Integrative mapping analysis of chicken microchromosome 16 organization

<p>Abstract</p> <p>Background</p> <p>The chicken karyotype is composed of 39 chromosome pairs, of which 9 still remain totally absent from the current genome sequence assembly, despite international efforts towards complete coverage. Some others are only very partially sequenced, amongst which microchromosome 16 (GGA16), particularly under-represented, with only 433 kb assembled for a full estimated size of 9 to 11 Mb. Besides the obvious need of full genome coverage with genetic markers for QTL (Quantitative Trait Loci) mapping and major genes identification studies, there is a major interest in the detailed study of this chromosome because it carries the two genetically independent <it>MHC </it>complexes <it>B </it>and <it>Y</it>. In addition, GGA16 carries the ribosomal RNA (<it>rRNA</it>) genes cluster, also known as the <it>NOR </it>(nucleolus organizer region). The purpose of the present study is to construct and present high resolution integrated maps of GGA16 to refine its organization and improve its coverage with genetic markers.</p> <p>Results</p> <p>We developed 79 STS (Sequence Tagged Site) markers to build a physical RH (radiation hybrid) map and 34 genetic markers to extend the genetic map of GGA16. We screened a BAC (Bacterial Artificial Chromosome) library with markers for the <it>MHC-B</it>, <it>MHC-Y </it>and <it>rRNA </it>complexes. Selected clones were used to perform high resolution FISH (Fluorescent <it>In Situ </it>Hybridization) mapping on giant meiotic lampbrush chromosomes, allowing meiotic mapping in addition to the confirmation of the order of the three clusters along the chromosome. A region with high recombination rates and containing PO41 repeated elements separates the two <it>MHC </it>complexes.</p> <p>Conclusions</p> <p>The three complementary mapping strategies used refine greatly our knowledge of chicken microchromosome 16 organisation. The characterisation of the recombination hotspots separating the two <it>MHC </it>complexes demonstrates the presence of PO41 repetitive sequences both in tandem and inverted orientation. However, this region still needs to be studied in more detail.</p

Centromere positions in chicken and Japanese quail chromosomes: de novo centromere formation versus pericentric inversions

Chicken (Gallus gallus domesticus, GGA) and Japanese quail (Coturnix coturnix japonica, CCO) karyotypes are very similar. They have identical chromosome number (2n = 78) and show a high degree of synteny. Centromere positions on the majority of orthologous chromosomes are different in these two species. To explore the nature of this divergence, we used high-resolution comparative fluorescent in situ hybridization mapping on giant lampbrush chromosomes (LBCs) from growing oocytes. We applied 41 BAC clones specific for GGA1, 2, 3, 11, 12, 13, 14, and 15 to chicken and quail LBCs. This approach allowed us to rule out a pericentric inversion earlier proposed to explain the difference between GGA1 and CCO1. In addition to a well-established large-scale pericentric inversion that discriminates GGA2 and CCO2, we identified another, smaller one in the large inverted region. For the first time, we described in detail inversions that distinguish GGA3 from CCO3 and GGA11 from CCO11. Despite the newly identified and confirmed inversions, our data suggest that, in chicken and Japanese quail, the difference in centromere positions is not mainly caused by pericentric inversions but is instead due to centromere repositioning events and the formation of new centromeres. We also consider the formation of short arms of quail microchromosomes by heterochromatin accumulation as a third scenario that could explain the discrepancy in centromeric indexes

Precise Centromere Positioning on Chicken Chromosome 3

Despite the progress of the chicken (Gallus gallus) genome sequencing project, the centromeric sequences of most macrochromosomes remain unknown. This makes it difficult to determine centromere positions in the genome sequence assembly. Using giant lampbrush chromosomes from growing oocytes, we analyzed in detail the pericentromeric region of chicken chromosome 3. Without knowing the DNA sequence, the centromeres at the lampbrush stage are detectable by immunostaining with antibodies against cohesin subunits. Immunostaining for cohesin followed by FISH with 23 BAC clones, covering the region from 0 to 23 Mb on chicken chromosome 3 (GGA3), allowed us to map the GGA3 centromere between BAC clones WAG38P15 and WAG54M22 located at position 2.3 and 2.5 Mb, respectively. This corresponds to the gap between 2 supercontigs at the 2.4-Mb position in the current GGA3 sequence assembly (build 2.1). Furthermore, we have determined that the current putative centromeric gap at position 11.6–13.1 Mb corresponds in fact to a long cluster of tandem chicken erythrocyte nuclear membrane repeats (CNM)

Extent of linkage disequilibrium in chicken

Many of the economically important traits in chicken are multifactorial and governed by multiple genes located at different quantitative trait loci (QTLs). The optimal marker density to identify these QTLs in linkage and association studies is largely determined by the extent of linkage disequilibrium (LD) around them. In this study, we investigated the extent of LD on two chromosomes in a white layer and two broiler chicken breeds. Pairwise levels of LD were calculated for 33 and 36 markers on chromosomes 10 and 28, respectively. We found that useful LD (i.e. an r2 value higher than 0.3) in Nutreco chicken breed E5 (inbred) can extend to around 1 cM on chromosomes 10 and 28, although in a second region on chromosome 28 it extends to about 2.5 cM. The extent in breed Nutreco E3 (outbred) was very short in chromosome 10 (15 kb) but very much larger on chromosome 28, particularly in one region of depressed heterozygosity. The layer breed E2 (inbred) showed an extent of useful LD up to 4 cM on chromosome 10; the extent on chromosome 28 could not be assessed due to an erratic pattern of LD on that chromosome, although in one region LD appears to be in the order of 0.8 cM. This indicates that there may be very large differences in patterns of LD between different chicken breeds and different genomic regions