The molecular basis of chromosome orthologies and sex chromosomal differentiation in palaeognathous birds

Abstract

Palaeognathous birds (Struthioniformes and Tinamiformes) have morphologically conserved karyotypes and less differentiated ZW sex chromosomes. To delineate interspecific chromosome orthologies in palaeognathous birds we conducted comparative chromosome painting with chicken (Gallus gallus, GGA) chromosome 1-9 and Z chromosome paints (GGA1-9 and GGAZ) for emu, double-wattled cassowary, ostrich, greater rhea, lesser rhea and elegant crested tinamou. All six species showed the same painting patterns: each probe was hybridized to a single pair of chromosomes with the exception that the GGA4 was hybridized to the fourth largest chromosome and a single pair of microchromosomes. The GGAZ was also hybridized to the entire region of the W chromosome, indicating that extensive homology remains between the Z and W chromosomes on the molecular level. Comparative FISH mapping of four Z-and/or W-linked markers, the ACO1/IREBP, ZOV3 and CHD1 genes and the EE0.6 sequence, revealed the presence of a small deletion in the proximal region of the long arm of the W chromosome in greater rhea and lesser rhea. These results suggest that the karyotypes and sex chromosomes of palaeognathous birds are highly conserved not only morphologically, but also at the molecular level; moreover, palaeognathous birds appear to retain the ancestral lineage of avian karyotypes.2

Intrachromosomal rearrangements in avian genome evolution: evidence for regions prone to breakpoints

“…Further species that have been of interest in zoo-FISH studies and would thus be likely candidates for studies in addition to the Falconiformes and Psittaciformes mentioned above include other Galliformes (Shibusawa et al, 2004a, b) and a range of species from orders such as Anseriformes and Passeriformes (Guttenbach et al, 2003). Comparisons with Paleognathous birds, whose chromosomes are thought to resemble closely the ancestral avian karyotype (Shetty et al, 1999;Nishida-Umehara et al, 2007) should provide a useful outgroup for understanding the chromosomal evolution of the Neoaves.…”

Disruption of FEM1C-W gene in zebra finch: evolutionary insights on avian ZW genes

“…The ancestral chromosomes of avian sex chromosomes are different from those of eutherian mammals and snakes (Fridolfsson et al 1998;Nanda et al 1999;Matsubara et al 2006), although recent evidence indicates surprising shared gene content of the avian Z and monotreme X chromosomes (Rens et al 2007). The gene content of the Z chromosome appears to have changed little during avian evolution, as shown by comparative painting analysis (Itoh and Arnold 2005;Nishida-Umehara et al 2007;Tsuda et al 2007), even in Ratites (Struthioniformes) which are considered to be an ancestral avian species whose sex chromosomes are morphologically relatively undifferentiated (Ogawa et al 1998;Shetty et al 1999). The sex determining system in birds is still unclear, although several candidate systems have been suggested.…”

Rowley Review

“…comm.). Cross-species chromosome painting and comparative gene mapping have shown that the large avian Z sex chromosome is highly conserved among groups, including the ratites (Shetty et al 1999;Nishida-Umehara et al 2007). (Chromosome painting involves fluorescently labelling an entire chromosome of one species and using that to probe to chromosomes of another species.…”

“…In the Chicken (c) the heteromorphic sex chromosomes are highly differentiated. Modified and reproduced with permission from Nanda et al (1998), andNishida-Umehara et al (2007). (d) Evolution of avian Z versus W sex chromosomes, with recombining regions shown in grey and non-recombining regions in black.…”