Genomic Structures of the kW1 Loci on the Z and W Chromosomes in Ratite Birds: Structural Changes at an Early Stage of W Chromosome Differentiation

Abstract

The W chromosome of ratite birds shows minimal morphological differentiation and retains homology of genetic linkage and gene order with a substantial stretch of the Z chromosome; however, the molecular structure in the differentiated region is still not well known. The kW1 sequence was isolated from the kiwi as a W-specific DNA marker for PCR-based molecular sexing of ratite birds. In ratite W chromosomes, this sequence commonly contains a ∼200-bp deletion. To characterize the very early event of avian sex chromosome differentiation, we performed molecular cytogenetic analyses of kW1 and its flanking sequences in paleognathous and neognathous birds and reptiles. Female-specific repeats were found in the kW1-flanking sequence of the cassowary (Casuarius casuarius), and the repeats have been amplified in the pericentromeric region of the W chromosomes of ratites, which may have resulted from the cessation of meiotic recombination between the Z and W chromosomes at an early stage of sex chromosome differentiation. The presence of the kW1 sequence in neognathous birds and a crocodilian species suggests that the kW1 sequence was present in the ancestral genome of Archosauria; however, it disappeared in other reptilian taxa and several lineages of neognathous birds.

The Staurotypus Turtles and Aves Share the Same Origin of Sex Chromosomes but Evolved Different Types of Heterogametic Sex Determination

“…Although ACO1 ( IREBP1 ) was previously mapped to the ostrich Z chromosome [26], [27], we cloned a cDNA fragment of this gene and mapped it to determine its precise location on the ostrich Z chromosome. We also mapped DMRT1 to ostrich chromosomes using the cDNA fragments isolated in our previous study [18]. We then constructed a cytogenetic map of the ostrich Z and W chromosomes with 16 functional genes by adding seven ostrich Z-linked genes ( ATP5A1 , CHD1 , GHR , NTRK2 , RPS6 , SPIN , and TMOD ), which were cloned and mapped in our previous studies (Figure S2) [18], [27].…”

“…We also mapped DMRT1 to ostrich chromosomes using the cDNA fragments isolated in our previous study [18]. We then constructed a cytogenetic map of the ostrich Z and W chromosomes with 16 functional genes by adding seven ostrich Z-linked genes ( ATP5A1 , CHD1 , GHR , NTRK2 , RPS6 , SPIN , and TMOD ), which were cloned and mapped in our previous studies (Figure S2) [18], [27]. Nucleotide sequence identities in the equivalent regions of cDNA fragments of 16 genes ranged from 79.6% to 94.4% between S. triporcatus and the ostrich (Table 2).…”

“…a ACO1 , aconitase 1, soluble; ATP5A1 , ATP synthase, H + transporting, mitochondrial F1 complex, alpha subunit, isoform 1, cardiac muscle; CHD1 , chromodomain helicase DNA binding protein 1; DMRT1 , doublesex and mab-3 related transcription factor 1; FER , (fps/fes related) tyrosine kinase; GHR , growth hormone receptor; HMGCR , 3-hydroxy-3-methylglutaryl-CoA reductase; KIF2A , kinesin heavy chain member 2A; NARS , asparaginyl-tRNA synthetase; NFIB , nuclear factor I/B; NTRK2 , neurotrophic tyrosine kinase receptor, type 2; RNF20 , ring finger protein 20, E3 ubiquitin protein ligase; RPS6 , ribosomal protein S6; SPIN , spindlin; TMOD , tropomodulin 1; VCP , valosin containing protein.bThe number in parenthesis indicates the number of identical bases/the number of bases in the overlapped region between cDNA fragments of two species.cThe nucleotide sequences were obtained from Tsuda et al [27].dThe nucleotide sequence was obtained from Ishijima et al [18].…”

“…Cell culturing and chromosome preparation were performed as described previously [12]. Fibroblasts of the ostrich used in our previous study [18] were recovered from liquid nitrogen and subsequently cultured for chromosome preparation. For gene mapping by fluorescence in situ hybridization (FISH), replication banding was performed to identify each chromosome precisely, as described previously [12], [19].…”

“…triporcatus and testis of the ostrich were homogenized and lysed with TRIzol Reagent (Life Technologies), and total RNA was extracted following the manufacturer’s instructions. Testis tissues of the ostrich used in our previous study [18] were recovered from liquid nitrogen. Molecular cloning of S .…”