Avian sex chromosomes in the lampbrush form: the ZW lampbrush bivalents from six species of bird

Abstract

The ZW bivalent has been identified and characterized in detail in its lampbrush form in oocytes of chicken, quail, turkey, pigeon, chaffinch and sparrow. The sex bivalent in all six species looks like a single highly asymmetrical chromosome. Most of it has the typical lampbrush organization. The terminal one-fifth is relatively thick and condensed and bears only a few pairs of lateral loops: this condensed terminal region is the W chromosome; the part with normal lampbrush morphology is the Z. The two are connected by a single near terminal chiasma. The fine scale morphology and arrangement of loops and markers on Z and W chromosomes are described for each species and lampbrush maps have been constructed. The identification of the lampbrush sex bivalent is based on the following criteria. The asymmetrical chromosome has two centromere regions. In the interstitial region of the asymmetrical chromosome where the junction between Z and W chromosomes is supposed to be, there are telomere-specific loops and telomeric DNA sequences and there is good morphological evidence for the presence of a chiasma. There are W chromosome specific DNA sequences in the region of the asymmetrical lampbrush chromosome that is thought to represent the W. Breed-specific variations in the morphology of the chicken W chromosome with respect to the sizes, numbers and arrangements of axial chromomeres and distributions of specified repeated DNA sequence families have been identified, offering one of the first examples of definitive correlation between a repeat family and a single chromomere. The lampbrush chromosomes of all the birds examined, except quail, terminate in distinctive free hanging loops. These are a novel feature in the sense that at the end of each chromatid there is a large transcription unit terminating in a cluster of telomeric DNA sequences.

FISH on avian lampbrush chromosomes produces higher resolution gene mapping

“…In the case of quail LBC4, it appears we have observed an example of unequal condensation/decondensation of chromosomal arms at the lampbrush stage. Earlier, this phenomenon was defined on sex chromosomes Z and W, which, in the lampbrush form, remain more condensed than other macrochromosomes (Solovei et al, 1993;Saifitdinova et al, 2003). Thus we can hypothesize that the terminal chromomere near the hybridization site of BAC clone bW008H20 corresponds to the heterochromatic short arm of quail chromosome 4.…”

“…Chicken and Japanese quail LBCs were isolated manually from oocytes of 1.0-1.5 mm diameter according to standard techniques (Solovei et al, 1993(Solovei et al, , 1994. After overnight fixation in 70% ethanol, preparations were dehydrated in a 70-80-96% ethanol series and air-dried.…”

Evolution of the avian sex chromosomes from an ancestral pair of autosomes

“…Several other observations support a homology of the avian Z and W chromosomes. First, and importantly, the terminal, nonrepetitive part of one of the W chromosome arms pairs with the terminal part of chromosome Zp during pachytene and diplotene of female meiosis (37)(38)(39)(40). Because pairing is associated with an obligate chiasmata, the region should be regarded as pseudoautosomal and must contain homologous, but as yet uncharacterized, sequences.…”

Meiotic drive and sex determination: molecular and cytological mechanisms of sex ratio adjustment in birds

“…The W chromosome is much smaller (figure 1e; Solari 1993) and, in the lampbrush form, more condensed than other chromosomes (Solovei et al 1993). In addition, avian sex chromosomes vary in centromere position (Panov & Bulatova 1972;Belterman & De Boer 1984;Solari 1993) and tandem repeats at terminal chromomeres (Matzke et al 1990;Krasikova et al 2006).…”

“…Interestingly, there is an extensive within-species variation in protein body sizes; for example, in rock dove Columba livia, house sparrow Passer domesticus and chaffinch Fringilla coelebs, the protein bodies vary from 1 to 10 mm (Solovei et al 1993). Importantly, protein bodies can also differ between sex chromosomes; for example, the protein body is larger on the W chromosome than on the Z chromosome in pigeons (Solovei et al 1993). Although the protein bodies are typical of the lampbrush stage, the cohesion proteins remain attached to the centromeres throughout the subsequent chromosome contraction stages keeping homologous chromosomes paired until the second division of meiosis (Hagstrom & Meyer 2003;Krasikova et al 2005Krasikova et al , 2006.…”