Evolution of the Spindlin Gene in Birds: Independent Cessation of the Recombination of Sex Chromosomes at the Spindlin Locus in Neognathous Birds and Tinamous, a Palaeognathous Avian Family

Abstract

Tinamous (Aves, Palaeognathae, Tinamiformes) are primitive birds, generally considered to be the sister group to the ratites. Tinamous possess a W sex-chromosome, intermediate in heterochromatization between the largely euchromatic W chromosome of the ratites and the highly condensed W chromosome of the neognathous birds. Of the four genes which are known to have diverged copies on the neognathous avian W and Z chromosome (ATP5A1, CHD1, PKC and SPIN) only the spindlin gene has W- and Z-chromosomal forms in the tinamiformes. This paper describes experiments which show that the sequences of these forms are more similar to each other and to the homologous undifferentiated spindlin gene sequences in the ratite genome than to the W or Z forms of the spindlin gene in other, neognathous species. This suggests that cessation of recombination at the spindlin locus of the ancestral W and Z chromosomes of the paleognathous tinamiformes and the neognathous avian species were independent events.

Second report on chicken genes and chromosomes 2005

“…Molecular analysis of four gametologous gene loci in a paleognathous bird, Tinamous, revealed only a single gene, SPIN, that had differentiated into Z and W forms. This is consistent with the view that suppression of recombination did not occur simultaneously for all gametologous loci but in different steps, maybe involving different genes in different species (de Kloet and de Kloet, 2003). Female ostriches possess a single copy of the Z-linked marker IREBP, whereas in emus IREBP is present on both the Z and W chromosomes.…”

Comparison of the Z and W sex chromosomal architectures in elegant crested tinamou (Eudromia elegans) and ostrich (Struthio camelus) and the process of sex chromosome differentiation in palaeognathous birds

“…These data strongly suggest that the sex chromosome differentiation occurred around the CHD1 locus independently in the Tinamiformes lineage after the divergence of the Palaeognathae and the Neognathae. The Z-and W-chromosomal forms have also been reported for the SPIN gene in four Tinamiformes species, including E. elegans (de Kloet 2002;de Kloet and de Kloet 2003). In E. elegans, the SPIN locus is contained in the W-specific heterochromatin region, and chromosomal recombination may be suppressed around this locus.…”

“…The Z-and W-chromosomal forms of the CHD1 gene, CHD1Z and CHD1W, are present in neognathous birds, and the presence of the sex-specific forms facilitates the use of this gene as a molecular marker for sexing (Ellegren 1996;Griffiths et al 1996Griffiths et al , 1998Fridolfsson and Ellegren 1999). The presence of the Z-and W-chromosomal forms has been reported for five other "gametologous" genes (ATP5A1, UBAP2, SPIN and HINT) in neognathous birds: These are relic genes shared between homologous sex chromosomes as a result of the cessation of recombination (García-Moreno and Mindell 2000;de Kloet and de Kloet 2003;Handley et al 2004). No W-specific forms of these genes have been reported in palaeognathous ratite birds having the extensively homomorphic Z and W chromosomes.…”

Molecular evolutionary genomics of birds

“…However, despite their independent origin, the evolutionary mechanisms of sex chromosome evolution in birds and mammals may be similar. Recombination between the proto Z and W chromosomes seems to have ceased in a stepwise fashion, causing the formation of at least two evolutionary strata with distinct degrees of sequence divergence between sequences shared between Z and W (Ellegren and Carmichael, 2001;de Kloet and de Kloet, 2003;Handley et al, 2004). This mimics the situation seen in mammals (e.g.…”