Sex determination in the representatives of the superorder Ratite birds

• greater rhea (Rhea americana) (order Rheiformes)
• common ostrich (Struthio camellus) (order Struthioniformes)
• emu (Dromaius novaehollandiae) (order Casuariiformes)
• southern cassowary (Casuarius casuarius) (order Casuariiformes)
• kiwi (Apteryx australis) (order Apterygiformes)

The sex in birds is determined by sex chromosomes Z and W. The female birds are heterogametic (contrary to humans) and their chromosome set contains combination of sex chromosomes ZW. The males are homogametic and their chromosome set contains two Z chromosomes (ZZ).  The sex determination in an unknown bird is based on proving the absence or presence of W-chromosome or a specific sequence of W-chromosome.

According to the taxonomic classification the avian species are classified into two primary groups – flightless ratites (the Paleognathae) and flying birds (the Neognathae). The Paleognathaes/Ratites include flying tinamous (Tinamiformes), the kiwis (Apterygiformes), the cassowaries (Casuariformes), the rheas (Rheiformes) and the ostriches (Struthioniformes).

Birds  are generally classified into two groups on the basis of skull morphology. In case of ratites, the skull base is of Palaeognathous type and in case of flying birds of the Neognathous type. This classification has been also confirmed at the molecular level by DNA hybridization and sequencing of mitochondrial RNA genes (Sibley & Ahlquist 1990, van Tuinen et al. 1998, 2000).  The DNA analysis was also used for creation of taxonomy of avian species. Between the two main groups of birds, significant differences in the structure of the sex chromosomes were found.

The flying birds (Neognathae) have very different female chromosomes W that are relatively smaller than the male chromosomes Z and are highly heterochromatized and replicate later, while the ratites (Paleognathae) retained the most primitive forms of avian sex chromosomes Z and W that are highly homomorphic (Takagi et al 1972;. de Boer 1980; Ansari et al. 1988). These two groups of birds diverged evolutionary 120 million years ago. (van Tuinen a Hedges, 2001). From phylogenetic point of view, the tinamous are positioned as a sister group of the ratites (van Tuinen et al 1998, 2000;. Cracraft 2001). In the tinamous, half to two-thirds of W chromosomes consist of heterochromatin. Therefore, the W chromosomes in tinamous are considered as transient stage between the heterochromatized W chromosomes in ratites and highly euchromatized W chromosomes in the Neognathaes. These findings have been also confirmed by cytogenetic studies at the level of meiotic chromosome pairing.

The FISH method (Fluorescent in-situ hybridization) was used for comparison of Z-chromosome in chickens and emus and revealed large homologies between the Z and W chromosomes at the molecular level in the emu (Dromaius novaehollandiae) (Shetty et al. 1999). The homology has been also confirmed by comparison of sex chromosomes in the emu, the ostrich and the double-wattled cassowary (Casuarius casuarius). The Z and W chromosomes in the ratites are largely homomorphic except for slight sex chromosome differentiation (Ogawa et al 1998.; Nishida-Umehara et al. 1999; Shetty et al. 2002) that is successfully used for determination of sex by the methods of molecular genetics.

.

References:

Ansari HA, Takagi N, Sasaki M (1988) Morphological differentiation of sex chromosomes in three species of ratite birds. Cytogenet Cell Genet 47:185-188

Cracraft J (2001) Avian evolution, Gondwana biogeography and the Cretaceous-Tertiary mass extinction event. Proc R Soc Lond B 268:459-469

de Boer LEM (1980) Do the chromosomes of the kiwi provide evidence for a monophyletic origin of the ratites? Nature 287:84-85

Huynen L, Craig D, Millar R, Lmbert DM (2002) A DNA test to sex ratite birds. Molecular Ecology, 11, 851-856

Nishida-Umehara C, Fujiwara A, Ogawa A, Mizuno S, Abe S, Yoshida MC (1999) Differentiation of Z and W chromosomes revealed by replication banding and FISH mapping of sex-chromosome-linked DNA markers in the cassowary (Aves, Ratitae). Chromosome Res 7:635-640

Ogawa A, Murata K, Mizuno S (1998) The location of Z- and W-linked marker genes and sequence on the homomorphic sex chromosomes of the ostrich and the emu. Proc Natl Acad Sci USA 95:4415-4418

Shetty S, Griffin DK, Graves JAM (1999) Comparative painting reveals strong chromosome homology over 80 million years of bird evolution. Chromosome Res 7:289-295

Shetty S, Kirby P, Zarkower D, Graves JAM (2002) DMRT1 in a ratite bird: evidence for a role in sex determination and discovery of a putative regulatory element.Cytogenet Genome Res 99:245-251

Sibley CG, Ahlquist JE (1990) Phylogeny and classification of birds: a study in molecular evolution. Yale University Press, New Heaven

Takagi N (1972) A comparative study of the chromosome replication in 6 species of birds. Jpn J Genet 47:115-123

Takagi N, Itoh M, Sasaki M (1972) Chromosome studies in four species of Ratitae (Aves). Chromosoma 36: 281-291

van Tuinen M, Sibley CG, Hedges SB (1998) Phylogeny and biogeography of ratite birds inferred from DNA sequences of the mitochondrial ribosomal genes. Mol Biol Evol 15:370-376

van Tuinen M, Sibley CG, Hedges SB (2000) The early history of modern birds inferred from DNA sequences of nuclear and mitochondrial ribosomal genes. Mol Biol Evol 17:451-457

van Tuinen M, Hedges SB (2001) Calibration of avian molecular clocks. Mol Biol Evol 18:206-213