about patterns of repetitious sequences? Previous- primarily identified coding regions Start sites for transcription? Regulatory motifs? Will D. mohavaensis look more like D. virilis than D. melanogaster? Other features? - Should we look for conserved non-coding regions? - How does our finished sequence compare to unfinished strain? - Other questions? 15