A genome-wide screen identifies a single beta-defensin gene cluster in the chicken: implications for the origin and evolution of mammalian defensins.

BACKGROUND: Defensins comprise a large family of cationic antimicrobial peptides that are characterized by the presence of a conserved cysteine-rich defensin motif. Based on the spacing pattern of cysteines, these defensins are broadly divided into five groups, namely plant, invertebrate, alpha-, beta-, and theta-defensins, with the last three groups being mostly found in mammalian species. However, the evolutionary relationships among these five groups of defensins remain controversial. RESULTS: Following a comprehensive screen, here we report that the chicken genome encodes a total of 13 different beta-defensins but with no other groups of defensins being discovered. These chicken beta-defensin genes, designated as Gallinacin 1-13, are clustered densely within a 86-Kb distance on the chromosome 3q3.5-q3.7. The deduced peptides vary from 63 to 104 amino acid residues in length sharing the characteristic defensin motif. Based on the tissue expression pattern, 13 beta-defensin genes can be divided into two subgroups with Gallinacin 1-7 being predominantly expressed in bone marrow and the respiratory tract and the remaining genes being restricted to liver and the urogenital tract. Comparative analysis of the defensin clusters among chicken, mouse, and human suggested that vertebrate defensins have evolved from a single beta-defensin-like gene, which has undergone rapid duplication, diversification, and translocation in various vertebrate lineages during evolution. CONCLUSIONS: We conclude that the chicken genome encodes only beta-defensin sequences and that all mammalian defensins are evolved from a common beta-defensin-like ancestor. The alpha-defensins arose from beta-defensins by gene duplication, which may have occurred after the divergence of mammals from other vertebrates, and theta-defensins have arisen from alpha-defensins specific to the primate lineage. Further analysis of these defensins in different vertebrate lineages will shed light on the mechanisms of host defense and evolution of innate immunity.

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

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.

Highly conserved linkage homology between birds and turtles: bird and turtle chromosomes are precise counterparts of each other.

The karyotypes of birds, turtles and snakes are characterized by two distinct chromosomal components, macrochromosomes and microchromosomes. This close karyological relationship between birds and reptiles has long been a topic of speculation among cytogeneticists and evolutionary biologists; however, there is scarcely any evidence for orthology at the molecular level. To define the conserved chromosome synteny among humans, chickens and reptiles and the process of genome evolution in the amniotes, we constructed comparative cytogenetic maps of the Chinese soft-shelled turtle (Pelodiscus sinensis) and the Japanese four-striped rat snake (Elaphe quadrivirgata) using cDNA clones of reptile functional genes. Homology between the turtle and chicken chromosomes is highly conserved, with the six largest chromosomes being almost equivalent to each other. On the other hand, homology to chicken chromosomes is lower in the snake than in the turtle. Turtle chromosome 6q and snake chromosome 2p represent conserved synteny with the chicken Z chromosome. These results suggest that the avian and turtle genomes have been well conserved during the evolution of the Arcosauria. The avian and snake sex Z chromosomes were derived from different autosomes in a common ancestor, indicating that the causative genes of sex determination may be different between birds and snakes.